On August 6, 1945, the world officially entered the nuclear age when the United States dropped an atomic bomb on the city of Hiroshima. Three days later, the U.S. dropped a second atomic bomb on Nagasaki, and on August 14, the Japanese government agreed to an unconditional surrender that all but ended World War II. These bombings halted the planning of Operations Olympic and Coronet, which entailed the full-scale Allied invasion of Japan, and also showed the true power of the American economy and workforce. In these respects, the United States was justified in using the most powerful weapon of the time to end the greatest conflict in human history.
Before the successful testing of the atomic bomb, the United States had an elaborate set of plans for the invasion of Japan that were developed in the spring and summer of 1945. According to Davis, in "The Invasion Not Found in the History Books," an article found in the Omaha World Herald, Operation Olympic would begin with "X-Day" on November 1, 1945, and would land fourteen American divisions with the support of the largest naval fleet ever assembled, a total of more than 450 American vessels. After seizing the southern third of Kyushu, the southernmost island of Japan, the Americans would use this area as a staging ground and airbase for Operation Coronet. "Y-Day" was the beginning of Operation Coronet, and was to take place on March 1, 1946. Twenty-five divisions would invade the island of Honshu, the Kanto Plain, and ultimately Tokyo itself.
The Japanese had well laid plans of their own to defend the islands from occupation, and this plan (Operation Ketsugo) was designed to make the cost of conquering Japan too high for the Allies to accept, which would (in their minds) lead to an armistice being signed rather than having to experience total defeat. In "Downfall:The End of the Imperial Japanese Empire," a 1999 book by Richard Frank, it's shown that the Imperial Japanese Air Force had amassed more than 10,000 aircraft, which were projected to destroy one-third of the invasion force before the landings in Kyushu.