He passed through Los Alamos, New Mexico, home during the Second World War to the Manhattan Project, the US government's programme to build an atomic bomb. An elderly guy who'd worked there as a machinist after being transferred from the Ford Motor factory in Detroit told him how him and his workmates had climbed into the mountains and watched the weapon being tested. After seeing the power of the explosion, they started a petition against it being unleashed against civilians which hundreds of workers signed. It reached the US Defense Secretary who refused to pass it on to the President.
I think it's pretty hard to argue that wiping cities off the face of the earth and slaughtering hundreds of thousands of people wasn't a war crime and that Harry Truman by rights shouldn't have been in the dock at Nuremberg. The bombing of Hiroshima and Nagasaki in 1945 was supported by the majority of the US population. A major factor in that was the way anti-Japanese racism had been whipped up during the war, describing the enemy in the Pacific as sub-human for example. I doubt there would have been the same feeling about the atomic bomb if it was German or Italian cities that had been targetted.