During WWII he served again under Stimson, now Secretary of War, as his Special Assistant on Atomic Matters, serving as liaison between Stimson and the director of the Office of Scientific Research and Development, Vannevar Bush.
During WWII he served again under Stimson, then Secretary of War under President Franklin D. Roosevelt, as his Special Assistant on Atomic Matters beginning in 1941. He served as liaison between Stimson and the director of the Office of Scientific Research and Development, Vannevar Bush.