By February 1942, the United States had entered World War II, and the Manhattan Project was building up. At the University of Chicago, Arthur H. Compton established its Metallurgical Laboratory. Its mission was to build nuclear reactors to create plutonium that would be used in atomic bombs. For advice on assembling the laboratory's Chemistry Division, Compton, a physicist, turned to Herbert McCoy, who had considerable experience with isotopes and radioactive elements. McCoy recommended Spedding as an expert on the rare earth elements, which were chemically similar to the actinide series that included uranium and plutonium. Compton asked Spedding to become the head of the Metallurgical Laboratory's Chemistry Division.