User blog comment:Thedoucheinthenorth/The Civil War - Why it is great/@comment-1600847-20121031011532/@comment-5135631-20121102151501

It never states that he worked for them knowingly. He wouldn't have just signed up to be a Thalmor agent, if he was an "asset" it would be because he did something against the Empire in the past.

Anyway, Ulfric is only one reason why I joined the Stormcloaks. The more important reasons are why Skyrim will be better off on it's own.