Manhattan Project

The Manhattan Project was the codename for the Allied effort, led by the United States with participation from the United Kingdom and Canada, to develop atomic weapons during World War II. It ran from 1942 to 1946 under the U.S. Army Corps of Engineers' Manhattan Engineer District, from which it took its name. Major General Leslie R. Groves directed the program, and physicist J. Robert Oppenheimer led the weapons laboratory at Los Alamos, New Mexico.

The project emerged from concerns—voiced most famously in the 1939 Einstein–Szilárd letter to President Franklin D. Roosevelt—that Nazi Germany might develop nuclear weapons first. After preliminary work under the S-1 Committee, the program was formally consolidated in 1942 and eventually employed roughly 130,000 people across sites including:

• Los Alamos, New Mexico – weapons design
• Oak Ridge, Tennessee – uranium enrichment
• Hanford, Washington – plutonium production

The first nuclear device was detonated at the Trinity test in the New Mexico desert on 16 July 1945. Three weeks later, on 6 and 9 August 1945, the United States dropped uranium and plutonium bombs on Hiroshima and Nagasaki, killing well over 100,000 people and contributing to Japan's surrender.

The project's political legacy is foundational to modern international relations. It inaugurated the nuclear age, triggered a Soviet weapons program that produced a successful test in 1949, and set the stage for Cold War deterrence doctrine. It also motivated subsequent arms-control efforts, including the 1963 Partial Test Ban Treaty and the 1968 Treaty on the Non-Proliferation of Nuclear Weapons (NPT). Debates over the moral and strategic justification of the bombings, the role of scientists in weapons development, and civilian oversight of nuclear technology—formalized in the U.S. by the Atomic Energy Act of 1946—remain central reference points in disarmament, nonproliferation, and nuclear ethics discussions today.