The first atomic explosion was conducted, as a test, at Alamogordo, New Mexico, on July 16, 1945. The energy released from this explosion was equivalent to that released by the detonation of 20,000 tons of TNT. Near 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 followed with a second bomb against the city of Nagasaki on August 9. According to U.S. 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 agreed to Allied terms of surrender on August 14th. These are the only times that a nuclear weapon has been used in a conflict between nations.
Fusion bombs, also called hydrogen or thermonuclear bombs, were developed and tested in the early 1950s, but these have never been used in warfare. A thermonuclear device depends on a fission reaction to produce extreme heat that causes hydrogen isotopes of deuterium and tritium to come together, or fuse, but the main energy source for thermonuclear devices comes from the fusion reaction, not the triggering fission reaction.