130,000 people, 30 secret cities, $2 billion, three years — and most of the workers had no idea they were building a weapon that would kill 200,000 people in two days.
In August 1939, Albert Einstein signed a letter to President Roosevelt warning that Germany might be developing atomic weapons and urging the U.S. to begin its own research. The fear was real: many of the world's top physicists — including several who ended up working on the Manhattan Project — were refugees who had fled Nazi Europe and knew exactly how far German science could go.
The Manhattan Project officially began in 1942 under the direction of Army General Leslie Groves. It was the largest and most expensive weapons program in history at the time: 130,000 workers, 30 secret facilities scattered across the country, and a total cost of about $2 billion — roughly $28 billion in today's dollars. It was run with such secrecy that Vice President Harry Truman didn't learn of its existence until he became president.
Most of the 130,000 workers had no idea what they were building. At the massive Oak Ridge facility in Tennessee, workers operated centrifuges to enrich uranium without understanding why. At Hanford in Washington state, workers built reactors to produce plutonium without knowing what plutonium was for. The operation was deliberately compartmentalized — only the scientists at Los Alamos knew the full picture.
The scientific brain of the project was Los Alamos, a remote mesa in New Mexico that didn't exist on any official maps. J. Robert Oppenheimer assembled an extraordinary team there: Niels Bohr, Enrico Fermi, Hans Bethe, Richard Feynman, and dozens of other brilliant physicists, working in near-total isolation under false names. Feynman passed the time cracking safes and annoying the security office.
The first nuclear bomb was tested at the Trinity site in New Mexico on July 16, 1945. The explosion, equivalent to 21,000 tons of TNT, vaporized the desert steel tower it sat on and turned the surrounding sand into a glassy substance scientists named trinitite. Watching from a distance, Oppenheimer later recalled a line from Hindu scripture: 'Now I am become Death, the destroyer of worlds.'
Three weeks later, on August 6, 1945, Little Boy was dropped on Hiroshima. Three days after that, Fat Man was dropped on Nagasaki. The two bombs killed approximately 200,000 people, the majority of them civilians. Japan surrendered on August 15, ending World War II. The debate over whether the bombings were necessary or justified has never fully resolved.
The Manhattan Project ended the war and opened the nuclear age — but its immediate legacy was complicated. Several of the project's scientists, including Oppenheimer, later became targets of McCarthy-era investigations for allegedly being security risks. The Soviet Union detonated its own atomic bomb in 1949, just four years later, partly thanks to intelligence passed by Klaus Fuchs, a Manhattan Project physicist who had been a Soviet spy throughout.