Los Alamos Laboratory · Attribution

Joseph Rotblat worked on the Manhattan Project's British contingent, then left it in 1944 on grounds of conscience — the only scientist to walk away from the bomb project — once it was clear Germany had abandoned its own atomic weapons program, the reason he had joined in the first place. He spent the following decades campaigning against nuclear weapons, co-founding the Pugwash Conferences on Science and World Affairs and serving as its secretary-general for a quarter century, and shared the 1995 Nobel Peace Prize with the organization he built.