An atomic bomb is a weapon of mass destruction in which enormous energy is released by nuclear reactions. The United States was the first country to drop the atomic bomb when President Harry S. Truman ordered the weapon used on the Japanese cities of Hiroshima and Nagasaki on August 6 and August 9, 1945, ending World War II. The atomic weapons used in these cases work through nuclear fission—literally splitting the nucleus of an atom—and exploded with a yield of 15-20 kilotons of TNT. In contrast, thermonuclear weapons, which were developed following World War II, work through nuclear fusion, creating an explosive yield exponentially greater. A test bomb detonated by the Soviet Union, the Tsar Bomba, produced an explosive yield of an estimated 50 megatons.