<P> A study presented at the annual meeting of the American Geophysical Union in December 2006 asserted that even a small - scale regional nuclear war could produce as many direct fatalities as all of World War II and disrupt the global climate for a decade or more . In a regional nuclear conflict scenario in which two opposing nations in the subtropics each used 50 Hiroshima - sized nuclear weapons (c. 15 kiloton each) on major population centers, the researchers predicted fatalities ranging from 2.6 million to 16.7 million per country . The authors of the study estimated that as much as five million tons of soot could be released, producing a cooling of several degrees over large areas of North America and Eurasia (including most of the grain - growing regions). The cooling would last for years and could be "catastrophic", according to the researchers . </P> <P> Either a limited or full - scale nuclear exchange could occur during an accidental nuclear war, in which the use of nuclear weapons is triggered unintentionally . Postulated triggers for this scenario have included malfunctioning early warning devices and / or targeting computers, deliberate malfeasance by rogue military commanders, consequences of an accidental straying of warplanes into enemy airspace, reactions to unannounced missile tests during tense diplomatic periods, reactions to military exercises, mistranslated or miscommunicated messages, and others . A number of these scenarios actually occurred during the Cold War, though none resulted in the use of nuclear weapons . Many such scenarios have been depicted in popular culture, such as in the 1962 novel Fail - Safe (released as a film in 1964), the film WarGames, released in 1983 and the film Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb, also released in 1964 . </P> <P> During the final stages of World War II in 1945, the United States conducted atomic raids on the Japanese cities of Hiroshima and Nagasaki, the first on August 6, 1945, and the second on August 9, 1945 . These two events were the only times nuclear weapons have been used in combat . </P> <P> For six months before the atomic bombings, the U.S. 20th Air Force under General Curtis LeMay executed low - level incendiary raids against Japanese cities . The worst air raid to occur during the process was not the nuclear attacks, but the Operation Meetinghouse raid on Tokyo . On the night of March 9--10, 1945, Operation Meetinghouse commenced and 334 Boeing B - 29 Superfortress bombers took off to raid, with 279 of them dropping 1,665 tons of incendiaries and explosives on Tokyo . The bombing was meant to burn wooden buildings and indeed the bombing caused fire that created a 50 m / s wind, which is comparable to tornadoes . Each bomber carried 6 tons of bombs . A total of 381,300 bombs, which amount to 1,783 tons of bombs, were used in the bombing . Within a few hours of the raid, it had killed an estimated 100,000 people and destroyed 41 km (16 sq mi) of the city and 267,000 buildings in a single night--the deadliest bombing raid in military aviation history other than the atomic raids on Hiroshima and Nagasaki . By early August 1945, an estimated 450,000 people had died as the U.S. had intensely firebombed a total of 67 Japanese cities . </P>

When was the last time a nuclear bomb was used