The Manhattan Project was the code name given to the US project to develop an atomic bomb – an explosive device based on the phenomenon of nuclear fission – during World War II. The Project was established at Chicago, California, and Columbia universities, as well as at Los Alamos, New Mexico, and other centers.
In the summer of 1939, before the outbreak of World War II, President Franklin D. Roosevelt (1882–1945) received the Einstein-Szilard letter, a plea from physicists Albert Einstein, Leo Szilard, and Eugene Wigner warning him of the possibility that Nazi Germany could develop an atomic bomb. Within a year, the United States was conducting research into the feasibility of such a weapon. The project grew steadily over time, and in 1942 President Roosevelt signed an order directing the creation of a nuclear weapon. Eventually falling under the direction of General Leslie Groves (1896–1970), the scientific research team that designed and built the device was led by physicist J. Robert Oppenheimer. By December 1942, a team headed by Enrico Fermi, which included, among others, Edward Teller, initiated the first self-sustaining nuclear chain reaction. On July 16, 1945, the first atomic bomb was detonated near Alamogordo, New Mexico.
The moment the Manhattan Project succeeded, it irrevocably altered human history and ushered in the Atomic Age. On August 6, 1945, the United States dropped an atomic weapon on Hiroshima, Japan, followed three days later by another weapon on Nagasaki. Four years later, the Soviet Union developed its own atomic weapon, and the Cold War that followed lasted for decades. By its end, several nations possessed nuclear weapons that could reach across continents, threatening nuclear annihilation at the push of a button. Though the Cold War has ended, the threat nuclear weapons pose to humanity remains.