User blog comment:Johnsmith2962/Skyrim: New DLC idea/@comment-96.63.7.168-20120715023817/@comment-5284153-20120715050639

Actually Tullius seems the most likely candidate for a fight against the Aldmeri. During the campaign he basically says that the civil war is just a precursor to driving out the Dominion. Tullius may be a general, he just goes wherever the Empire tells him to go. If the Empire decides to go against the Dominion, they'll probably assign him to lead the troops.

Even though Hammerfell seceded, that does not make them enemies. If they have a common enemy, they can make an alliance.