Scheduled for November 1945, Operation Olympic was to be the first stage of an Allied invasion of Japan, with the goal of securing the southern third of Kyushu, the southernmost of Japan’s main home islands. The seized territory would provide airbases for land-based aircraft, and serve as the staging area for an even bigger invasion, Operation Coronet in the spring of 1946, directed at Honshu, the largest and most populous of Japan’s home islands.
The operation was to commence with amphibious landings on three Kyushu beaches, but as was discovered after the war, the Japanese had accurately predicted US intentions and landing sites – Japanese geography was such that the only viable beaches for large amphibious landings were the ones selected by Allied planners for operations Olympic and Coronet.
The Allies would still have prevailed in the end: the resources committed to the operation dwarfed those of the D-Day landings in France, and included 42 aircraft carriers, 24 battleships, 400 destroyers and destroyer escorts, tactical air support from the Fifth, Seventh, and Thirteenth Air Forces, and 14 divisions for the initial landing. Casualties, however, would likely have been horrific. Depending on the degree of Japanese civilian resistance – and Japanese authorities were busy training even women and children to fight the invaders with spears and pointy sticks -, worst-case scenarios envisioned over a million Allied and tens of millions of Japanese casualties.
Olympic’s planners were unaware of the highly secretive Manhattan Project, and when the US successfully tested an atomic bomb in July, 1945, their game-changing potential was not fully understood by planners. Envisioned simply as “really big bombs”, they had nebulous ideas of using atomic bombs during the November invasion in support of the amphibious landings. Their use instead against Hiroshima and Nagasaki in August 1945, shocked the Japanese government back to its senses, ended the war, and eliminated the necessity for Operation Olympic and its expected butcher’s bill.