Chapter 2: Project ICEBERG
the Task Defined1
Three weeks after receipt of the Joint Chiefs of Staff directive ordering the Okinawa invasion, Admiral Nimitz’ headquarters published and distributed the ICEBERG Joint Staff Study. This study served as a planning guideline for the units assigned to the campaign and defined for them the objectives, the allotment of forces, and roughly outlined the scheme of maneuver ashore.
Although Operation CAUSEWAY, the invasion of Formosa, had been cancelled in favor of ICEBERG, the principal commanders for CAUSEWAY were retained for the Okinawa landing and redirected their staffs’ efforts towards planning for the assault on the newly assigned target. Admiral Raymond A. Spruance, whose Task Force 50 (TF 50) contained the Fifth Fleet and the Central Pacific Task Forces, was made responsible for the Ryukyus operation. His staff, previously charged with preparing plans for the Iwo Jima invasion scheduled for 20 January 1945, was now given the concurrent assignment of planning for Okinawa.
Certain assumptions governed task planning and the assignment of assault and garrison forces for ICEBERG. Adherence to the scheduled 1 March 1945 invasion date (L-Day) for Okinawa was based on the presupposed seizure of Iwo Jima at a date early enough to permit release of naval gunfire and air support units for the second operation. It was further assumed that ICEBERG commanders would be able to secure the prompt release from General MacArthur of assault shipping, support shipping, supporting naval forces, and Army troops assigned to the Philippines operation which had been earmarked for use later at Okinawa. Finally, before Okinawa was invaded, Allied air and surface superiority had to be gained in the target area.
This last point was one of the most important in the overall concept of the operation, for it was believed that air attacks on Japan, together with the conquest of Iwo Jima, would force a concentration of Japanese air strength on the bases which ringed the Home Islands. It would be necessary, therefore, to destroy enemy air installations at Japanese staging areas in Kyushu and Formosa, and neutralize those at Okinawa, since it was a basic assumption that enemy aircraft. would vigorously oppose any invasion attempt. For this reason, the scheme of maneuver ashore included plans for the
early securing of airfields on Okinawa and their equally early use by Allied land-based aircraft. Japanese sea communications in the Ryukyus area were to be severed before the operation by surface and air attacks on enemy shipping and by a maximum effort mounted by American submarines.
According to the ICEBERG staff study, operations ashore were to be conducted in three phases. To be accomplished in the first phase were the capture of the southern portion of Okinawa and small adjacent islands and the initial development of base facilities. In Phase II, Ie Shima and the remainder of Okinawa were to be seized and the base build-up continued with the construction of installations in favorable locations designated in the development plan. Phase III required the exploitation of Allied positions in the Nansei Shoto and, when Admiral Nimitz directed, the seizure and development of additional positions with forces then locally available. (See Map 4.)
It was envisioned that an army of two corps, each composed of three reinforced infantry divisions, would be required in the initial assault. In addition, two divisions were to be assigned as area reserve. Okinawa’s proximity to the heart of the Empire as well as to other major Japanese bases, and the expectation of fanatic resistance by enemy troops on a battleground of such large dimensions, presaged a prolonged period of fierce combat. For these reasons, a new command relationship was established for the Okinawa operation differing, in some respects, from that which had been effective in previous Pacific campaigns.
As strategic commander of the invasion forces, Admiral Nimitz directed that the chain of command would descend to Admiral Spruance, thence to Vice Admiral Richmond K. Turner who would command Task Force 51 (Joint Expeditionary Force), and then to Lieutenant General Simon B. Buckner, Jr., USA, who would command the Army, Navy, and Marine units comprising the Expeditionary Troops. When Spruance had determined that the amphibious phase of the invasion had ended, he would pass the command of all forces ashore to Buckner. As Commanding General of the Tenth Army, Buckner would assume responsibility for the defense and development of positions captured on the island. When the situation permitted, he would also relieve Admiral Spruance of the responsibility for the defense and development of the Ryukyus as a whole and, at that time, he would be directly responsible to CinCPOA for the captured island positions and for the waters within a 25-mile radius. Concurrently, responsibility for the establishment of an Island Command and a military government on Okinawa would be General Buckner’s also.
Allied Commanders and Forces2
Many units of Admiral Nimitz’ command not directly assigned Task Force 50 were to support the Okinawa landing
Map 4: ICEBERG Scheme of Maneuver
from bases widespread in the Pacific Ocean Areas. Additionally, from their airdromes in China and the Southwest Pacific, Army Air Forces elements were to assist the ICEBERG effort, both prior to and during the course of the campaign. In all, about 548,000 men of the Marine Corps, Army, and Navy, together with 318 combatant and 1,139 auxiliary vessels—exclusive of numerous small personnel craft of all types3—and a profusion of strategic and tactical aircraft were to strike some of the last blows dooming the Japanese attempts to gain supremacy in Asia and the Pacific.
In the Fifth Fleet were the Covering Forces and Special Groups which included the Fast Carrier Force (TF 58, Vice Admiral Marc A. Mitscher) and the British Carrier Force (TF 57, Vice Admiral Sir H. Bernard Rawlings, RN). These two forces were to conduct air strikes and neutralize Japanese air power prior to the landing, and prevent enemy air and surface interference with the Allied landing and subsequent occupation of Okinawa.
The units more directly concerned with the landing were components of Turner’s Task Force 51. Its complex composition reflected its many assignments incident to the capture, occupation, and defense of Okinawa. Any enemy attempt to disrupt the movement to the target or landing on the beach would be handled by the force’s support elements. These naval units would also undertake air support and minesweeping operations once the beachhead had been gained. Assignments for these tasks were allocated, in turn, to the Amphibious Support Force (TF 52, Rear Admiral William H. P. Blandy) which provided direct air and naval support, and to the Gunfire and Covering Force (TF 54, Rear Admiral Morton L. Deyo).4 The Northern Attack Force (TF 53, Rear Admiral Lawrence F. Reifsnider) and the Southern Attack Force (TF 55, Rear Admiral John L. Hall, Jr.,) contained the transports which were to lift the assault troops to the objective and the tractor units which were to land them on L-Day.
The assault of Okinawa and its surrounding islands was to be accomplished by the landing forces of Buckner’s Expeditionary Troops (TF 56). The assault force of the Northern Attack Force was Major General Roy S. Geiger’s III Amphibious Corps (IIIAC), composed of the 1st Marine Division (Major General Pedro A. del Vane) and the 6th Marine Division (Major General Lemuel C. Shepherd, Jr.). The Army XXIV Corps (Major General John R. Hedge) would be lifted by the Southern Attack Force and would consist of the 7th Infantry Division (Major General Archibald V. Arnold) and the 96th Infantry Division (Major General James L. Bradley).
