Begins consulting for the NDRC, a US government agency established to coordinate scientific research for military purposes. This marks the beginning of his extensive involvement in wartime projects.

Becomes deeply involved in the Manhattan Project at Los Alamos National Laboratory, where he provides critical mathematical and computational expertise for the development of the atomic bomb. He became a key person in the project.

Makes crucial contributions to the design of the implosion-type atomic bomb, specifically in understanding and optimizing the implosion lens system. He was able to model this in a way that helped with the development of the atomic bomb.

Was involved in the selection process of the Japanese cities against which the first atomic bombs were to be dropped. The exact extent of his involvement and his reasoning remain controversial.