User blog comment:Draevan13/Ulfric's suspicious Thalmor Dossier/@comment-5558140-20121209052441/@comment-1600847-20121210160715

The part that I wonder about is that the dossier says: "Contact was established, and he has proven his worth as an asset." That means they contacted him and he agreed to help them. I have a feeling Bethesda planned to make his story more detailed (the Civil War was supposedly the original Main Quest), but never got around to it. Here we have a dossier stating he willingly co-operated with the Thalmor, but his background isn't explored in great detail in-game, nor can you act on this dossier, i.e giving it to the Stormcloaks who then want to replace him as leader, or something along those lines.