1. To hasten the end of the war: By August 1945, the war in the Pacific had been ongoing for over three years, and it had become increasingly clear that Japan was not willing to surrender. The US military estimated that a conventional invasion of Japan would have resulted in massive casualties on both sides. They believed atomic bombs would bring about a swift end to the war and save lives.
2. To avoid a prolonged and costly invasion: The Allies had already invaded Okinawa in April 1945 and encountered fierce resistance from Japanese forces. The casualties suffered during the Battle of Okinawa convinced US military planners that a ground invasion of the Japanese mainland would be incredibly bloody. By dropping atomic bombs, they hoped to bypass a costly invasion and achieve victory without additional loss of life.
3. To demonstrate US power: The United States wanted to showcase the destructive power of its new atomic weapons as a way of deterring potential future adversaries and asserting itself as a dominant global power. By demonstrating its nuclear capabilities, the US hoped to prevent the Soviet Union from expanding its influence and challenging US hegemony in the post-war world.
4. To achieve unconditional surrender: The Allies' goal in World War II was to achieve unconditional surrender from Japan, meaning that Japan would have to accept all of the terms set by the Allies. By dropping atomic bombs, the US sought to force Japan to surrender on the Allies' terms, without any negotiations or concessions.
5. To justify the massive investment in the Manhattan Project: The United States had invested billions of dollars in developing and producing the atomic bombs, and there was pressure to use them in order to justify the cost and effort. By dropping the bombs, the US also sought to demonstrate the value and effectiveness of its scientific and technological capabilities.