'Olympic' was the US unrealised plan within 'Downfall' for the initial invasion of the Japanese home islands by Lieutenant General Walter C. Krueger’s 6th Army, and scheduled for implementation on 1 November 1945 (summer/15 August 1945).
The planners had no knowledge of the Allied development of atomic weapons, which ultimately removed the need for the invasion, but all their estimates were that the undertaking would be an exceptionally bloody undertaking, for the Japanese could be expected to resist with more than their usual fanatical courage once the Americans came ashore on the Japanese home islands.
The planning of the operation was a monumental undertaking because of the size of the forces involved, and because of the logistical problems of delivering and supporting so many men and machines over so great a distance from the US industrial base on the other side of the Pacific Ocean in the continental USA.
High command modifications to ease the task of the final assault on Japan were implemented on 5 April 1945, when the Pacific Theater became a joint command under General Douglas MacArthur (ground forces) and Admiral Chester W. Nimitz (naval forces), though the question of command for strategic air support was left open, the matter resting on co-operation between MacArthur and Nimitz on the one hand, and General Henry H. Arnold (from 16 July Major General Curtis E. LeMay), commanding the 20th AAF (Brigadier General Roger M. Ramey’s [from 25 April Brigadier General Joseph Smith’s] XX Bomber Command and LeMay’s own XXI Bomber Command) in the Mariana islands group, on the other. This potential command problem was removed in July 1945, when General Carl A. Spaatz was appointed to head the new Strategic Air Force in the Pacific, whose subordinate commands were Lieutenant General James H. Doolittle’s 8th AAF and the 20th AAF, the latter commanded from 2 August by Lieutenant General Nathan F. Twining and headquartered in the Mariana islands group rather than Washington as had previously been the case. Subordinate commands of the 20th AAF were LeMay’s XXI Bomber Command in the Mariana islands group, Smith’s XX Bomber Command to be based on Okinawa, and Brigadier General Ernest Moore’s VII Fighter Command on Iwo Jima.
Tactical air support of the Pacific Theater’s ground forces remained the responsibility of General George C. Kenney’s Far East Air Forces, with headquarters on Okinawa and having under command the 14 bomber groups and 10 fighter groups of Lieutenant Ennis C. Whitehead’s 5th AAF, Major General Thomas D. White’s 7th AAF and Major General Paul B. Wurtsmith’s 13th AAF.
The US command structure was thus optimised for 'Olympic', the invasion of Kyushu at the south-western end of the Japanese home islands, which was authorised on 25 May. Overall planning was vested in MacArthur’s staff, which called for a continuing strategic air campaign against Japan’s war industries and internal lines of communication while the US Navy’s Fast Carrier Task Force undertook the destruction of Japan’s surviving naval and air forces. To this end the five huge bomber bases in the Mariana islands group were to be supplemented by new bases on Okinawa, which by 1 November was to be able to accommodate the 240 squadrons of 60 air groups as well as the logistical bases for the Strategic and Far East Air Forces.
The British and commonwealth contribution to the Allied air armada included 'Tiger' Force (detached from the RAF Bomber Command) with some 480 to Avro Lancaster heavy bombers (about half to be used as tankers) and the 20 squadrons of the Australian 1st Tactical Air Force.
For the descent on Kyushu, MacArthur proposed to use formations already in the Pacific and experienced in war against the Japanese. The overall plan ready by the time of Japan’s surrender in August 1945 envisaged that Lieutenant General Walter C. Krueger’s 6th Army would land at the south end of Kyushu and occupy the island as far to the north as the line from Tsuno to Sendai. This would provide the US Navy with an excellent base in the Kagoshima Wan, and also enable forward air bases to be built for the support of 'Coronet', the main step to Honshu island planned for implementation in 1946. With this primary strategic objective attained, the 6th Army would release four divisions for the Honshu invasion force by Lieutenant General Courtney H. Hodges’s 1st Army and Lieutenant General Robert L. Eichelberger’s 8th Army.
