The day before the Nagasaki bombing, the Soviet Union entered the war against Japan and commenced an attack on the Chinese province of Manchuria, which was still held by the Japanese. The combination of the atomic bombings with the potential threat of a full-scale invasion of Japan by the USSR was enough to remove any hope that Japan may have held for continuing the war. On August 15, 1945, Emperor Hirohito announced Japan’s capitulation in accordance with the Potsdam Declaration. A formal surrender was signed on September 2 aboard the battleship USS Missouri.
The campaign against Japan at the end of the war and the use of the atomic bomb have long been the subject of debate and controversy around the world, especially outside the United States. Critics contend that Japan was already on the verge of surrender by late summer 1945 and that the atomic bombings were superfluous, needlessly killing hundreds of thousands of people. The same was said about the mass incendiary bombing attacks on Tokyo and other cities, which killed even more people than did the atomic bombs, although without as many long-term effects.
On the other hand, proponents of the bombings say that battles such as Iwo Jima and Okinawa demonstrated that the Japanese population was prepared to fight to the last man and that only a weapon as overpowering as the atomic bomb could have ended the war without unfathomable casualties. Indeed, the only alternative to the bombings would have been a ground invasion using U.S. troops, which would have been extremely costly in both American and Japanese lives. The argument has thus been made that such a ground invasion would have cost far more Japanese lives than the Nagasaki and Hiroshima bombings combined.
Moreover, evidence suggests that external political factors played a significant part in the decision to drop the atomic bombs. The Manhattan Project had been very expensive, and the Truman administration felt pressured to demonstrate that the weapon actually could be used effectively for military purposes. Furthermore, tensions were growing with the Soviet Union over the division of Eastern Europe, and the United States may have wished to demonstrate its newfound power.
Finally, the atomic bomb was a new, untested technology. The worldwide cultural taboo surrounding such weapons did not exist at the time, and in general there was less understanding of the long-term effects of their use—only one atomic bomb had ever been tested successfully. Frighteningly, as powerful as the Hiroshima and Nagasaki bombs were, their destructive power was small in comparison with the nuclear weapons of today.