Roosevelt avoided the State Department and conducted high-level diplomacy through his aides, especially Harry Hopkins, whose influence was bolstered by his control of the Lend Lease funds.[283]

Nuclear program
See also: History of nuclear weapons and Nuclear weapons of the United States
In August 1939, Leo Szilard and Albert Einstein sent the Einstein–Szilárd letter to Roosevelt, warning of the possibility of a German project to develop nuclear weapons. Szilard realized that the recently discovered process of nuclear fission could be used to create a nuclear chain reaction that could be used as a weapon of mass destruction.[284] Roosevelt feared the consequences of allowing Germany to have sole possession of the technology and authorized preliminary research into nuclear weapons.[m] After the attack on Pearl Harbor, the Roosevelt administration secured the funds needed to continue research and selected General Leslie Groves to oversee the Manhattan Project, which was charged with developing the first nuclear weapons. Roosevelt and Churchill agreed to jointly pursue the project, and Roosevelt helped ensure that American scientists cooperated with their British counterparts.