Later, Placzek was the only Czech with a leading position in the Manhattan project, where he worked from 1943 till 1946 as a member of the British Mission ; first in Canada as the leader of a theoretical division at the Montreal Laboratory and then (since May 1945) in Los Alamos, later replacing his friend Hans Bethe as the leader of the theoretical group. Since 1948, Placzek was a member of the Institute for Advanced Study in Princeton, United States.