Nuclear Weapon

"If you don't know what an atomic bomb is, then imagine the worst thing possible. Atomic bombs were worse than that."

- Vault Dweller's memoirs

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter; a modern thermonuclear weapon weighing little more than a thousand kilograms can produce an explosion comparable to the detonation of more than a billion kilograms of conventional high explosive.

Pre divergence
Nuclear weapons were first used on the Japanese cities of Nagasaki and Hiroshima in August 1945 at the end of the Second World War. These two explosions could be seen as the start of the eventual end of the world that would come 132 years later, because without them humans would never have come to possess the power to wipe out entire cities. The atomic bomb, a purely fission-based weapon, and the hydrogen bomb, a fission-fusion thermonuclear weapon, were both developed in the Fallout universe, with hydrogen bombs being considerably more dangerous because of the sheer size of their explosive yields.

Post divergence
In the Fallout world, megaton-class thermonuclear weapons had largely been retired by the major nuclear powers in favor of much smaller-yield warheads by the time of the Great War. An average strategic warhead in 2077 (with a few exceptions, such as the weapons which fell on Washington D.C.) had a yield closer to that of the nuclear bombs that fell on Hiroshima and Nagasaki in World War II, between 10-20 kilotons, but with a massive increase in radioactive fallout in place of thermal shock, much like a neutron bomb in our own world. However, despite the apparent reduction in raw explosive power, this arsenal was far more dangerous to the Earth's ecosystem, as it deposited far greater amounts of fallout in the atmosphere than had been assumed by pre-War models.

China, the United States, the Soviet Union, the European Commonwealth's member states and other countries around the world possessed massive nuclear stockpiles and when the Great War began on October 23, 2077, they sent them skyward, on bombers and intercontinental ballistic missiles. Entire continents were swallowed in flames and fell under the boiling oceans, while the EMP released from the blasts destroyed electronics worldwide.

Fallout
In Fallout, the Glow is testament to the horror of nuclear war, a radioactive hellhole destroyed by a direct nuclear hit. In the same game, the Vault Dweller also discovers an unused nuke sitting in the Master's vault, to be used as last resort against an undefeatable enemy.

Fallout 2
A nuclear bomb also rests on the Enclave Oil Rig, and is, once again, used to obliterate the main enemy of the game (detonated by an explosion of the on-board nuclear reactor).

Fallout 3
Nuclear weapons feature prominently in Fallout 3, in the form of Megaton's nuke, the Fat Man and its unique variant, the Experimental MIRV, which are two tactical nuclear catapults, a bunker full of nuclear bombs, Vertibirds with nuclear carpet bombs, a massive robot throwing nuclear bombs at everything that moves with lethal efficiency, various orbital weapons platforms such as Highwater-Trousers, and Bradley-Hercules - the Enclave-controlled satellite which destroys Liberty Prime in the Broken Steel expansion pack.

Fallout Tactics
A nuclear ICBM warhead appears first (called Plutonius) in Kansas City, worshipped by a ghoul cult. It is later used to gain entrance to Cheyenne Mountain installation, the Vault 0.

Van Buren
The B.O.M.B.-001 space station, the endgame location, was an orbital ballistic missile launch platform, that Victor Presper wanted to use to reshape the world as he envisioned it.

Inconsistencies
The yield of the weapons in the games is never explicitly stated. While it is stated that entire continents were scorched by nuclear weapons, their effects in the game are not even remotely similar to that description.

Additionally, the way the weapons are portrayed in the games is inconsistent - while in the "classic" Fallout games nuclear weapons are feared, respected and exceedingly rare (not to mention that arguably the most intelligent being in the Fallout world, the Master, is unwilling to unleash the power of atom again), in Fallout 3 nuclear weapons are commonplace and devoid of their traits from previous games. You can detonate a city with a nuclear bomb in the first few hours of the game, blow up cars in nuclear explosions and have a personal tactical nuclear launcher.