Albert Einstein is known to have expressed regret and deep concern about his involvement in the development of the atomic bomb during World War II. Einstein, along with other prominent scientists, signed a letter to President Franklin D. Roosevelt in 1939, urging the United States to research atomic weapons out of concern that Nazi Germany might develop them first.
Einstein's regret stemmed from the destructive power of the atomic bomb and the potential for its use in warfare. He believed that the development and use of such weapons would have devastating consequences for humanity. Einstein's pacifist and humanitarian beliefs conflicted with the destructive nature of the atomic bomb, leading to his feelings of regret.
After the bombings of Hiroshima and Nagasaki in 1945, Einstein's concerns intensified. He became an advocate for nuclear disarmament and dedicated himself to promoting peace and preventing the further use of nuclear weapons. Einstein's regret can be seen as a reflection of his recognition of the tremendous destructive power of the atomic bomb and his desire to prevent its future use.