He went to Liverpool on a scientific grant and was caught by the WWII. In Britain he was collaborating with James Chadwick and during the war he was involved in the Manhattan project but left when he had found out that it led to the arms race.
He became one of the prominent critics of nuclear arms race and founded with Bertrand Russell the Pugwash organization. Despite the Iron Curtain and the Cold War he advocated establishing links between scientists from the West and East. Similarily to the Hippocratic Oath he thought that scientists should have their own moral code of conduct.