The Manhattan Project was the codename for the American effort to develop and test nuclear weapons during World War II.
Run by General Leslie Groves, the construction of the actual bomb was overseen by Robert Oppenheimer, who was head of the Los Alamos Laboratory where it was developed.
In 1939 a letter written by Leo Szilard and signed by Albert Einstein was delivered to US President Franklin D. Roosevelt. The letter urged the United States to develop uranium stockpiles and commence research efforts, especially as Nazi Germany might do the same.
Two bomb types were developed: Little Boy, a uranium bomb, and Fat Man, a plutonium bomb. The work was carried out with extreme secrecy; many of those working on the project had no idea what they were working towards. Despite the security, Soviet spies managed to penetrate the project, and were aware that the US had developed the bomb.
On July 16, 1945, the Trinity test became the first detonation of a nuclear weapon. Less than a month later, President Harry Truman authorized the use of nuclear weapons on Hiroshima and Nagasaki, to date the only use of nuclear weapons in history. The bombs brought about the quick end of World War II without the need for a catastrophic invasion of Japan, but with an exceptionally high loss of civilian life in the two destroyed cities.
Clancy's comment: Mm ... and things sure have changed since then.