In the Fourth Era, a rebellion lead by Jarl Ulfric Stormcloak went to war against the Imperial powers within Skyrim, resulting in what is now called the Skyrim Civil War. After several large scale invasions of the different cities in the cold North, the Stormcloaks emerged victorious, as the Empire withdrew most of it military under the command of a new general and a new emperor.
Eventually, however, the Empire tried to reenter Skyrim, and the Stormcloaks reluctantly gave up half of Skyrim to the Empire. The small rebellion, nowadays, is a recognised nation, controlling the Eastern half of Skyrim, and it is not inconceivable that they would try to take over more land in the future. One thing is for sure, no Thalmor agents have managed to remain in Eastern Skyrim for a very long time.
