User blog comment:JoshofBlades/Give the Empire a Chance./@comment-4098609-20111209132236/@comment-71.41.149.55-20111209163615

There is a book in the Thalmor Embassy that explains that Ulfric is a Thalmor agent, and was tricked into thinking his information was helping in the conquest of the Imperial city. Thus, he is also a traitor to the Empire since he was part of the Legion at the time.