One other major Marine echelon in the Tenth Army was Major General Francis P. Mulcahy’s joint air task
command, Tactical Air Force (TAF), which was to provide land-based air support for the operation once its squadrons were ashore. The elements initially assigned to TAF were to come primarily from the 2nd Marine Aircraft Wing (2nd MAW). Although TAF was established under the Tenth Army on 21 November 1944, its staff was not really organized until late in December. By that time, much of the earlier, basic, and important preinvasion planning had been completed without TAF participation. As a matter of fact, the last of the personnel assigned to TAF staff did not even report until after the assault echelon had already left for the target.5 Although he had not taken part in ICEBERG planning, General Mulcahy was kept fully abreast of Tenth Army activities and decisions by his chief of staff, Colonel Perry O. Parmelee, who daily visited Buckner’s headquarters and attended briefings and conferences there.6
A most important element of TAF was its fighter arm, the Air Defense Command (ADC), headed by Brigadier General William J. Wallace who had formerly been AirFMFPac Chief of Staff. Wallace’s squadrons were to begin operations from previously designated airfields on Okinawa as soon as they had been captured by the ground troops. Initially, General Wallace’s command consisted of a headquarters squadron and a service squadron, and three MAGs with a total complement of nine fighter, two night fighter, and four air warning squadrons, The radar installations of the units last named would give early warning of enemy air attacks. An Army Air Forces fighter wing was also part of ADC, but only one group was to join TAF before the campaign was brought to a close.
General Mulcahy’s Bomber Command was made up wholly of AAF flight and support elements, none of which arrived on Okinawa before the beginning of June. Photographic coverage of enemy installations, interpretation of the pictures thus obtained, and an aerial photographic survey of the island for mapping purposes were to be the missions of an AAF photo-reconnaissance squadron which was also part of the TAF organization.
Rounding out the Tenth Army air force were two Marine torpedo-bomber squadrons which were to conduct antisubmarine warfare operations together with the carrier-based naval aircraft at the target. The Marine squadrons were also prepared to conduct bombing attacks on ground targets and any other missions when the need for them arose.
Marine aviation, other than that which was organic to TAF, was to play an important part in the invasion. Artillery spotting was the assigned mission of Marine observation squadrons
attached to the Marine divisions and corps. Scheduled to control all aircraft in support of the ground forces were Colonel Vernon E, Megee’s Landing Force Air Support Control Units (LFASCUS) . When directed by Admiral Turner, LFASCUS, set up ashore at the headquarters of Tenth Army and its two corps, would take over control from their shipboard naval counterparts.
In addition to the tactical units assigned to the Tenth Army for the assault and consolidation phases of the operation, General Buckner was to have direct command of the defense and service troops assigned for the garrison phase, Major General Fred C. Wallace, USA, was designated Island Commander, Okinawa,7 while the Naval Forces, Ryukyus, were to be commanded by Rear Admiral Calvin H. Cobb, who would assume his command upon completion of the amphibious phase of the operation, Although strategic air force and naval search squadrons were to be based on Okinawa, they would remain under the operational control of the Commanding General, Army Air Forces, Pacific Ocean Area, and Commander, Fifth Fleet, respectively.
Infantry units were assigned also to the Western Islands Attack Group (TG 51.1, Rear Admiral Ingolf N. Kiland) which had the 77th Infantry Division (Major General Andrew D. Bruce) as its landing force; the Demonstration Group (TG 51.2, Rear Admiral Jerauld Wright) whose landing force was the 2nd Marine Division (Major General Thomas E. Watson) ; and the Floating Reserve Group (TG 51.3, Commodore John B. McGovern) which carried 27th Infantry Division (Major General George W. Griner, Jr.).
Joint Preparations and Planning8
Intensive joint planning attested to the immensity of the future operation. Smooth Army, Navy, and Marine Corps coordination of operational, logistical, and administrative matters was imperative. Since the Tenth Army, under CinCPOA, would consist of an Army corps and a Marine amphibious corps, and a large naval contingent, General Buckner believed that it was important for him to have a joint staff. He therefore requested Admiral Nimitz to authorize a Marine and naval augmentation of his staff. When this request was granted, approximately 30 Marine and 30 Navy officers, and enlisted assistants from each of these services, were assigned and integrated within the Tenth Army staff. “There was no Marine or naval section of the staff. “9 One of the Marine officers was Brigadier General Oliver P. Smith, who became the Marine Deputy Chief of Staff; he had been the Assistant Division Commander of the
1st Marine Division in the Peleliu campaign. His counterpart on the Tenth Army staff was Brigadier General Lawrence E. Schick, who filled the billet of Army Deputy Chief of Staff.
When General Smith arrived at Tenth Army headquarters, he found that CinCPOA had already approved the Marine augmentation for the Army staff. The Marine general believed that this augmentation was overly large, for:–
This padding would result in Marine officers doing clerical duty at Army Headquarters as there were manifestly not enough bona fide billets to take care of all the Army officers on the staff as well as the Marine and naval officers.10
After considerable discussion with the Army officer responsible for the assignment of staff billets, General Smith managed to have the number of Marine officers on the Tenth Army staff reduced by nearly 30 percent.11
A tactical concept based upon the directive stated in the ICEBERG joint staff study, and later incorporated in the TF 50 operation plan, required “early use of sufficient airdrome capacity in Okinawa, together with unloading facilities adequate to support its development and to maintain positive control of the air in the area.”12 In a study of all landing beach areas in southern Okinawa, those beaches on the west coast which lay north and south of Hagushi were deemed to be best suited to sup- port the ICEBERG landing. Admiral Turner’s operation plan assumed that there would be bitter Japanese air reaction to the Okinawa invasion; that enemy submarines would be very active in the target area; that the Japanese surface fleet might possibly sortie out from its bases in Japan; and, that attempts might be made to reinforce the garrison on Okinawa. The first three assumptions proved correct; the fourth was not tested because, in accordance with the JCS directive ordering the invasion of Okinawa, Allied air and surface superiority had been gained prior to L-Day.
Based on Admiral Turner’s plan, the Tenth Army staff drew up Plan Fox, which committed the assault forces to a landing on the west coast. Plan Fox also included the pre-L-Day capture of Keise Shima, since a study of this small island indicated the feasibility of its use as a fixed emplacement for artillery which would first augment the naval and air bombardment of the main objective before the landing, and afterwards provide support during the land campaign. This plan, approved by Buckner, was presented to Turner at the initial joint conference held at Pearl Harbor on 1 November 1944.
Following this presentation, Turner stated his views of the operation and outlined what would be the requirements of the Navy during the course of ICEBERG. He believed that, prior to the landings on Okinawa, the adjacent islands had to be neutralized. Once this had been done, the major landings on Okinawa would be more secure and the fleet could be replenished in a safe anchorage
without danger from enemy surface vessels or submarines.
