Atomic bombs were the first nuclear weapons to be developed, tested and used. In the late 1930s, European and American physicists realized that uranium fission could be used to create an extremely powerful explosive weapon. In August 1939, German-American physicist Albert Einstein sent a letter to U.S. President Franklin D. Roosevelt describing this discovery and warning against its potential development by other countries. The U.S. government launched the top secret Manhattan Project in 1942 to develop an atomic device. The leader of the Manhattan Project was U.S. Army Brigadier General Leslie R. Groves. His team, working in several locations but largely in Los Alamos, New Mexico, under the leadership of American physicist J. Robert Oppenheimer, designed and built the first atomic bombs. The first atomic explosion was carried out, as a test, at Alamogordo. , New Mexico, July 16, 1945. The energy released by this explosion was equivalent to that released by the detonation of 20,000 tons of TNT. Toward the end of World War II, on August 6, 1945, the United States dropped the first atomic bomb on the Japanese city of Hiroshima. It was followed by a second bomb against the city of Nagasaki on August 9. According to US estimates, 60,000 to 70,000 people were killed by the Hiroshima bomb, called "Little Boy", and about 40,000 by the bomb dropped on Nagasaki, called "Fat". Man." Japan accepted the Allies' terms of surrender on August 14. These were the only times a nuclear weapon was used in conflict between nations. Fusion bombs, also called hydrogen bombs or thermonuclear bombs, were were developed and tested in the early 1950s, but they were never used in combat A thermonuclear device relies on a fission reaction to produce extreme heat that causes hydrogen isotopes to fuse or fuse together. deuterium and tritium, but the main source of energy for thermonuclear devices comes from the fusion reaction and not from triggering the fission reaction. For more information on this type of bomb, see Hydrogen bomb..