RE: Quick "Guacho" (Caribbean Kick Ass Soup)
As a quick synthesis you can sum up the facts that cause the war in two branches:
1) USA found and held the world's last Oil Stash (I don't know what the English world for "yacimiento" is). World plummets into wide scale riots for lack of oil. Canada annexes to the USA (there is a very funny comment of the President of the USA regarding Canada at the Sierra Army Depot in one of the holotapes). The UN sanctions the USA; the USA wipes its ass with the sanctions. China starts marauding near the newest USA oil rig (not the Enclave Oil Rig, this one was in Alaska I think.). China invades North America from Alaska by crossing the Behring Strait. The USA, thanks to the newly invented Powered Armor and to the lack of fuel that crippled the Chinese invasion, kicks the Chinese back into their continent...shit soon follows.
2) ACE, the computer at the Brotherhood Facility in San Francisco, tells you of the possibility that computers started the war because they were incredibly intelligent but were unable to get sensorial experiences.
Tie your own strings from there, mate.
