Einstein reacted to the bombing of Hiroshima and Nagasaki with deep regret and remorse. Although he had written a letter to U.S. President Franklin D. Roosevelt urging the development of atomic weapons as a deterrent against Nazi Germany, he later expressed regret over the use of such weapons. After the bombings, Einstein became a strong advocate for nuclear disarmament and peace.
In a famous statement, Einstein said: “If I had known that the Germans would not succeed in developing an atomic bomb, I would have never lifted a finger.” He recognized the devastating consequences of the use of nuclear weapons and was deeply disturbed by the destruction and loss of life in Japan. The events led him to dedicate much of his later life to advocating for peace, the control of nuclear arms, and the ethical responsibilities of scientists.