1. Alien Crash Site & the coming expansion Mothership Zeta (The Most Obvious)
2. The Forced Evolutionary Virus (http://fallout.wikia.com/wiki/FEV): No one really knows who developed or where it came from, given that the predecessor Virus PVP also came from the same company.
3. Advanced Weaponry and Power Armor: Again from the same company West - Tec.
4. The Enclave's original intention to leave planet Earth. (Where would they go? How Would they get there? It seems reasonable to assume that they had some means of travel and information on other habitable worlds)
5. Brainwashing techniques by U.S Military (Again ingame record from a terminal)
6. Vault-tec technology: In effect your living inside a spaceship. A self contained vault will have precisely the same life support systems as a spaceship after all.
Now consider the following theory:
1. Aliens visit earth shortly after WWII (Roswell anyone?) and make contact with the U.S Government, probably a cabal of high ranking officers and government personnel with ties to major U.S corporations, forming a cabal.
2. The cabal makes some sort of agreement with the Aliens in exchange for advanced technology, albeit the most basic by the alien's standards, and only the fundamental principles, not any actual working tech.
3. Cabal begins researching the technology via West-Tec and other companies, spurring breakthroughs in powered armor, robotics, viral agents, weaponry, medical systems etc.
4. Advances also achieved in Fusion Tech and associated technologies, probably resulting in "cleaner" nuclear weapons which would explain in the fact that Earth is actually still habitable after a global nuclear war. - other countries may have got their hands on the tech in due course via espionage or even the aliens themselves making deals with other nations.
5. Dependence on the Aliens' results in different Research paradigms that causes technologies present in today's world from never being developed i.e microwave cellular communications (the internet?), LCD Tvs, Personal PCs etc.
6. Despite use of Fusion technology, demand for natural resources does not decrease, but actually increase, spurred on by greater demand for various goods that still need large inputs example: Need for oil for plastics production.
7. Resource wars begin, with nations now armed with Energy weapons, robots, and the like, escalation in tensions.
8. Global Conflagration in 2077, major population centers destroyed, irradiated, but due to the nature of nuclear weapons based of the Alien technology, resulting destruction caused is not as deadly to life as expected, also causes unforeseen consequences such as various mutations - rad scorpions, ghouls, etc.
Now we can also perhaps come to a scenario where the aliens themselves by manipulating world governments and / or causing destabilization by providing above technologies. It seems every government by then had become pretty totalitarian - talk of democracy notwithstanding. After all the U.S government with the help of Corporations brainwashes its own people, add to this their seeming disregard for human life when corporations test their various products - see Nuka Cola quantum for example, as well as aggressive actions such as annexation of Canada and violent suppression of population, executions, interrogations etc.
Possible Motivations for aliens can be various, but probably destroying human civilization as we know it so they can begin colonization in due course, or maybe some sort of experimentation etc.
I ask this because I kind off neatly ties things together and provides a credible theory as to why things went down as they did. It does seem a little too X-files cliched, but that also fits into the 50s appeal given shows like the Twilight Zone: http://en.wikipedia.org/wiki/The_Monsters_Are_Due_on_Maple_Street
Thinking about making a quest mod based on the above after Zeta Releases so any comments appreciated.