Operation Downfall was the proposed Allied plan for the invasion of the Japanese home islands near the end of World War II.
The operation had two parts, Operation Olympic, intended to capture the southern third of the southernmost main Japanese island, Kyūshū, and Operation Coronet, the planned invasion of the Kantō Plain, near Tokyo, on the main Japanese island of Honshu. Olympic was scheduled for November 1945, to be followed by Coronet in early 1946.
If Downfall had taken place, it would have been the largest amphibious operation in history, surpassing D-Day.[1] The planned operation was canceled when Japan surrendered following the atomic bombings of Hiroshima and Nagasaki, the Soviet declaration of war, and the invasion of Manchuria.[2]
Supreme Commander, Allied Forces Pacific
General Douglas MacArthur
Should these four corps prove insufficient to accomplish the tasks assigned, elements earmarked for Coronet would be used to reinforce Sixth Army at the rate of three divisions per month beginning about 30 days after the initial landings.
General Walter Krueger[3]
Peripheral landings
40th Infantry "Sunburst" Division (Landing on Yakushima and Koshikijima Islands)
158th Infantry Regiment (Landing on Tanegashima)
I Corps (Landing at Miyazaki)[3]
Major General Innis P. Swift
25th Infantry "Tropic Lightning" Division
33rd Infantry "Illinois" Division
41st Infantry "Sunsetters" Division
V Amphibious Corps (Landing at Kushikino)[3]
Major General Harry Schmidt, USMC
3rd Marine Division (Bougainville, Guam, Iwo Jima)
4th Marine Division (Kwajalein, Saipan/Tinian, Iwo Jima)
5th Marine Division (Iwo Jima)
XI Corps (Landing at Ariake)[3]
Major General Charles P. Hall
43rd Infantry "Winged Victory" Division
Major General Charles W. Ryder
81st Infantry "Wildcat" Division
98th Infantry "Iroquois" Division
Follow-up units:
Admiral William F. Halsey
20 fleet and light aircraft carriers
9 battleships
26 cruisers
75 destroyers
incl. British Pacific Fleet
6 fleet and light carriers
Admiral Raymond A. Spruance
36 escort carriers
11 battleships
26 cruisers
387 destroyers and destroyer escorts
394 AKA, AP, APA, APD, APH
Admiral Thomas C. Kinkaid
General George C. Kenney (119,000 men) – 14 bomber groups, 10 fighter groups
United States Strategic Air Forces in the Pacific
General Carl A. Spaatz
Major General Curtis E. LeMay
Twentieth Air Force (Lt. Gen. Nathan Twining) (77,000 men)
1,000 B-29 Superfortresses
Eighth Air Force (Lt. Gen. Jimmy Doolittle)
Commonwealth forces
Tiger Force (detached from RAF Bomber Command):
480–580 Avro Lancaster bombers (about half to be used as tankers for in-flight refuelling)
Australian First Tactical Air Force
20 fighter/attack squadrons from the Royal Australian Air Force
Field Marshal Shunroku Hata
Lieut. General Yokoyama Isamu (600,000 men)
Northern Kyūshū — 56th Army
Lieut. General Ichiro Shichida (365,000)[4]
312th Division
351st Division
124th Independent Mixed Brigade
57th Division (20,000 men)
4th Tank Brigade
Southeastern Kyūshū — 57th Army
Lieut. General Nishihara Kanji (150,000 men)
Tanegashima—109th Independent Mixed Brigade (5,900 men)
Miyazaki—154th Division, 156th Division, 212th Division[4] (55,000 men)
Ariake—86th Division, 98th Independent Mixed Brigade, 1 regiment, 3 infantry battalions (29,000 men)
25th Division, 5th Tank Brigade, 6th Tank Brigade[4]
Southwestern Kyūshū — 40th Army
Lieut. General Nakazawa Mitsuo (85,000 men)
303rd Division (12,000 men) (Sendai)
146th Division, 125th Independent Mixed Brigade (S. Satsuma Peninsula)
77th Division[4] 1 tank regiment
216th Division[4] 4 brigades
General Masakazu Kawabe
Sixth Air Army — Kyūshū
5,000 aircraft assigned as kamikazes, 5,000 aircraft available for kamikaze service, 7,000 aircraft in need of repair
100 Koryu-class midget submarines, 250 Kairyu-class midget submarines, 1,000 Kaiten manned torpedoes, 800 Shinyo suicide boats
General Courtney H. Hodges[3]
1st Marine Division (Guadalcanal, Peleliu, Okinawa)
2nd Marine Division (Tarawa, Saipan, Okinawa)
6th Marine Division (Okinawa)
7th Infantry "Bayonet" Division
27th Infantry "New York" Division
96th Infantry "Columbia" Division
Lieut. General Robert L. Eichelberger
31st Infantry "Dixie" Division
37th Infantry "Buckeye" Division
6th Infantry "Red Star" Division
32nd Infantry "Red Arrow" Division
38th Infantry "Cyclone" Division
XIII Corps (Reserve afloat)[3]
United States Army Forces Pacific reserve[3]
97th Infantry "Trident" Division
Thirty days after the initial assault, each army would be reinforced by a corps of 3 divisions. Five days later an airborne division and a United States Army Forces Pacific Reserve Corps of 3 divisions would be made available. Strategic reserve for the entire operation would consist of a corps of 3 divisions located in the Philippines and divisions from the United States to permit reinforcement at the rate of 4 per month.[3]
For First Army
Unnamed follow-on corps
For Eighth Army
Unnamed follow-on corps
United States Army Forces Pacific reserve
Unnamed follow-on corps
Strategic reserve
British Commonwealth Ground Forces
Commonwealth Corps (Lt. Gen. Charles Keightley)
3rd Infantry Division (United Kingdom)
6th Infantry Division (Canada)
10th Infantry Division (Australia)
All Japanese formations on Honshu were badly understrength and lacking in equipment. The American First Army's landings would likely have been opposed by the Japanese 52nd Army and the Eighth Army's landings by the Japanese 53rd Army.
Field Marshal Hajime Sugiyama
General Shizuichi Tanaka
51st Army – Tsuchiura, Ibaraki
221st Division – Kashima
115th Independent Mixed Brigade – Shibasaki
116th Independent Mixed Brigade – Hokota
7th Independent Armored Brigade – Ogawa
3rd Imperial Guards Division – Naruto
234th Division – Sōsa
3rd Independent Armored Brigade
8th Artillery Headquarters
84th Division – Odarawa
316th Division – Isehara
117th Independent Mixed Brigade – Numazu
2nd Independent Armored Brigade – Tsudanuma
11th Artillery Headquarters – Hiratsuka
Tokyo Bay Garrison – Choshi, Chiba
321st Division – Tokyoa