Atomic bombings of Hiroshima and Nagasaki
Partly because of the atomic bombings, Japan adopted the Three Non-Nuclear Principles after the war. These principles said that Japan could not create or try to get nuclear weapons. A scientist Albert Einstein wrote a letter about the atomic bomb on August 2, 1939. But he said later, "I made one great mistake in my life, when I signed a letter to President Roosevelt recommending that atom bombs be made."