Atomic bombs were the first nuclear weapons to be developed, tested and used. In the late 1930s, European and American physicists realized that fission of uranium could be used to create an extremely powerful explosive weapon. In August 1939, German-American physicist Albert Einstein sent a letter to US President Franklin D. Roosevelt describing this discovery and warning of its potential development by other nations. In 1942 the United States government established the top secret Manhattan Project 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 direction of American physicist J. Robert Oppenheimer, designed and built the first atomic bombs. The first atomic explosion was conducted, as a test, in Alamogordo, New Mexico, on 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. A second bomb followed on the city of Nagasaki on August 9. According to US estimates, between 60,000 and 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." On August 14, Japan accepted the Allied terms of surrender. These are the only times a nuclear weapon has been used in a conflict between nations. Fusion bombs, also called hydrogen or thermonuclear bombs, they were developed and tested in the early 1950s, but were never used in warfare A thermonuclear device depends on a fission reaction to produce extreme heat that causes the hydrogen isotopes of deuterium and tritium to combine or. they fuse, but the main source of energy for thermonuclear devices comes from the fusion reaction, not from the initiating fission reaction. For more information on this type of bomb, see Hydrogen bomb.