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.