Sid Meier's Alien Crossfire is an expansion pack to Sid Meier's Alpha Centauri. It adds seven new factions, two of them the aliens who left Planet before the humans arrived, new technologies, new city structures, and new units.
The two new alien races are radically different from any other human faction, right down to the unit art. The player can mix and match any seven factions from the expansion pack and the original game, but the aliens have to go together or not at all. The other four human factions include a seafaring race, and an even more zealous environmental race then the original Gaians.
There is also a handy faction editing tool included.