Two provisions of Plan Fox particularly concerned the Fifth Fleet commander. Because of the suspected presence of Japanese mines and submarines immediately west of Okinawa, should the Hagushi beaches be used for the invasion, the landings here would perforce require the fleet to steam into a hazardous area. The second apprehension arose because 1 March had been scheduled as L-Day. He feared that unfavorable weather conditions, which generally prevailed in March, might possibly affect the conduct of the landings and unduly prolong the unloading of supplies on exposed beaches. Available meteorological data justified this concern, for from October to March the Ryukyus experienced strong northerly winds with a mean velocity of 17–19 miles-per-hour as well as frequent gales. A generally moderate wind, averaging 11 miles-per-hour, marked the beginning of the summer monsoon period and characterized the weather of Okinawa in April, which was a more suitable time for the invasion.13 In any case, Turner requested that the possibility of landings along the east coast be restudied. At the same time, he suggested that the value of a feint landing be determined and, if valid, should be incorporated in the plan finally adopted for ICEBERG.
After a lengthy discussion of the problems inherent in the proposed plan, the conferees concluded that a landing on the western beaches on 1 March was fraught with considerable risk. The alternatives were either a 30-day delay of the operation or a landing on the southeast coast on the date originally scheduled for the assault. All other possible courses of action were re-examined, with the result that the Hagushi beaches were recommended again as the site for the landings. Final approval was withheld by Turner because he retained doubts as to the practicality of landing and supporting the proposed assault force of four divisions over the Hagushi beachhead. In spite of the objections of Admiral Turner, the Plan Fox estimate was distributed on 5 November.14 When completed on 9 November, another detailed study upheld the original contention that Hagushi held the only beaches in southern Okinawa adequate to receive four divisions abreast and, subsequently, to handle sufficient logistical support for the operation.
In the face of these convincing arguments, Admiral Turner accepted the plan with the proviso that both Kerama Retto and Keise Shima were to be captured prior to the main landing. With minor exceptions, General Buckner concurred with these modifications, and the revised plan was forwarded to Turner on 11 November. The original target date of 1 March was changed twice within the next month, first to 15 March and finally to 1 April. The first change was made on 19 November in anticipation of bad weather at the target at the beginning of March. On 7 December, Admiral Nimitz advanced L-Day two more weeks when doubts arose as to whether the shipping assigned to General MacArthur’s Lingayen Gulf operations
could be returned in time to permit its reemployment at Okinawa.15
Since the naval planning staff recommended a sustained seven- or eight-day bombardment of the assault beaches, the resulting expenditure of Navy supplies and ammunition would force the bombardment group to either withdraw from the area for resupply and refueling or to conduct these operations under dangerous conditions in the open sea offshore of the objective. Basically, it was this consideration that prompted Turner’s insistence on the pre-L-Day capture of the entire Kerama group. At first, these islands appeared to be only worthy as targets for amphibious raids in which the raiding parties would retire after destroying enemy coastal artillery. Later plans for their capture grew out of Admiral Turner’s proposal that, once taken, the Keramas provide a protected anchorage for the establishment of a small-boat pool and a seaplane base.
Because the Kerama assault was now to be a full-scale invasion instead of a raid, the assignment of a larger force was indicated and Major General Thomas E. Watson’s 2nd Marine Division was chosen initially. This unit, designated IIIAC Reserve, had been slated for early commitment in support of operations on Okinawa, and so the task of capturing the Keramas was given instead to the 77th Infantry Division while the Marine division was assigned tentatively to a feint landing off southeastern Okinawa.16
As the scope and importance of preliminary operations grew, the reserves which had been made available to General Buckner originally decreased in number, and it was found necessary to secure from CinCPOA release of the area reserve division (27th Infantry Division). This unit was then designated as the Tenth Army floating reserve and was replaced by the 81st Infantry Division which remained in New Caledonia under Admiral Nimitz’ control.
The alternate plan for the operation, Plan BAKER, was approved on 3 January 1945. It envisioned first the capture of
Kerama Retto, followed by a sweep of the Eastern Islands by General Watson’s Marines. Both of these actions were to be conducted prior to the assault of Okinawa itself. A mixed Marine and Army corps artillery group was to support both the XXIV and III Amphibious Corps assault of the east coast.
On L-Day, General Geiger’s Marines would land between Chinen Point and Minatoga, secure the high ground behind the beaches, and, following the Army landing two days later, tie-in with XXIV Corps at Yonabaru. After effecting this juncture, both corps were to make a rapid advance across the island during which time the Marines were to take the airfield on Oroku Peninsula and the Army was to capture the unfinished field at Yonabaru. Included in the alternate plan were provisions for the capture of Ie Shima, feints against Chimu Wan on L plus 3 or 4, and, overall, the maintenance of flexibility of action in the commitment of Army reserves to either of the corps zones or for the protection of XXIV Corps’ northern flank.
Although the principal advantages of Plan Baker were that the approach to the east coast of Okinawa was more direct and the weather here was vastly superior to that of the west coast, they were outweighed by the disadvantages. These included: (1) the difficulty of providing optimum naval gunfire support because of the interposition of the Eastern Islands and off-shore islets, (2) the paucity of good beaches, (3) the length of time it would take to uncover airfields, located, for the most part, on the west coast, and, (4) because of Plan Baker landing zone assignments, the possibility that Japanese forces might be able to concentrate considerable strength against IIIAC troops before they could even contact the XXIV Corps. General Smith was convinced at this time that “in the advent of bad weather on the west coast, landings would have been delayed rather than resort to the east coast landing as provided in the alternate plan.”17
General Geiger became involved in the planning for ICEBERG in November 1944, when he was directed to report to General Buckner for planning purposes. Upon receipt of this order, the IIIAC commander immediately reported by dispatch. Shortly thereafter, IIIAC headquarters received a copy of the tentative Plan Fox together with all available intelligence on the prospective target, and a request that Geiger prepare a tentative corps operation plan.
When the IIIAC plan was completed, and at the request of Buckner, Geiger, accompanied by his chief of staff, Colonel Merwin H. Silverthorn,18 his G-2, Lieutenant Colonel Sidney S. Wade, his G-3, Colonel Walter A. Wachtler, his G-4, Colonel Francis B. Loomis, Jr., and other members of his staff, departed Guadalcanal for Pearl Harbor, arriving at Schofield Barracks on 9 December. After personally contacting their opposite numbers on the Tenth Army staff, the IIIAC staff officers prepared to present their plan to General Buckner.