The detailed plan for 'Olympic' foresaw the operation progressing in one preliminary and four main phases. In the preliminary phase the 22,000 men of Brigadier General Donald J. Myers’s 40th Division and 7,600 men of Brigadier General Hanford MacNider’s 158th RCT were to take the islands to the south and west of Kyushu to provide advanced anchorages, early warning positions and seaplane bases (on X-5 the 158th RCT was to take Tanega-shima, while on X-4 the 40th Division was to take the Koshiki-retto and neighbouring islands, moving on X-day to seize Uji-gunto, Kusagaki-jima, Kuro-shima and Kuchino Erabu-shima). After these preliminary moves, the main assault was to be launched simultaneously by three corps, Major General Harry Schmidt’s V Amphibious Corps landing the 99,000 men of Major General Graves B. Erskine’s 3rd Marine Division, Major General Clifton B. Cates’s 4th Marine Division and Major General Keller E. Rockey’s 5th Marine Division on the western flank at Kushikino, Major General Charles P. Hall’s XI Corps landing the 113,000 men of Major General Leonard F. Wing’s 43rd Division, Major General William H. Arnold’s Americal Division and Major General Vernon D. Mudge’s 1st Cavalry Division as well as the 112th Cavalry in the centre in the Ariake Wan between Kanoya and Subishi, and Major General Innis P. Swift’s I Corps landing the 95,000 men of Major General Charles L. Mullins’s 25th Division, Major General Percy W. Clarkson’s 33rd Division and Major General Jens A. Doe’s 41st Division on the eastern flank just to the south of Miyazaki.
During this first phase of the main assault, the 6th Army’s floating reserve (Major General Charles W. Ryder’s IX Corps, comprising Major General Paul J. Mueller’s 81st Division and Major General Arthur McK. Harper’s 98th Division) was to demonstrate off Shikoku, the island between Kyushu and Honshu.
The second phase of the assault was to be the consolidation and expansion of the three initial beach-heads to allow the construction of forward airstrips, and the opening of the Kagashima Wan for US shipping. To aid the successful completion of the latter task, the divisions of the IX Corps were to be prepared to come ashore if necessary on X+3 at the extreme southern end of Kyushu, on the Satsuma peninsula, for an advance on Kagoshima at the head of the Kagoshima Wan and a link-up with the V Amphibious Corps.
The third phase was then to comprise the consolidation of the separate beach-heads to secure a cohesive lodgement as far north as the line from Sendai on the west coast to Tsuno on the east coast, giving the 6th Army an area some 60 miles (96.5 km) wide and on average 50 miles (80.5 km) deep.
The fourth phase, to be undertaken only if necessary, was an exploitation on Kyushu or in the Inland Sea to secure the US lodgement.
Also available to the 6th Army were Major General Joseph M. Swing’s 11th Airborne Division and Major General Roscoe B. Woodruff’s 77th Division on Luzon, and if the 6th Army’s 14 divisions could not complete their task Krueger could be reinforced from the forces earmarked for the Honshu landings at the rate of three divisions per month from X+30 onward.
Naval support for these land operations was the responsibility of Nimitz’s Pacific Fleet command, for the purposes of 'Olympic' divided into the 3rd Fleet under Admiral William F. Halsey, 5th Fleet under Admiral Raymond A. Spruance and 7th Fleet under Admiral Thomas C. Kinkaid.
The 3rd Fleet (20 fleet and light carriers, nine battleships, 26 super heavy, heavy, light and light anti-aircraft cruisers, and 75 fleet destroyers) was tasked with strategic support of the landings (operating against targets in the Kurile islands group, Hokkaido and Honshu) with its complement of fast carrier task groups and escorting battleships, cruisers and destroyers, while the 5th Fleet (36 escort carriers, 11 battleships, 26 heavy and light cruisers, and 387 fleet destroyers and destroyer escorts) was to convoy and support the 7th Fleet (394 assault transports and 977 landing ships and craft).
