Hi, I'm trying to design a board game with a system of 2 large AI factions fighting while players are intervening on behalf or against either side. I was wondering if anyone knew of any board games that had a system similar to this?
You could try the GMT COIN Series. For example, Cuba Libre provides AI sheets to control any of the four warring factions; so it's possible to play, say, with two human players in a four-sided conflict.
To make sure I get the question right: You basically want a game where factions are independent of players and players can decide to support either, depending on their current goals?
Sounds intersting, but I can't think of many games doing that and the only ones that come to mind where players are somewhat uncoupled from the actual "characters" in the game are interestingly enough all lighter games:
To some extent Takenoko is doing this, with the Panda eating and the gardener planting the bamboo and the players trying to use both of them to fulfill their goals.
In Black Fleet while every player has their own merchant and pirate ships there is also the Navy ships that each player can control on their turn to go after the other players.
Similar thing in Colt Express where the marshal would be a character that all players can take control of temporarily to hinder their opponents or to make the way free for themselves, overall again everyone has their own character though.
In Colosseum the emperor and senators are also moved by all players trying to get them into their own colosseum.