The Manhattan Engineer District (MED), or Manhattan Project, was the government program run by the U. S. Army Corps of Engineers during World War II that oversaw the development of the atomic bomb. Prompted by a warning letter from Albert Einstein that Germany was already working on an atomic weapon, the Manhattan Project was initiated by order of President Franklin Delano Roosevelt in 1939. The U. S., Canada and Great Britain maintained research sites at a cost of more than $2 billion, primarily in Washington state (where the study of plutonium capabilities were centered); Oak Ridge, Tennessee (uranium research); and at Los Alamos, New Mexico. The first atomic bomb, nicknamed Little Boy, was a uranium-based fission weapon developed in Oak Ridge; the "more complex plutonium core" variety, nicknamed Fat Man, was developed in Los Alamos. They were the two types eventually used on the first military atomic drops at Hiroshima and Nagasaki in 1945. More than 130,000 men and women were employed in the program between 1939-1946.