The latest issue of GamePro features a 10-page world exclusive preview of Halo Wars. Among other things, the article has revealed that the game will support up to six players on Xbox Live for versus multiplayer and there is also an online campaign cooperative mode that enables two players to to team up against the Covenant.
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I have to say, as a fan of RTS games, that this looks to be a complete and epic failure. The graphics look shoddy, and I don't really think that the storyline will translate well from the FPS to an RTS. But, time will tell.