[@Shorticus] The Legion are a Grogar Cult centered around the Worship of the AI controlling the massive facility from which they were born from. Unlike many grogar clans, the Legion remained true to the AI and do its bidding. Both plan to purge North America of those that would refuse to bow to the Mad AI. 

As for the Separatists and Militia(may merge them into a single group), I haven't develop much of them yet, but one idea of them is they are a nationalist Faction who seek to create their own state.