On 6 August 1945, the U.S. dropped an atomic bomb on the Japanese city of Hiroshima in the first nuclear attack in history. In a press release issued after the atomic bombing of Hiroshima, Truman warned Japan to surrender or "...expect a rain of ruin from the air, the like of which has never been seen on this earth." Three days later, on 9 August, the U.S. dropped another atomic bomb on Nagasaki, the last nuclear attack in history. More than 140,000–240,000 people died as a direct result of these two bombings. The necessity of the atomic bombings has long been debated, with detractors claiming that a naval blockade and aerial bombing campaign had already made invasion, hence the atomic bomb, unnecessary. However, other scholars have argued that the bombings shocked the Japanese government into surrender, with Emperor finally indicating his wish to stop the war. Another argument in favor of the atomic bombs is that they helped avoid Operation Downfall, or a prolonged blockade and bombing campaign, any of which would have exacted much higher casualties among Japanese civilians. Historian Richard B. Frank wrote that a Soviet invasion of Japan was never likely because they had insufficient naval capability to mount an amphibious invasion of Hokkaidō.

America dropped what on August 6, 1945?