Geiger planned to employ the 1st and 6th Marine Divisions in the assault, with General del Vane’s division on the right or south flank. The choice of these
divisions was logical since they were both located in the Solomons and there would be no problem in establishing liaison. The 2nd Division, based on Saipan, would be the floating reserve of the army, according to the IIIAC plan. The question then arose regarding what steps would be taken if the Japanese were encountered in strength as IIIAC advanced eastward across Okinawa, for there was no doubt that an additional division would have to be inserted in the line before the east coast was reached. General Smith took this question up with the Tenth Army commander, who agreed that IIIAC would have first call on the 2nd Marine Division.19
General Watson’s division was scheduled to make the feint landings on the southeast coast of Okinawa on L-Day and L plus 1, and it was not contemplated that Geiger would need it before the third day of the operation. The IIIAC staff presented their plan orally to General Buckner on 19 December, when it was approved. According to General Smith, who was present on this occasion, Geiger’s staff members “did a very creditable job. ...”20
Scheme of Maneuver21
Basically, the scheme of maneuver ashore was designed to attain early use of the airfields so that land-based air supremacy over the target could be gained and held. An additional dividend derived from the capture of the airfields would be their use as staging bases for continuing mass air raids on both Japan and those areas within flying range of Okinawa under enemy control. As in the case of earlier amphibious landings in the Pacific, certain preliminary softening-up steps had to be taken before the main assault was launched.
Kerama Retto was to be seized by the 77th Infantry Division (Reinforced) on 26 March 1945, or six days before L-Day. Following the first day of operations in the Kerama Retto and beginning the night of the 26th, Marines of the FMF Amphibious Reconnaissance Battalion were to reconnoiter the reef islets of the island group. First they were to investigate Keise Shima for the presence of enemy troops, and in the following days and nights prior to L-Day, they were to land on Aware Shima, Mae Shima, and Kuro Shima. To support the landing on Okinawa, a field artillery group of XXIV Corps Artillery was to land and be emplaced on Keise Shima prior to L-Day. While these operations were underway, Okinawa would receive increased air and naval gunfire bombardment which would mount in intensity until the first assault waves neared the beaches. At this time, the fire would lift from the beach area and continue inland.
The Army and Marine divisions were to land on the Hagushi beaches, General Geiger’s corps on the left. The mouth of the Bishi Gawa marked the beginning of the corps boundary, which
roughly followed the course of the river to a point just north of Kadena; here, the line headed almost due east to bisect the island. (See Map 4.)
Once landed north of Hagushi town, the Marine assault divisions were to move rapidly inland, coordinating their advance with that of XXIV Corps. On the Marine left flank was the 6th Division; the 22nd Marines on the left and the 4th Marines, less its 2nd Battalion in division reserve, on the right. The 29th Marines, the third infantry regiment of the 6th Division, was corps reserve and was to be ready to land on any of the beaches. It was also to be prepared to revert one battalion landing team to the 6th Division on order. General Shepherd’s initial mission was the capture of Yontan airfield while protecting the northern flank of the Tenth Army.
General del Vane’s division, landing to the right of the 6th, was to assist in the capture of Yontan by quickly seizing the high ground northeast of China. The attack was then to continue, with major emphasis placed on maintaining contact with General Hodge’s corps and assisting his advance. The 1st Marine Division scheme of maneuver placed the 5th and 7th Marines in the assault, 7th on the left, and the 1st Marines in division reserve.
Adjoining the 1st Marine Division was to be the 7th Infantry Division, with one regiment in division reserve but under the operational control of XXIV Corps. The other Army assault division was to be 96th, which was to land with two regiments abreast and a third in corps reserve.
Artillery support for the Marines was to come from IIIAC Corps Artillery and those artillery units organic to the divisions. General Geiger’s guns were to land on his order to support the attack and, once ashore, corps artillery would coordinate all supporting arms in the Marine sector. XXIV Corps Artillery, less the group on Keise Shima, would land on General Hedge’s order and support the attack with long-range interdiction, counterbattery, and harassing fires.22
Following the initial landing, operations were designed to isolate the Phase I objective, which consisted of that part of the island lying south of a general line drawn across the Ishikawa Isthmus, through Chimu, and including the Eastern Islands. In order to prevent enemy reinforcement from the north and to fulfill its assignment in Phase I, IIIAC was to gain control of the isthmus as swiftly as possible. To seal off the Japanese in the south, General Hodge’s troops were to drive across the island, his right flank units holding a line that ran through Futema to Kuba Saki. Once the central portion of the island had been captured and secured, the direction of attack would be faced to the south and continued until all of the objectives of the first phase had been achieved.
Phase II, the seizure of northern Okinawa and the capture of Ie Shima, was to be executed with Tenth Army troops locally available when Buckner was satisfied that Phase I had been accomplished. The first major military objective
in the north was Motobu Peninsula, which was to be taken by means of simultaneously launched attacks from sea and land. Once the peninsula had been gained, a shore-to-shore assault would be made against Ie Shima. The end of Phase II would be signaled when the rest of northern Okinawa had been captured.
While higher echelon air planning for ICEBERG detailed both strategic and tactical missions, the Tenth Army was more immediately concerned with the latter. Carrier-based tactical aviation, aboard the TF 52 escort carrier group (TG 52.1, Rear Admiral Calvin T. Durgin), was to provide the invasion force with air support until General Mulcahy’s squadrons were established ashore and could take over. At this time, TAF would also be responsible for overall air defense.
When this responsibility was assumed, TAF operations would be based on the following order of priority: (1) attainment of air superiority by annihilation of enemy aircraft in the air and on the ground, and destruction of enemy air installations; (2) interdiction and destruction of enemy troop and supply movements immediately within or heading towards the target area; and (3) execution of combined air-ground attacks on specific frontline objectives. The importance of the first priority lay in Tenth Army recognition of the yet-existing Japanese air strength and the threat it posed to the invasion force.
As soon as Air Defense Command fighter squadrons were established ashore on captured airfields, they were to begin fulfilling their assigned missions. From these fields, ADC was to provide air defense to ground units on the island and naval forces in its environs. Combat air patrols, close air support, and other related flight missions were considered the means by which the defense was to be maintained. Although it was a function of ADC, close air support is not normally a part of air defense; it is more closely associated with a ground offensive concept. Despite this fact, however, Okinawa’s terrain and the nature of the Japanese defenses were to provide Marine aviators of the Air Defense Command with ample opportunities to display close air support techniques born of experience accumulated in earlier Pacific campaigns.
Logistic Support Planning23
Fortunately for those preparing ICEBERG, much in the logistical plans for the cancelled Formosa operation could
be salvaged and adapted for the invasion of Okinawa with but few changes. Without competent logistics planning of the highest order, and utilization of a resupply and shipping support schedule designed to function with clockwork precision, the target date for the Okinawa operation could not have been met. This would have caused all related planned strategy to have been either nullified or advanced to a later date.
