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First atomic bomb test, near Alamogordo, New Mexico, July 16, 1945.Courtesy of Los Alamos National Laboratory, New MexicoWeapon whose great explosive power results from the sudden release of energy upon the splitting, or fission, of the nuclei of heavy elements such as plutonium or uranium (see nuclear fission). With only 1133 lb (515 kg) of highly enriched uranium, a modern atomic bomb could generate a 15-kiloton explosion, creating a huge fireball, a large shock wave, and lethal radioactive fallout. The first atomic bomb, developed by the Manhattan Project during World War II, was set off on July 16, 1945, in the New Mexico desert. The only atomic bombs used in war were dropped by the U.S. on Hiroshima on Aug. 6, 1945, and on Nagasaki three days later. In 1949 the Soviet Union tested its first atomic bomb, followed by Britain (1952), France (1960), China (1964), India (1974), and Pakistan (1998). Israel and South Africa were suspected of testing atomic weapons in 1979. See also hydrogen bomb; Nuclear Non-proliferation Treaty; nuclear weapon.
Cenotaph in Peace Memorial Park, Hiroshima, Japan; Atomic Bomb Dome is visible through the arch.Bob GlazeArtstreetCity (pop., 2002 est.: 1,113,786), southwestern Honshu, Japan. Founded as a castle town in the 16th century, it was from 1868 a military centre. In 1945 it became the first city ever to be struck by an atomic bomb, dropped by the U.S. in the last days of World War II. Rebuilding began in 1950, and Hiroshima is now the largest industrial city in the region. It has become a spiritual centre of the peace movement to ban nuclear weapons; Peace Memorial Park is dedicated to those killed by the bomb, and Atomic Bomb Dome is the ruin of the only building to survive the blast.
Secret documents detailing the U.S. role in Indochina from World War II to 1968. The U.S. Defense Department commissioned the study; a project associate, Daniel Ellsberg, who was opposed to U.S. participation in the Vietnam War, leaked details of the documents to the press. In June 1971 The New York Times began publishing articles based on the study. The U.S. Justice Department, citing national security, obtained a temporary court order halting publication. The U.S. Supreme Court ruled that the government had failed to justify restraint of publication, and the documents were published widely, fueling debate over the country's Vietnam policy.
© 2010 Encyclopædia Britannica, Inc.


