Albert Einstein was a life-long pacifist, till the rise of fascism in Europe in the 1930′s when he fled to the United States. As a physicist, he became convinced that physics could play a part in the creation of a weapon which could bring the end to this war, and perhaps all wars. A letter was written, mainly by Leó Szilárd in consultation with fellow Hungarian physicists Edward Teller and Eugene Wigner, and was sent to Einstein. He agreed with the concept, and whilst staying at the Long Island home now known as the Belvedere (currently a bed and breakfast), affixed his name to the document, walked down the road to the tiny town of Peconic’s post office, and sent it to Washington, where it found its way to President Roosevelt. Roosevelt authorized the research, and the Manhattan Project was begun, which was to result in the world’s first atomic weapon.
It is said that Einstein later regretted signing the letter. That I believe, is a sentiment shared by many of us.