Warmatrix

Atomic bomb test during Operation Buster-Jangle in Nevada

Nuclear weapons changed warfare like no other weapon before. They made destruction of whole cities, countries and even the entire planet possible, reducing all conventional weapons to a sideshow. Only the horrors of a nuclear war and threat of counter strikes have kept their use down to just two times.
German chemists and physicists discovered nuclear fission in 1938 CE, but the nazi government largely ignored their work and focused on conventional weapons instead. Other scientists also recognized the potential of a nuclear weapon and created awareness in other countries. In World War II Britain, the USA and the USSR started to work on creating an 'atomic bomb' too. The Manhattan Project of the USA, started in 1941 CE, was the first to produce experimental, yet usable weapons. A fission bomb works by compressing enriched uranium or plutonium into a critical mass. In this, atoms splitting apart release energy and also release neutrons that split neighboring atoms. This starts a chain reaction that proceeds exponentially and frees all fission energy in a fraction of a second. The first two atomic bombs produced by the Manhattan Project had blast yields of 15 kilotons TNT (63 TJ) and 21 kilotons TNT (88 TJ) respectively.
In August 1945 CE these two atomic bombs were dropped on Hiroshima and Nagasaki. They killed around 75,000 and 40,000 people respectively in the explosions and immediate aftermath, plus probably several thousands in later decades through radiation-afflicted cancer, and caused illness in several hundred thousands more. The deaths and destruction prompted Japan to surrender and ended World War II. The motives of the USA are still disputed; the American argument was that the bombs saved many lives that would have been lost during an invasion of Japan, though it is clear that cutting Japan's supply lines was already bringing the country to its knees. Others surmise that the real reason was to curb the influence of the USSR in east Asia and Europe.
For a few years the USA had a monopoly on nuclear weapons, but in 1949 CE the USSR catched up and tested their first bomb. Britain followed suit in 1952 CE; France in 1960 CE; China in 1964 CE; Israel around 1967 CE; India in 1974 CE; South Africa in 1979 CE; Pakistan in 1998 CE; North Korea in 2006 CE. The first bombs were delivered by aerial bombers, dropping them from high altitude. Other delivery methods were developed but were never widely adopted: artillery shells, land mines, antisubmarine depth charges and torpedoes. During the Cold War the strategic situation changed drastically with the development of cruise missiles and especially ballistic missiles, which could deliver atomic bombs quickly over large distances. Meanwhile bombers evolved too, changing to low flying, fast moving penetration bombers, capable of dropping nuclear weapons that simultaneously grew smaller and lighter.
Because of their awesome destructive power, nuclear weapons are almost exclusively strategic weapons, not tactical ones, though some people in the military envision relatively small tactical nuclear weapons. The case of a full-scale nuclear war transcends strategy by threatening to affect the whole planet with not just explosions, but also radioactive fallout and a dust cloud that can cause a nuclear winter.
The world came close to a nuclear war at the Cuban Missile Crisis of 1962 CE, but fortunately the USSR backed down from its initial moves, while the USA dismantled their Jupiter missile launch sites in Turkey. Afterwards the USA maintained the doctrine of Mutually Assured Destruction, a kind of nuclear deterrence, where the threat of a counterattack had to keep the enemy from a first strike. This led to a massive arms race that gave rise to arsenals large enough to destroy the entire surface of the Earth many times over. Several peace treaties, starting the with Non Proliferation Treaty of 1968 CE, have limited the spread and increase of nuclear weapons. However pressure from arms industries, continuing technological advances and the ambitions of medium sized powers to acquire nuclear muscle keep upsetting the balance. In the 1980's CE the world's supply of nuclear weapons was estimated at some 40,000 warheads with a combined explosive yield of 13 gigatons TNT (54 EJ).
Since the two pioneering atomic bombs of World War II, nuclear weapons have evolved further. In 1952 CE the USA developed the first 'hydrogen bomb', a thermonuclear weapon. Here much energy comes from the nuclear fusion of hydrogen atoms, though fission of heavier elements is used to kickstart the process. Pure fission weapons cannot exceed some 500 kilotons TNT (2.1 PJ) of explosive power, yet hydrogen bombs have no theoretical limit. The first fusion weapon yielded 10 megatons TNT (42 PJ); the largest fusion weapon ever detonated, the Soviet Tsar bomba in 1961 CE had a power of 50 megatons TNT (210 PJ). Other types of nuclear weapons have been developed, like the boosted fission weapon that is a fusion-boosted fission bomb and the neutron bomb that has a relatively small explosive yield but kills by excessive radiation. The radiological a.k.a 'dirty' bomb, which causes little immediate destruction but spills radioactive material around, for now remains confined to the realms of theory.
Though the two bombs on Japan remain the only cases when a nuclear weapon has been used in war until this day, a total of 16,000 of them are ready for use and another 14,000 held in reserve. They are stored in 14 countries, with 9 countries capable of producing them (South Africa has abolished nuclear weapons). Though nuclear weapons are under tight control of the government of the countries that own them, there is always the risk of accidents, false alarms, theft and sabotage, sustaining the risk of their unintended use.