Hammerfell actually isn't under siege anymore. They signed a treaty, and the Dominion pulled out. That doesn't mean the Thalmor isn't doing what they do best though, sabotage and subterfuge. However we don't have anything that says that's the case.
The Thalmor are so powerful because they're the Thalmor. Sabotage and Subterfuge. The Dominion is their arm, and they're the brain.
True. I just see them as a vile thing. I mean, I support the Empire since I can see their plight, and why Mede signed the WGC, but anytime I kill a Thalmor, and doing Diplomatic Immunity, it gives me so much satisfaction. The Dominion is the worst type of people. They basically assaulted Hammerfell because they refused to bow to them. Kind of makes me baffles how any non-Altmer can sympathise with the Thalmor's agenda. But I suppose that anyone with a political idea is bound attract one follower.
@Titan- The problem is, I doubt the Empire could align with Skyrim, given their outspoken views on the Thalmor, where as the Empire is trying to bide its time and resources for the inevitable Great War ll.