The logistics plan for Okinawa “was the most elaborate one of its kind developed during World War II, involving prearranged movement of both assault and cargo shipping over vast ocean distances.”24 The plan required establishment of a 6,000-mile-long supply line, stretching across the Pacific, with 11 different ports-of-call,25 to support the mounting of 182,821 troops encumbered with some 746,850 measurement tons26 of cargo loaded into 434 assault transports and landing ships.
A great limitation imposed upon preinvasion logistical planning was the shortage of shipping and the delay in the return from the Philippines of the vessels which were to be Used for Okinawa. Seeking a solution to lift and timetable problems was not the only concern of the Tenth Army logistics staff, “for the mere loading of more ships led only to congestion at the receiving end unless the development of unloading facilities kept pace.”27
It had been decided that the Hagushi beaches were sufficiently large to handle the supply tonnage required by the assault echelon of two corps and their support troops; however, it was impossible to prophesy exactly how soon after the landings the beachhead would be secured and the advance continued inland, or how soon thereafter base development could begin and the supplies for this aspect of Phase I would be required and available. Nor was it possible to forecast the possibility that Phase II would be completed before the accomplishment of Phase L Nonetheless, estimates of troop progress had to be made in order to prepare a logistics plan at all.
The main features of the ICEBERG logistics plan required an initial supply level to be taken to Okinawa by the assault troops who were mounted at such distantly scattered points as Leyte, Guadalcanal, Espiritu Santo, Banika, Pavuvu, Saipan, Eniwetok, Oahu, and the west coast of the United States. Upon completion of the assault phase of the landing, a staggered series of supply shipments would replenish the Tenth Army in accordance with a schedule established earlier. This timetable had been based on the estimated time required to conduct combat operations ashore and, in turn, on how quickly the beach and port capacity could be expanded.
Beginning on 20 February 1945, ICEBERG replenishments were to leave the west coast every 10 days for regulating points at Ulithi, Eniwetok, and Saipan, the first shipments to arrive at each place on L minus 5 (27 March). The supplies would remain at these points until they were called-up by General Buckner. It was planned to continue these automatic resupply shipments for a period of 210 days beyond L-Day. The Tenth Army was also to have emergency reserves located at Saipan, Tinian, and Guam.
The prediction of supply requirements depended upon completed tactical plans, a firm troop basis, and other necessary items of information which either were nonexistent or had not yet been made available to the logistics planners. Adding to the logistics dilemma was the factor of time, for it look 120 days for supplies to be requisitioned, procured, and shipped from the Pacific Coast of the United States to the objective.
To facilitate the preparation and shipment of resupply items in accordance with the scheduling of the various invasion echelons, Army commanders established a standard unit of supply, or “block requisitions,” tailored specifically to the organization of each of the support and assault elements. The composition of the individual block requisition was determined by estimating the logistic support required by a particular unit for a given number of days regardless of the combat situation.
In contrast to this approach, Marine supply agencies, drawing on their experience, felt that the combat situation as envisioned in the planning stages should govern the nature of the supplies requisitioned, and the number, types, and frequency of shipments. Tenth Army considered the Marine system to be more flexible than the Army’s because the requisitioning agencies were better able to make the several automatic resupply shipments conform to their view of how the campaign would progress.
Each service was responsible for initial support of its own elements in the Okinawa task force, with the exception of troops mounting in the South and Southwest Pacific. Area commanders there would be charged with logistical support of units assigned to ICEBERG. After the landing had been accomplished, and when directed by Admiral Turner, Island Command would take over as the Tenth Army central support agency charged with funneling supplies to all of the assault forces.
Early in January28 it became obvious that ICEBERG had been allocated insufficient shipping to accomplish the tactical mission, to support base development, and to lift to the target those air units which were to be committed early in the campaign. An inadequate transport quota for engineer units, whose services would be needed in the
early development of airfields, roads, and waterfront facilities, was improved slightly by scheduling the immediate return of assault LSTS to Saipan after the initial landings to shuttle eight naval construction battalions (Seabees) to the target. In the same manner, other LSTs would be sent to Leyte to pick up any XXIV Corps equipment not carried in assault shipping.29
Of the overall inadequate shipping situation and its effect on the combat divisions, the former G-4 of the Tenth Army recalled that, if needed, ICEBERG was to get all shipping available in the Pacific, because:–
the amount of assault shipping assigned for the operation was far below that required to properly lift the assault elements of the Tenth Army. This resulted in [the Tenth Army being given] authority to modify Combat Loading Doctrine so that the most essential equipment and supplies could accompany the assault echelon. Additional items that should have been in the assault echelon were loaded in a subsequent shipping echelon.30
The overall assault lift was augmented by other means also. Vessels to be used for the Luzon and Iwo Jima landings were made available later for Okinawa through adherence to a stringently monitored and thoroughly regulated shipping schedule. Additional space for Tenth Army troops was gained by reducing the tonnage requirements of IIIAC, substantially at the expense of the 2nd Marine Division. It was reasoned that since the division was not going to be committed immediately, it could acquire whatever additional shipping it needed within a short time following the initial assault. Further lift capacity was gained by loading landing ships to their rated limits, by the addition to the invasion flotilla of newly constructed attack transports (APAs) with greater cargo-carrying characteristics, and by an increased allocation of landing ships, tank, (LSTs) and landing ships, medium (LSMs).
The shipping allocation for the garrison forces was governed by the estimated capacity of Okinawan beach and port unloading facilities, Past experience, however, resolved the size of the lift necessary to transport an assault echelon of three reinforced Marine divisions, three reinforced Army divisions, a Marine amphibious corps headquarters and corps troops, and an Army corps headquarters and corps troops. Thus, the required assault tonnage was a firm figure from the beginning and was deducted from that allotted to the ICEBERG forces overall. The remainder was assigned as the lift for Tenth Army support troops, which included air, naval, and airfield construction units.
After the Marianas and Palau operations, it was found that one transport group (12 APAs and 3 cargo ships, attack (AKAs)), made up of three transport divisions, had sufficient lift capacity for a combat-loaded reinforced infantry division. For the ICEBERG lift, however, a new shipping echelon, the transport squadron (TransRon) was formed to carry a proportionate share of assault forces, corps troops, and elements
from corps and army headquarters. The TransRon was nothing but the old transport group augmented by three APAs and three AKAs.
Each transron was to be accompanied by one APH, which was a troop transport specially rigged as a hospital and equipped to treat casualties and then evacuate them from the battle zone.31 There were to be six hospital ships (AHs) assigned to ICEBERG; one was to be on station L minus 5 with the Kerama Retto invasion group, three were assigned to the main attack forces and were to arrive off Hagushi on L plus 1, while the other two were scheduled to reach Okinawa three days later.
