After the first use of nuclear weapons in 1945, Infeld, like Einstein, became a peace activist. Because of his activities, he was unjustly accused of having communist sympathies. In 1950 he left Canada and returned to communist Poland. He also could not return to the US at that time. In the US while previously working directly with Einstein who formally invented nuclear weapons by his famous formula he was even like the Rosenbergs in possible danger of receiving the death penalty in case he would be accused of spying for the Soviet Union.