The Manhattan Project was the effort, led by the United States with participation from the United Kingdom and Canada, which resulted in the development of the first atomic bomb during World War II. From 1942 to 1946 the project was under the command of Major General Leslie R. Groves Jr. of the US Army Corps of Engineers. The Army component of the project was designated the Manhattan District or Manhattan Engineer District (MED), but "Manhattan" gradually superseded the official codename for the project.