Improved casualty evacuation was planned for this invasion by assigning four hospital landing ships (LST(H)s) to each of the two naval attack forces in the major assault. Assigned to each vessel was a naval medical officer who functioned as an evacuation control officer and, as such, was responsible for screening the wounded as they arrived, giving treatment and classifying them with reference to their estimated recovery time, and transferring the casualties in accordance with the provisions of a system related to their recovery classification. Accordingly, hospital ships would evacuate those men wounded seriously enough to require hospitalization for two months or more. Casualties requiring treatment for a minimum of two and a maximum of eight weeks would be evacuated in APHs during the initial assault phase and, after that, would receive further treatment in hospitals established on Okinawa. Those men who could be returned to duty within two weeks after being wounded would be treated and held in the hospital transports or landing ships until they had fully recovered or until the land-based hospitals had been established.
The LST(H)s were to remain on station until released by Admiral Turner, at which time the medical officers aboard would land and assign casualties directly to the ships from aid stations set up on the beaches. When General Buckner assumed command ashore, he would become responsible for the establishment and administration of medical services on the island, and for air evacuation of casualties, when airfields became operational.
The equipment and supplies to be taken to Okinawa by the corps and the divisions had been specifically designated by Tenth Army order. After cargo space in assigned shipping had been allocated to this material, any other available space would be filled by additional items which the corps and division commanders had decided the troops could carry. Logistical planning on the division level was influenced by the supposition that the beaches would be heavily defended and that the inland advance stubbornly resisted. As a result, only “hot cargo,” predetermined blocks of high-priority supplies, was to be landed on L-Day. Included in a block of cargo
were one CinCPOA unit of fire32 for all weapons and rations and water for one day. Moreover, all organic division motor transport would be taken to the target in available shipping space because the prospect of prolonged operations over a relatively large land mass envisioned wide-spread use of vehicles.33
To assist in Marine logistical planning and preparations, Fleet Marine Force, Pacific, established the 2nd Field Service Command on Guadalcanal. Here relatively close liaison could be maintained with Marine ICEBERG elements mounting from the Solomons. This service command was empowered to coordinate the efforts of the supply agencies of both the 1st and 6th Marine Divisions and to deal with Army and Navy sources of supply directly. In the same manner, the Marianas-based 1st Field Service Command assisted the 2nd Marine Division. Re-equipment of General del Vane’s division on Pavuvu was relatively simple since its primary supply source, the 4th Base Depot, under the 2nd Field Service Command, was on the other major island in the Russells, Banika. General Shepherd’s division experienced some difficulties, however, because its supply source was a transfer rather than a stocking agency and had to obtain its requisitioned items from the 4th Base Depot. As a result of the cumbersome and time-consuming administrative procedures involved in processing requisitions through the several service echelons in the area, the 6th Division experienced many delays in the delivery of much of its needed equipment and supplies.34 Both assault divisions, however, embarked for the target with but few shortages, none of which affected combat readiness and efficiency.
By the time that the TAF logistics section had been activated, AirFMFPac had already issued warning orders and was in the process of preparing subordinate units for the impending campaign. The basis for logistic support of Marine aviation units was different, in certain ways, from that of Marine ground elements. While items peculiar to the Marine Corps were drawn by both ground and air units from the same sources, all technical aviation materiel was received through Navy supply channels or, in some cases, from the Army. Since this was the case, the TAF logistics staff established liaison with representatives of Commander, Aircraft, Pacific Fleet (ComAirPac), the agency responsible for fulfilling the fuel and installation requirements at the Okinawa air fields the TAF units were to occupy. The supply section of Commander, Naval Air Bases, Okinawa (ComNABS) was made
the ComAirPac type command logistics representative for these matters.
All other supply requirements were to be handled by the supply section of the Navy’s Pacific service command. Liaison was also established with Army Air Forces logistics representatives to determine the nature and extent of support required by Army elements in General Mulcahy’s command. Arrangements were then made to obtain special combat clothing and equipment for the AAF personnel to be assigned to TAF. Based upon the latter’s recommendations, automatic resupply shipments for the Army squadrons were adjusted to coincide with the schedule established for the Marines.
The organization and general administration of the supply system on Okinawa was to be an Island Command function, in which it would receive and distribute Tenth Army supplies. The Marine groups in TAF, however, would support their own squadrons and would draw Marine Corps supplies from the 2nd Wing or other designated Marine sources. Air base commanders would provide aviation fuel and lubricants to squadrons operating from their strips; all technical aviation supplies were to be requisitioned through ComNABS, Okinawa.
Service units organic to the AAF fighter and bombardment groups would support the flying squadrons of each. All supplies other than the technical items peculiar to AAF planes would be requisitioned from sources designated by the Island Commander. Until an Air Service Command Depot was established on Okinawa, the one at Guam would supply the remainder.
Base Development and Military Government Planning35
A second logistic mission given to ICEBERG, separate yet related to the assault effort, was the immediate development of Okinawa as an advanced air and fleet base. In order to support all of the aircraft assigned to the invasion, eight airfields and one seaplane base were to be built almost immediately and during the later phases of the operation this number would be increased. Also, two ports were to be developed—one, Nakagusuku Wan, by the Navy and the other, Naha harbor, by the Army. Since Okinawa was to serve merely as a staging base for final operations against Japan, it was not contemplated that the installations on the island were to be of permanent construction. Ie Shima was included in the base development program as the island was to hold four airfields and to garrison ground and antiaircraft artillery defense troops.
Base development would proceed right on the heels of the assault troops as two of Okinawa’s airfields were to be seized, improved, and made operational by L plus 5, while two more fields were to be available by L plus 20. The preparation of Okinawa as a mounting and staging point was to be undertaken concurrently. First priority was given the early development and activation of airfields; next in order of importance was the construction of bulk fuel storage facilities; and the third most important matter
was the development of waterfront installations. Reflecting the urgency of these tasks, every effort was made to schedule the shipments of supplies required to support base improvement so that they would arrive at the island when they were needed. Accordingly, garrison troops and the materials which they were to employ were to arrive in 17 successive echelons. The timing of their arrival was governed not only by the preplanned work schedule but also by the projected unloading capacity of the captured beaches.
