The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada.

From 1942 to 1946, the project was directed by Major General Leslie Groves of the U.S. Army Corps of Engineers. Nuclear physicist J. Robert Oppenheimer was the director of the Los Alamos Laboratory that designed the bombs. The Army program was designated the Manhattan District, as its first headquarters were in Manhattan; the name gradually superseded the official codename, Development of Substitute Materials, for the entire project. The project absorbed its earlier British counterpart, Tube Alloys, and subsumed the program from the American civilian Office of Scientific Research and Development.

The Manhattan Project employed nearly 130,000 people at its peak and cost nearly US$2 billion (equivalent to about $27 billion in 2023). The project pursued both highly enriched uranium and plutonium as fuel for nuclear weapons. Over 80 percent of project cost was for building and operating the fissile material production plants. Enriched uranium was produced at Clinton Engineer Works in Tennessee.