Claim: Franklin D. Roosevelt started the development of a nuclear weapon.

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions , either fission -LRB- fission bomb -RRB- or a combination of fission and fusion -LRB- thermonuclear weapon -RRB- . Both reactions release large quantities of energy from relatively small amounts of matter . The first test of a fission -LRB- `` atomic '' -RRB- bomb released the same amount of energy as approximately 20,000 tonTNT . The first thermonuclear -LRB- `` hydrogen '' -RRB- bomb test released the same amount of energy as approximately 10 e6tonTNT . A thermonuclear weapon weighing little more than 2400 lb can produce an explosive force comparable to the detonation of more than 1.2 e6tonTNT . A nuclear device no larger than traditional bombs can devastate an entire city by blast , fire , and radiation . Nuclear weapons are weapons of mass destruction , and their use and control have been a major focus of international relations policy since their debut .   Nuclear weapons have been used twice in war , both times by the United States against Japan near the end of World War II . On August 6 , 1945 , the U.S. Army Air Forces detonated a uranium gun-type fission bomb nicknamed `` Little Boy '' over the Japanese city of Hiroshima ; three days later , on August 9 , the U.S. Army Air Forces detonated a plutonium implosion-type fission bomb codenamed `` Fat Man '' over the Japanese city of Nagasaki . The bombings resulted in the deaths of approximately 200,000 civilians and military personnel from acute injuries sustained from the explosions . The ethics of the bombings and their role in Japan 's surrender are subjects of debate .   Since the atomic bombings of Hiroshima and Nagasaki , nuclear weapons have been detonated over two thousand times for testing and demonstration . Only a few nations possess such weapons or are suspected of seeking them . The only countries known to have detonated nuclear weapons -- and acknowledge possessing them -- are -LRB- chronologically by date of first test -RRB- the United States , the Soviet Union -LRB- succeeded as a nuclear power by Russia -RRB- , the United Kingdom , France , the People 's Republic of China , India , Pakistan , and North Korea . Israel is also believed to possess nuclear weapons , though , in a policy of deliberate ambiguity , it does not acknowledge having them . Germany , Italy , Turkey , Belgium and the Netherlands are nuclear weapons sharing states . South Africa is the only country to have independently developed and then renounced and dismantled its nuclear weapons .   The Treaty on the Non-Proliferation of Nuclear Weapons aims to reduce the spread of nuclear weapons , but its effectiveness has been questioned , and political tensions remained high in the 1970s and 1980s . Modernisation of weapons continues to occur .