What was the "Manhattan Project"?

Explanation

The atomic bombs Little Boy (6 August) and Fat Man (9 August) were dropped on the Japanese cities Hiroshima and Nagasaki in 1945. They killed 120,000 people on impact and around twice as many in the time following. The decision to drop the atomic bombs, which were developed in the desert of New Mexico as part of the secret Manhattan Project, was made by then president Harry S. Truman. By then, the US had been involved in World War II for 3.5 years and lost some 400,000 soldiers.