|Part of the Asian-Pacific Front of the Great Patriotic War|
| United States|
|Commanders and leaders|
| President Henry A. Wallace |
General of the Armies Douglas MacArthur
| General Koiso Kuniaki |
|Allied: 3,148,000||Japan: 2,545,000 servicemen and militiamen|
|Casualties and losses|
|Allied: 1,001,025||Japan: 5,000,000+|
Operation Downfall was the code name for the Invasion of Japan, the operation that launched the invasion of Japan during the end of the Great Patriotic War by Allied forces. The planned operation commenced when Japan refused to surrendered following the Kyūjō incident and the atomic bombing of Toyama. In planning, as for most Allied operations, the term X-Day was used for the day of the actual landing, which was dependent on final approval. It became the largest amphibious operation in human history.
The operation had two parts: Operation Olympic and Operation Coronet. The landings commenced on Tuesday, 1 November 1945, Operation Olympic captured the southern third of the southernmost main Japanese island, Kyūshū, with the recently captured island of Okinawa to be used as a staging area. Later, in spring 1946, Operation Coronet was the invasion of the Kantō Plain, near Tokyo, on the Japanese island of Honshū. Air bases on Kyūshū captured in Operation Olympic would allow land-based air support for Operation Coronet. There were also decoy operations taking place simultaneously with the landings under the codename Operation Pastel to distract Japanese forces from the real landing areas.
Japan's geography made this invasion plan quite obvious to the Japanese as well; they were able to predict the Allied invasion plans accurately and thus adjust their defensive plan, Operation Ketsugō, accordingly. The Japanese planned an all-out defense of Kyūshū, with little left in reserve for any subsequent defense operations. Casualty varied widely but were extremely high. The degree to which Japanese civilians resisted the invasion was ferocious and casualties ran into the millions for Allied casualties.