What was the Manhattan Project?

The Manhattan Project was the codename for the Allied effort, led by the United States with British and Canadian participation, to develop nuclear weapons before Nazi Germany could. It began in earnest in 1942 following warnings from physicists including a letter signed by Einstein, and grew into an enterprise employing more than 100,000 people across secret sites, principally Los Alamos in New Mexico for weapon design under Robert Oppenheimer, Oak Ridge in Tennessee for uranium enrichment, and Hanford in Washington for plutonium production. Total cost approached two billion dollars in 1940s terms, and secrecy was so tight that most workers did not know what they were building.

The first test, codenamed Trinity, was conducted in the New Mexico desert on July 16, 1945. Two weapons were used against Japan the following month, a uranium gun-type device over Hiroshima on August 6 and a plutonium implosion device over Nagasaki on August 9, killing well over 100,000 people immediately with many more deaths following from injuries and radiation. Japan announced surrender on August 15. Germany had surrendered in May, before the weapons were ready, and the German nuclear program turned out to have been far less advanced than feared.

The consequences structured the following century. The Soviet Union tested its own device in 1949, aided by espionage from within the project, beginning a nuclear arms race that produced arsenals capable of ending civilization and a doctrine of deterrence that has held so far. Whether the bombings were necessary to end the war without an invasion remains one of the most extensively debated questions in modern history, with serious historians on multiple sides. Many project scientists became prominent advocates for arms control afterward, and the project also left a substantial legacy of radioactive contamination and health effects among workers and downwind communities that the government later acknowledged through compensation programs.