To establish this schedule, a series of echelonment conferences were held between the staffs of the Tenth Army and the different type commanders who were furnishing troops for the operation. In any large amphibious operation, it is neither possible nor feasible, because of shipping limitations, to transport to the target in the assault convoy both those troops required to undertake the campaign to its end and the troops, equipment, and supplies required to develop the captured base. Even if all required shipping had been made available for an operation of the size of Okinawa, it would have been patently undesirable to schedule the simultaneous arrival at the target of both assault and garrison troops. Until the assault forces had landed, unloaded their shipping, and gained enough room on the beaches for the landing of the garrison elements and equipment, the shipping in which garrison troops were embarked would have had to lie off Okinawa, where it would have been vulnerable to enemy submarines and aircraft. For these reasons, it was imperative that echelonment plans covering the movement of thousands of assault, service, and construction troops had to be precise.
In addition to its other functions, Island Command was also to establish a military government on Okinawa. Since this was to be the first Pacific operation in which large numbers of enemy civilians would be encountered by combat troops, it was expected that the island would serve as a valuable testing ground of civil affairs and military government procedures which would be applied later when Japan itself was occupied.
In 1943, the JCS gave the Navy basic responsibility for establishing military government on certain outlying islands of the Japanese Empire, once they had been captured. Included in this group were the Ryukyus. Because the Tenth Army would be in overall control of the Okinawa land campaign, Admiral Nimitz believed that General Buckner should be responsible for military government on the island. Accordingly, once the War Department concurred in this transfer of authority, CinCPOA was able to get the 1943 JCS order reversed.
Because of its European commitments, the Army was unable to furnish all of the civil affairs personnel needed to round out the entire Tenth Army military government component. Therefore, the Navy supplied Brigadier General William E. Crist’s command with naval officer and enlisted personnel so that Military Government would have well-balanced teams.
Direct naval participation in military government planning for Okinawa began in July 1944, when work was begun in New York City by the research staff of the Chief of Naval Operations’ military government section. The pooled
efforts of the staff resulted in the Civil Affairs Handbook for the Ryukyu Islands, a publication which proved to be of inestimable value to Tenth Army civil affairs administrators during both the ICEBERG planning phase and the rehabilitation period after Okinawa had been secured.36
The ICEBERG joint staff study originally anticipated that, within the Okinawan population to come under Tenth Army control, a small element would be “antipathetic” and would have to be “placed under detainment pending screening and probable internment.” No figures were available to determine how many mainland Japanese civilians on Okinawa might possibly be captured, but preparations had to be made for the construction of an internment camp whose facilities were flexible enough to provide for upwards of 10,000 island natives and Japanese civilian internees. It was expected that by L plus 40 this number would skyrocket to an approximate total of 306,000 captured civilians, whose food, clothing, and housing would have to come from captured stocks of salvageable material, since there was no room aboard assault ships for supplies of this nature. By the time ICEBERG had reached the garrison phase, 12 military government camps were to be in operation, each unit staffed and equipped to handle 2,500–10,000 civilians.
Assigned to General Crist’s jointly staffed military government section were such varied Army and Navy units as a military police battalion, a truck company, 20 Navy dispensaries, and 6 Navy hospital units. In addition to these and some purely administrative elements, 350 officer and 890 enlisted civil affairs personnel were organized into four types of teams, each of which had been tailored for specific functions. One of the teams was assigned to each of the assault divisions and, after landing, was to conduct preliminary reconnaissance missions relating to military government as the attack advanced. Teams in another group, attached to the two corps and all divisions also, were to take charge of civil affairs behind the front lines as civilians were encountered by the combat forces. A third type of team was made up of refugee camp administrators, while in the fourth category there were six teams, each of which was to take charge of one of the six military government districts into which Okinawa was to be divided.
The Chief Military Government Officer was to be directly subordinate to the Island Commander and would function as his deputy. The importance of this close relationship and the emphasis placed on intensive civil affairs planning was justified later during the campaign, when, by 30 April, there were approximately 125,000 civilians under military government jurisdiction on Okinawa. This figure climbed steadily following this date, reached 147,829 by 31 May, 172,670 by 15 June, and totaled 261,115 on 30 June.37
Intelligence Planning38
In October 1944, the statement that “information as to enemy defensive installations on Okinawa Jima is meager,”39 was indisputable. Despite the early lack of information concerning the island, the various intelligence gathering and processing agencies in the Pacific, as well as those in the United States, began to sift through available material and soon were able to clarify the enemy situation for ICEBERG forces. In keeping with the established principle of coordinated planning, the corporate activities of all intelligence agencies in the various Pacific commands quickly resulted in the production of urgently needed basic intelligence.
Currently valid military information of the Japanese situation was difficult to obtain because of the location of Okinawa within the Empire’s well-protected, strategic, inner defense line. For the most part, captured documents, interrogations of prisoners as well as of former island inhabitants, and old Japanese publications provided the basis for the intelligence estimates initially issued.40 In addition, the Navy was able to make use of both captured and previously available hydrographic charts for navigational studies of the waters surrounding Okinawa.
For a terrain study, a determination of the location and nature of enemy defenses, and an estimate of enemy strength, most of the data at hand was inadequate and an aerial photographic mission over the target had to be laid on. In conjunction with other information of the enemy received right up to L-Day, the thorough interpretation and evaluation of these photographs enabled Tenth Army to issue detailed intelligence studies which contained an accurate estimate of the Japanese situation.
Aerial photos were required also for use in the production of a map of the target. It was difficult to obtain adequate photographic coverage at first because of the distance of Okinawa from the closest Allied air base, some 1,200 nautical miles. This factor limited the conduct of such missions to either carrier aircraft, whose ships could carry them close to the target, or B-29s. Other obstacles to the amassing of a complete intelligence picture of Okinawa were the notoriously poor weather over the target, the vastness of the land mass to be photographed, and the schedule of carrier strikes against the target—few of which were timed to coincide with immediate Tenth Army intelligence requirements.
On 29 September 1944, the first ICEBERG photographic mission was flown by B-29s. While they covered all of Okinawa, and the outlying islands to a degree, the results of this flight were
limited by clouds which obscured about half of the area photographed, mainly the northern portion of the major island. Because of this inadequate photo coverage, the first map produced and distributed had many blank portions in which there was little or no topographic detailing. Modifications of this first map were made later in the campaign, when captured Japanese maps provided more thorough contouring information.
During the first fast-carrier strikes on Okinawa Gunto of 10 October 1944, large scale vertical and oblique aerial photographs were acquired, giving 90 percent coverage of the area. From 29 September 1944 to 28 March 1945, a total of 224 photo-reconnaissance sorties were flown over the target. Information gained from these photographs was collated and analyzed, and the resultant intelligence summaries were distributed to Tenth Army units.
