Planetside 2 is an MMO FPS that basically is a war between three factions spread out over three continents or maps. The game is persistent, and the object is to control territory. Never, not even with Battlefield 2/3 etc have I found a more immersive war-game

You can pilot bombers/fighters/etc if that is your thing

drive or support heavy armor

or, be part of an infantry squad.

that said, this is the only game I've seen that will have full on 100+ vs 100+ fights over a base at a time. It really is quite massive...and fun.