The Manhattan Project was the program based in the United States which tried to make the first nuclear weapons. The project went on during World War II and was run by the U.S. Army. The head of the project was General Leslie R. Groves, who had led the building of the Pentagon. The top scientist on the project was Robert Oppenheimer, a famous physicist. The project cost $2 billion and created many secret cities and bomb-making factories, such as a laboratory in Los Alamos, New Mexico, a nuclear reactor in Hanford, Washington, and a uranium processing plant in Oak Ridge, Tennessee.
The Manhattan Project had to find solutions to two difficulties. The first difficulty is how to make the special isotopes of uranium or plutonium. This process is called separation and is very slow. The United States built very big buildings with machines for separation. They made enough fissionable special isotopes for a few nuclear weapons. The second difficulty was how to make a bomb that will produce a big nuclear explosion every time. A weapon with a broken design will often make a much smaller nuclear explosion. This is called a "fizzle".

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