Rohan, Gondor, Isengard and Mordor. Duh.
And somehow you'll be able to call on Elves, Ents and other stuff for help. Initially I heard a rumor of no multiplayer ability, but that seemed a little bit of a stretch for an RTS. And with this franchise nonetheless. It's easy bank. But they are tweaking the multiplayer aspect at the moment. (Read that in an intv. somewhere with the producer).
appears to be the Witch-King of Angmar vs. Theoden on the fields of Pelennor! (though it's more like a 1 vs. 1, but you get the idea.)