In the early 1940s, Americans "watched" a world in chaos – as Nazi Germany and Imperial Japan invaded border after border. Despite the vow of isolationism, every citizen – from leader to laborer – pondered how long the United States could remain observers. On December 7, 1941, when the Japanese attacked Pearl Harbor, the question was answered.

Years before the attack on Pearl Harbor, government officials and the scientific community were concerned about Germany's chemical warfare strategy. In World War I, the Germans first used poisonous gas against the French at the Battle of Ypres (1915). As World War II approached, German scientists Otto Hahn and Fritz Strassmann successfully split the uranium atom (later coined fission). Hahn had served in the German gas warfare service in the first World War. Renowned physicist, Albert Einstein, warned President Franklin Delano Roosevelt that sustained fission, also known as a nuclear chain reaction, could lead to the construction of "extremely powerful bombs."

While the American scientific community had been engaged in atomic research since the 1930s, Einstein's warning and Hitler's progress set into motion the President's 1939 Advisory Committee on Uranium to understand what domestic experts had discovered and the standing of their research. The government began to formalize and organize efforts with the establishment of the National Defense Research Committee after Hitler invaded Paris in June 1940. When Hitler marched into the Soviet Union in Summer 1941, the newly formed Office of Scientific Research and Development, authorized to enter into contracts and agreements, awarded Columbia University a contract to study uranium enrichment using the gaseous diffusion method.

A month after the U.S. entered the war, President Roosevelt approved atomic weapon production through an unprecedented military operation. The U.S. would develop the bomb pursuing two methods – uranium separation and plutonium production.

The undertaking would become known as the Manhattan Project, from its origins in research laboratories in New York City. Major General Leslie Groves of the U.S. Army Corps of Engineers would lead the massive enterprise. In addition to research and development activities at universities across the country (Columbia, Chicago, and Berkley), the project would establish production facilities in East Tennessee (later to be called Oak Ridge); at Hanford, Washington; and at Los Alamos, New Mexico. These remote locations supported the physical, security and secrecy requirements to execute the mission.

Oak Ridge provided enriched uranium while Hanford produced plutonium. The Los Alamos Laboratory became the site for the development and assembly of the nuclear weapons.

Two types of atomic bombs were developed – a relatively simple gun-type weapon using uranium-235 and a more complex implosion-type weapon that used plutonium.

The uranium-235 was obtained by the use of three methods of enrichment: electromagnetic isotope separation, gaseous diffusion, and liquid thermal diffusion. Most of this work was performed at Oak Ridge, Tennessee.

Pilot-scale and production reactors were constructed at Oak Ridge and Hanford, Washington, respectively, in which uranium was irradiated and transmuted into plutonium. The plutonium was then chemically separated from the uranium.

Following production, enriched uranium and plutonium were shipped to Los Alamos. Engineers, scientists, and technicians at Los Alamos produced three weapons: Gadget, Little Boy, and Fat Man.

Los Alamos scientists successfully tested Gadget, a plutonium implosion bomb, at the Trinity site at Alamogordo, New Mexico, on July 16, 1945. On August 6, 1945, Little Boy, fueled by enriched uranium from Oak Ridge facilities, was dropped on Hiroshima, Japan. Three days later, Fat Man atomic bomb (plutonium-fueled from Hanford facilities) was dropped on Nagasaki, Japan.

On August 14, Japan surrendered unconditionally. World War II was over.

The Manhattan Project officially ended in 1946; its facilities and technologies became part of the Atomic Energy Commission.