The Manhattan Project was the code name for the American-led effort to develop a functional atomic weapon during World War II. The controversial creation and eventual use of the atomic bomb engaged some of the world’s leading scientific minds, as well as the U.S. military—and most of the work was done in Los Alamos, New Mexico, not the borough of New York City for which it was originally named.

The Manhattan Project was started in response to fears that German scientists had been working on a weapon using nuclear technology since the 1930s—and that Adolf Hitler was prepared to use it.

America Declares War

The agencies leading up to the Manhattan Project were first formed in 1939 by President Franklin D. Roosevelt after U.S. intelligence operatives reported that scientists working for Adolf Hitler were already working on a nuclear weapon.

At first, Roosevelt set up the Advisory Committee on Uranium, a team of scientists and military officials tasked with researching uranium’s potential role as a weapon. Based on the committee’s findings, the U.S. government started funding research by Enrico Fermi and Leo Szilard at Columbia University, which was focused on radioactive isotope separation (also known as uranium enrichment) and nuclear chain reactions.

The Advisory Committee on Uranium’s name was changed in 1940 to the National Defense Research Committee, before finally being renamed the Office of Scientific Research and Development (OSRD) in 1941 and adding Fermi to its list of members.

That same year, following the Japanese attack on Pearl Harbor, President Roosevelt declared that the U.S. would enter World War II and align with Great Britain, France and Russia to fight against the Germans in Europe and the Japanese in the Pacific theater.

The Army Corps of Engineers joined the OSRD in 1942 with President Roosevelt’s approval, and the project officially morphed into a military initiative, with scientists serving in a supporting role.

The Manhattan Project Begins

The OSRD formed the Manhattan Engineer District in 1942 and based it in the New York City borough of the same name. U.S. Army Colonel Leslie R. Groves was appointed to lead the project.

Fermi and Szilard were still engaged in research on nuclear chain reactions, the process by which atoms separate and interact, now at the University of Chicago, and successfully enriching uranium to produce uranium-235.

Meanwhile, scientists like Glenn Seaborg were producing microscopic samples of pure plutonium, and Canadian government and military officials were working on nuclear research at several sites in Canada.

On December 28, 1942, President Roosevelt authorized the formation of the Manhattan Project to combine these various research efforts with the goal of weaponizing nuclear energy. Facilities were set up in remote locations in New Mexico, Tennessee and Washington, as well as sites in Canada, for this research and related atomic tests to be performed.

Robert Oppenheimer and Project Y

Theoretical physicist J. Robert Oppenheimer was already working on the concept of nuclear fission (along with Edward Teller and others) when he was named director of the Los Alamos Laboratory in northern New Mexico in 1943.

Los Alamos Laboratory—the creation of which was known as Project Y—was formally established on January 1, 1943. The complex is where the first Manhattan Project bombs were built and tested.

On July 16, 1945, in a remote desert location near Alamogordo, New Mexico, the first atomic bomb was successfully detonated—the Trinity Test—creating an enormous mushroom cloud some 40,000 feet high and ushering in the Atomic Age.

Scientists working under Oppenheimer had developed two distinct types of bombs: a uranium-based design called “the Little Boy” and a plutonium-based weapon called “the Fat Man.” With both designs in the works at Los Alamos, they became an important part of U.S. strategy aimed at bringing an end to World War II.

The Potsdam Conference

With the Germans sustaining heavy losses in Europe and nearing surrender, the consensus among U.S. military leaders in 1945 was that the Japanese would fight to the bitter end and force a full-scale invasion of the island nation, resulting in significant casualties on both sides.

On July 26, 1945, at the Potsdam Conference in the Allied-occupied city of Potsdam, Germany, the U.S. delivered an ultimatum to Japan—surrender under the terms outlined in the Potsdam Declaration (which, among other provisions, called for the Japanese to form a new, democratic and peaceful government) or face “prompt and utter destruction.”

As the Potsdam Declaration provided no role for the emperor in Japan’s future, the ruler of the island nation was unwilling to accept its terms.

Hiroshima and Nagasaki

Meanwhile, the military leaders of the Manhattan Project had identified Hiroshima, Japan, as an ideal target for an atomic bomb, given its size and the fact that there were no known American prisoners of war in the area. A forceful demonstration of the technology developed in New Mexico was deemed necessary to encourage the Japanese to surrender.

With no surrender agreement in place, on August 6, 1945, the Enola Gay bomber plane dropped the as-yet untested “Little Boy” bomb some 1,900 feet above Hiroshima, causing unprecedented destruction and death over an area of five square miles. Three days later, with still no surrender declared, on August 9th, the “Fat Man” bomb was dropped over Nagasaki, the site of a torpedo-building plant, destroying more than three square miles of the city.

The two bombs combined killed more than 100,000 people and leveled the two Japanese cities to the ground.

The Japanese informed Washington, which following Roosevelt’s death was under the new leadership of President Harry Truman, of their intention to surrender on August 10th, and formally surrendered on August 14, 1945.

Legacy of the Manhattan Project

With the development of weapons designed to bring about the end of World War II as its stated mission, it’s easy to think that the story of the Manhattan Project ends in August 1945. However, that’s far from the case.

Following the end of the war, the United States formed the Atomic Energy Commission to oversee research efforts designed to apply the technologies developed under the Manhattan Project to other fields.

Ultimately, in 1964, then-President Lyndon B. Johnson put an end to the U.S. government’s effective monopoly over nuclear energy by allowing for private ownership of nuclear materials.

The nuclear fission technology perfected by the Manhattan Project engineers has since become the basis for the development of nuclear reactors, power generators, as well as other innovations, including medical imaging systems and radiation therapies for various forms of cancer.


Manhattan: The Army and the Atomic Bomb. U.S. Army Center of Military History.
The Manhattan Project—Its Story. U.S. Department of Energy: Office of Scientific and Technical Information.
Leo Szilárd, a traffic light and a slice of nuclear history. Scientific American.
J. Robert Oppenheimer (1904—1967). Atomic Archive.

HISTORY Vault: World War II

Stream World War II series and specials commercial-free in HISTORY Vault.