The 5th Fleet’s primary attack force was the 1st Fast Carrier Force, and within the 7th Fleet the amphibious operation was entrusted to Vice Admiral Richmond K. Turner’s 7th Amphibious Force. The Allies could deploy 14 fleet carriers (including the six carriers of Admiral Sir Bruce Fraser’s British Pacific Fleet under the tactical command of Vice Admiral Sir Bernard Rawlings), six light carriers and 36 escort carriers for the operation, together with very powerful battleship and cruiser forces.
The Japanese defence of the area threatened by 'Olympic' was entrusted to Field Marshal Shunroku Hata’s 2nd General Army, with its headquarters at Hiroshima on the island of Honshu. This army group headquarters controlled Lieutenant General Eitaro Uchiyama’s 15th Area Army in southern Honshu, and Lieutenant General Isamu Yokoyama’s 16th Area Army on the target island of Kyushu, with its headquarters at Fukuoka. Totalling some 600,000 men, the 16th Area Army could call on 14 infantry divisions and two tank brigades in a command structure of three armies and two other forces. In the north of Kyushu was Lieutenant General Ichiro Shichida’s 56th Army (Lieutenant General Masao Yano’s 57th Division, Lieutenant General Kazuaki Ohara’s 145th Division, Lieutenant General Mamoru Tada’s 312th Division and Lieutenant General Ken Fujimura’s 351st Division as well as the 4th Tank Brigade and the 124th Independent Mixed Brigade).
The assault areas of the V Amphibious and IX Corps in the south-western part of Kyushu were held by the 85,000 men of Lieutenant General Mitsuo Nakazawa’s 40th Army (Lieutenant General Hisashi Ishida’s 303rd Division of 12,000 men at Sendai, Lieutenant General Shigero Iwakiri’s 206th Division at Fukiage, Lieutenant General Fumio Tsuboshima’s 146th Division, Lieutenant General Ryoji Nakano’s 216th Division and Lieutenant General Masayasu Nakayama’s 77th Division as well as the 125th Independent Mixed Brigade on the southern part of the Satsuma peninsula, four independent infantry brigades, and one tank regiment).
The assault areas of the I and XI Corps in the south-eastern part of Kyushu were held by the 150,000 men of Lieutenant General Kanji Nishihara’s 57th Army (Lieutenant General Akisaburo Futami’s 154th Division, Lieutenant General Keishichiro Higuchi’s 156th Division and Major General Tokutaro Sakurai’s 212th Division with 55,000 men in Miyazaki, 29,000 men in Lieutenant General Wataro Yoshinaka’s 86th Division, the 98th Independent Mixed Brigade, one infantry regiment and three infantry battalions in Ariake, 5,900 men of the 109th Independent Mixed Brigade in Tanega-shima, Lieutenant General Reiji Kato’s 25th Division, and the 5th Tank Brigade and 6th Tank Brigade).
The strip of Kyushu between the 56th Army in the north and the 40th Army and 57th Army in the south was allocated to the 'Chikugo' Force and 'Higo' Force.
The Japanese air strengths in August 1945 were some 3,800 army aircraft (including 3,000 kamikaze machines) controlled by General Masamitsu Kawabe’s Air General Army, and 5,145 navy aircraft (including 2,500 trainers to be used as kamikaze machines) controlled by Vice Admiral Kinpei Teraoka’s 3rd Air Fleet, Vice Admiral Matome Ugaki’s 5th Air Fleet and Vice Admiral Minoru Maeda’s 10th Air Fleet. Fuel stocks were extremely low, and the 5th Air Fleet had priority in Kyushu, where the 71st Air Flotilla, 72nd Air Flotilla and 73rd Air Flotilla were tasked with air defence, and the 12th Air Flotilla and 13th Air Flotilla with attacks on the US fleet as it approached Kyushu.
Other naval assets in the theatre were 100 'Koryu' class midget submarines, 250 'Kairyu' class midget submarines, 1,000 'Kaiten' type manned torpedoes, and 800 'Shinyo' type suicide boats. A major Japanese problem was to be found in the lack of any overall command structure.