|The following is based on Fallout: Brotherhood of Steel and has not been confirmed by canon sources.|
The World After the War
The background of the Fallout world is fairly basic to understand and visualize. Assume that the "World of Tomorrow" display at the 1939 World's Fair became a reality - flying cars, robotic assistants, and self-cleaning homes. Now assume that world was nearly destroyed by nuclear war. That's the world of Fallout.
Fallout takes place many years after the apocalypse, as humanity struggles to pick up the pieces. Some groups have barely managed to survive above ground, either scavenging the scorched ruins of cities or surviving and adapting through mass mutation. Communities that were sealed in underground vaults during the war are now emerging to re-populate the blasted landscape. Striving to organize and sustain the human race, these tattered remnants of civilization are threatened by psychotic mutants, rogue machines, raiders, and all manner of hostile creatures.
Welcome to the wasteland.
The Great War
The United Nations failed in 2052 as the planet's natural resources dried up, causing many smaller nations to go financially bankrupt and fail. Europe and the Middle East were cast into a long, drawn-out war over the few remaining productive oil fields.
In late 2053, the United States closed its borders when a new super plague was discovered, and a terrorist nuclear weapon destroyed Tel Aviv. In early 2054, the U.S. responded by creating Project Safehouse. This project, financed by junk bonds, was responsible for creating large underground survival shelters, commonly known as Vaults. This project wasn't completed when it ran out of money nine years later.
In 2060, the Middle Eastern oil fields ran dry, ending the European war.
In the winter of 2066, China attacked Alaska over what were most likely the last drops of oil in the world. The U.S. responded with force, but it would be ten years before the conflict would end. In a desperate maneuver, both superpowers invaded neighboring countries in an effort to bolster their dwindling resources. The U.S. annexation of Canada was concluded by 2076. Canadian timber provided fuel for U.S. military needs, and Alaska was reclaimed by early 2077.
In the early morning hours of Saturday, October 23, 2077, the sky was filled with nuclear missiles. No one knows who started the war, but after two hours of nuclear devastation, no one cared.
Because of frequent false alarms, few made it to the Vaults when the final alarm sounded. Most people were trapped outside the closing Vault doors.
When nuclear war became a tangible fear amongst the American populace, the government worked with a private corporation to research and construct vast underground shelters. These shelters were referred to as Vaults, and the corporation that developed them was named Vault-Tec.
What were the Vaults? They were underground installations meant to preserve Americans in the event of a nuclear war. Or were they? As it was discovered, the Vaults were never intended to save the population of the United States. With a population of almost 400 million by 2077, the U.S. would need nearly 400,000 Vaults, and Vault-Tec was commissioned to build only 122 such Vaults. The real reason for these Vaults was to study pre-selected segments of the population to see how they react to the stresses of isolationism and how successfully they re-colonize after the Vault opens.
And then there's Vault-Tec. Like many major military contractors, as Vault-Tec grew in finances and responsibility, it began to develop highly experimental and secure technologies. In time, Vault-Tec was so thoroughly integrated with top-secret military research, it was practically a division of the U.S. government - almost. The company still retained some elements of privatization. So the principals of Vault-Tec - executives, scientists, and so on - had to be aware of the true function of the Vaults, and they had access to the technology. They were the developers. Regardless of the U.S. government's directives, Vault-Tec was not going to submit to a lottery and live underground with a bunch of human cattle in facilities that were intentionally defective. Thus Vault-Tec created their own facilities, removed from the larger Vault network - and kept secret from the U.S. government.
Vault-Tec didn't stop there. These secret, private installations were used to research the latest vault mechanisms as well as technologies that would benefit mankind in a post-nuclear environment. This, of course, included some extensive research with FEV.
No one can say for certain how Vault-Tec obtained FEV. Some think they had it all along, gaining it in a deal with the government, others think it was stolen from the government while VT was repairing one of the more guarded vaults.
Forced Evolutionary Virus
FEV was initially developed in response to the "New Plague" that tore through the world population prior to the Great War. The basic concept was to push any biological entity's ability to adapt to harsh circumstances - such as a super plague (which may or may not have been a genetically engineered weapon). Over time, the FEV research led to far greater exploits.
Some early tests with the virus seemed to create super animals - lab monkeys with greater body mass and higher intelligence. The military immediately saw some possible applications and took control of FEV research, dedicating several major installations to studying the virus and further applications. The initial test animals quickly regressed into a far less intelligent, aggressive state, eventually showing massive deformities and insanity. This led researchers to conclude that FEV was unstable at best - but experimentation continued.
When the bombs dropped, one of the major military FEV research centers was hit, cracking the tanks and allowing FEV to slowly permeate the wasteland ecosystem, carried by rats, dogs, and even humans that came into contact with the virus. Some creatures that are exposed become sterile. Some become tougher and live much longer lives. Some simply become bigger and far more dangerous.
Radiation is responsible for most of the mutations in the wasteland - ghouls, giant animals, and so on - think 1950s sci-fi physics, and that's what happened to most of the animals and people. FEV is responsible for the more bizarre critters in the Fallout universe (Master, Floaters, and Centaurs), but not most of the mutants the player encounters.
Just before the war, an isolated group of scientists in the West-Tech research facility began experimenting on soldiers with an advanced form of the Forced Evolutionary Virus. The results were promising, but the surviving soldiers got wise. They rebelled, killing all of the scientists and locking down the facility. Carting themselves and some high-tech equipment (including energy weapons and power armor) to a new base of operations, these soldiers became the Brotherhood of Steel. They had no intention of becoming a peacekeeping force - they just wanted to survive and preserve their military technology. They didn't even care about the rest of the wastes except for caravans of supplies, and that's it.
But the story of FEV doesn't end there either. Some intrepid explorers, puzzled and threatened by the strange FEV-mutated creatures emerging out of the wastes, stumbled into the facility and were assaulted by the robotic security system and genetic nightmares within. One, a man named Harold, was transformed by the virus even as he fell unconscious during a massive battle - he awoke back in the wasteland, radically changed. Another man, Richard Grey, the leader of the party, was knocked into a vat filled with FEV. He was transformed completely, became a huge genetic nightmare known as "The Master", and continued the facility's research in the hope of creating a master race of creatures. He became the primary enemy in Fallout, and the organisms he created were kidnapped humans transformed by an advanced version of the FEV virus, called super-mutants.
The Mutant Army
The Master created the mutant army with the intention of dominating the entire human race and transforming them into super-mutants as well, by dipping them into vats of the virus. If his plan had succeeded, it could have made humanity as we know it extinct, but the virus dipping process was unstable, and the results were unpredictable - which meant that some mutants were intelligent, capable beings (this is very rare) while most others were stupid and psychotic. What's worse, the sterility the FEV caused, made these mutants an evolutionary dead-end - and so, fortunately for the human race, in Fallout, the Vault Dweller destroyed the vats, killed The Master, and scattered the mutant army.
The remaining mutants scattered, many of them moved east. Some were integrated into small societies, but many were feared and shunned.
But a smaller sub-section of the mutant army did remain together, under the command of a visionary lieutenant named Attis. Highly intelligent and vindictive towards the Brotherhood, Attis took his group into hiding, searching for clues regarding the history of the FEV virus. Learning about a secret Vault-Tec installation with new and possibly improved versions of the FEV virus, Attis led his group to the area where the rumors started, the ruined city of Los. His ultimate goal, of course, is the continuation of The Master's plan - to rebuild the mutant vats and eventually transform the entire human race into mutants.