Einstein wrote a letter to U.S. President Franklin D. Roosevelt in 1939, urging the United States to begin research into the development of atomic weapons. The letter was prompted by concerns that Nazi Germany might be working on developing an atomic bomb. The letter warned Roosevelt of the potential danger posed by Germany's progress in nuclear physics and the possibility of them using atomic power as a weapon.
Although Einstein was a pacifist, he recognized the importance of the United States developing its own atomic bomb as a means of counteracting Germany's efforts during World War II. This letter played a significant role in initiating the U.S. atomic bomb program, which later became the Manhattan Project.
It is important to note that Einstein later regretted his involvement in this decision, particularly after the bombs were used on Hiroshima and Nagasaki. He became a strong advocate for nuclear disarmament and peace in the years following the war.