Fallout Tactics is story takes place on the timeline between (squad-based combat games) the stories 0f Fallout and Fallout 2.
With nuclear apocalypse looming over the world, several vaults were constructed to contain the best and brightest of humanity. By being shielded from the imminent death, the offspring of these people could reclaim and repopulate the Earth. However, before the entire network could be completed, nuclear war broke out. One of the military vaults located in California emerged from the war determined to restore civilization. Using their superior weapons, they were able to claim the surrounding wasteland. The members of this vault formed the Brotherhood of Steel, an organization dedicated to restoring civilization and reclaiming or developing new and better technologies.
A split soon formed in the Brotherhood, however. One faction supported allowing tribals (human outsiders) to join the organization to prevent a lack of troops. The other faction wanted to keep the Brotherhood pure and not accept outsiders. The faction against expansion won out, and the other faction was sent across the mountains on great airships to destroy the remnants of the mutant army defeated in the first Fallout game. A lightning storm struck down the ships, however, and they were dispersed and forced to crash-land. One of the surviving airships crashed near the ruins of post-war Chicago. After regrouping, and free from the Brotherhood members in California who wanted nothing to do with the tribals, the crash survivors established a first base near Chicago and founded a new Brotherhood that would grow and expand by recruiting outsiders and expanding across the land.