In the week preceding L-Day, escort carrier-based photographic aircraft flew daily missions over the island. Careful interpretation of the photos thus obtained permitted bomb damage assessments and, at the same time, comparison of these photos with ones taken earlier enabled the interpreters to locate many enemy installations previously concealed by effective camouflage. From a close study of successive sorties, it was possible to determine each displacement of the enemy’s defensive positions, to hazard guesses of his relative strength, and to compile a preliminary target information list for distribution to artillery units.41
After L-Day and while the fighting was still in progress, the island was completely rephotographed, the results of which enabled a more accurate map to be printed and distributed.42 A scale of 1:25,000 was used for the basic map originally issued from which maps of the initial zones of action, scaled at 1:10,000, were produced for the use of the lower echelon assault units. At the same time, smaller scale maps were reproduced for use as road maps in traffic control planning.
The Tenth Army made rubber relief maps on a scale of 1:10,000, which were issued to General Geiger’s troops in sufficient quantity to permit distribution down to and including assault battalions. The mapping sections of IIIAC, and the 1st and 6th Marine Divisions worked together to produce plastic terrain models of the corps zone of action. Made to a scale of 1:5,000 and constructed with a 2:1 vertical exaggeration, these models facilitated the briefing of commanders and their troops for the prospective operation. Wholesale distribution of these relief models was made soon after the troops embarked for the target, at which time some 600 copies of a 1:5,000 map of the landing beaches, specially prepared by the 1st
Marine Division, were issued to its assault units.
To supplement aerial photographs, the USS Swordfish, a specially equipped submarine, was dispatched to Okinawa from Pearl Harbor on 22 December 1944 with the mission of photographing Okinawa’s beaches and Japanese defensive installations on the island coasts. After making her last known radio transmission on 3 January 1945, the submarine was never heard from again and was reported missing. As a result, no beach photographs were taken before L-Day for, as succinctly stated in the TF 51 AR, “no information from submarine reconnaissance was available.”43
In October 1944, enemy strength on Okinawa was set at 48,600. It was estimated that two well-trained and experienced infantry divisions, and a tank regiment, comprised the major defense force on the island. At this time, it was recognized that an additional threat to the landings was posed by the size of the civilian population located in southern Okinawa. This manpower potential of more than 300,000 individuals would swell the enemy strength figure if they were used to form a home guard or militia, or to conduct guerrilla activities. In January 1945, the Tenth Army estimate assumed that the Japanese reinforcement capability could increase the regular force figure to 66,000 by L-Day, at which time enemy defense forces on Okinawa would be two and a half infantry divisions. If the enemy exerted his maximum reinforcement capability, he could then oppose the landing with four infantry divisions constituting the principal combat elements of the defense. Total Japanese strength would then be 87,000 men.
All possible Japanese courses of action were considered, and troop dispositions for each course were analyzed in light of what was known of current Japanese tactical doctrine and its evolution to date. All indications pointed to the fact that the enemy would most likely organize the southern third of Okinawa for a defense in depth while the bulk of his troops were withheld as a mobile reserve. This course of action would present a potentially more dangerous situation to the landing force than would the more commonly experienced alternative of a determined defense of the beaches.
An interpretation of aerial photographs in February revealed that the enemy force on Okinawa comprised two infantry divisions and an independent mixed brigade, service and support troop reinforcements for the infantry, all totaling an estimated 56,000–58,000 men. It also appeared that, while the far northern sector was defended by a single battalion only, the main force was disposed in the south in the projected XXIV Corps area. In the III Amphibious Corps zone of action, it was estimated that two infantry regiments defended. Conceivably, these six or seven thousand men could be reinforced by local auxiliaries.
While the small garrison in the north was given the capability of mounting counterattacks against the invader left flank, it was expected that the most
violent enemy reaction would come from the heavily defended south, on the XXIV Corps’ right flank, where the Japanese mobile reserve would be maintained in considerable strength, It was anticipated that, as soon as the Japanese had appraised the landing force’s dispositions, a counteroffensive in force would be mounted by the enemy reserve.
The estimate of Japanese strength was again revised in mid-February, this time downwards to 37,500–39,500, when information was received that a full division had been withdrawn from Okinawa. In view of this reduction, and supplemented by indications that the enemy was concentrating in the Nakagusuku Bay area, it was presumed that the two Marine divisions would be opposed in their zones by no more than one infantry regiment deployed in position, and that the total number of Japanese troops in the overall sector would be more than 10,000.
This numbers guessing game continued when, a month later, the estimate of Japanese defense forces was revised upwards to 64,000. It appeared that the enemy had been able to reinforce the garrison with an understrength infantry division as well as with some miscellaneous units of unknown origin, in all about 20,000 men. It was believed that an additional force of 4,000–6,000 men had arrived in March, having been lifted by shipping which successfully evaded the Allied blockade. The Tenth Army assumed that, if the March enemy reinforcements were the advance elements of another division, it was reasonable to assume further that by 1 April the landing force would be opposed by at least 75,000 men. In the week preceding L-Day, while the assault elements sortied for the target, still another estimate of enemy strength in the IIIAC zone was issued. In this supplementary revision., it was stated that the principal Japanese opposition now would come from two reinforced infantry regiments with a strength of 16,000 men.
Air and naval capabilities assigned to the Japanese remained relatively unchanged all during the planning phases of ICEBERG, At all times it was expected that the enemy would be capable of mounting heavy and repeated air attacks against invasion shipping. It was expected that this vigorous e air effort would include continued employment and intensification of the suicide bombing tactics which first had appeared during the invasion of Leyte in October 1944. The Japanese were credited with an air strength of approximately 3,000 planes which were based within range and capable of blunting the Okinawa landing. Along with this air capability, the enemy was believed able to mount an airborne counterattack, for “as air action is practically the only assistance he can give the Okinawa garrison from outside [the island], he may expend considerable aircraft and endeavor to land several thousand troops within our beachhead.”44
It was known that the Japanese had suicide motor torpedo boat units at Okinawa and it was assumed that midget submarines were based there also. Added to the possible tactical employment of these suicide organizations was the potential use of suicide swimmers
whose mission was also disruption of the invasion fleet at anchor off the objective. Although the Japanese Navy was a mere shadow of its former self, it still retained operational forces strong enough to pose a threat to the landing’s success. For that reason, it was deemed necessary to maintain a strong surface cover at the objective. While the southern part of Okinawa was ideally suited for the tactical use of tanks, the enemy was not given an armored capability. This was because the relation of estimated tank strength to the total estimated garrison strength was too low, and it was not felt that this support arm would offer any great opposition.
Three months after Admiral Nimitz had received the JCS directive for Okinawa’s invasion, General Buckner issued the initial operation order setting the ICEBERG juggernaut’s wheels into motion. During the course of this planning period, each Tenth Army general and special staff section prepared that portion of the operation order for which it was responsible while maintaining liaison with the subordinate units which were preparing to put words into action. Although most of the ICEBERG assault, support, and garrison forces did not issue their own operation orders until January 1945, warning orders had already alerted them to the impending invasion.