Its not. Only the south of the province was severely affected by the war with the Dominion. Sentinel and its northern (and generally more important) cities were not severely affected. In the 20 years since, much has likely been rebuilt.
Also fun fact about Ulfric: He's fighting for Talos worship because he was promised it by the Empire. They said he'd be able to worship freely and openly, and soon after the Markarth incident (what he had to do to get what he was promised) he was arrested by the Thalmor.
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Don't ever post again if you're just going to spew completely false nonsense.
Speaking of false nonsense, you seem to be the first person I've heard suggesting that rebuilding a war-battered land is easily done in 20 years or that nothern Hammerfell is totally intact after five years of warfare (check how well France came out of WW1 please), or that Ulfric is actually fighting for Talos worship rather than the title of High King. Would you mind providing any sources on this?
Except hes not. Using that idiotic logic, General Tullius is a Thalmor asset.
Idiot logic, eh?
The Thalmor need Talos worship to stop for what is essentially religious reasons and that effectively means they'll have to eventually take over mainland Tamriel. The biggest player in that area is the Empire. There are smaller players around but the big player is the Empire. Not Skyrim as a country, not Hammerfell, and certainly not Black Marsh (since the Argonians aren't particularly imperialistic) or Morrowind (since it got plot nuked to hell). Thus they need to get rid of the Empire to stand a chance of achieving their strategic goals. They can achieve NOTHING as long as the Empire can walk the Legion in as a backstabbing relief force if and when they feel like it.
Since the Empire is the main enemy, anything that weakens the main enemy is a positive and anything that strengthens the main enemy is a negative. It really is that simple and binary. Tulius is giving the Stormcloaks a beating, which means they're not serving their purpose of weakening the Empire. Tulius is thus effectively making the Empire stronger, arguably at the cost of an irrelevant country that won't be a main target for Thalmor for many, many years. You've said it yourself, the Thalmor can't invade Skyrim. It's literally impossible until they have Hammerfell and Cyrodiil. It goes both ways, of course. Skyrim can do even less to hurt the Thalmor than the Empire can.
If Skyrim pulls out of the Empire then that leaves the main enemy, the Empire, much weaker than before. That's good for Thalmor and you have to be a right fool to think otherwise. On the other hand, you also have to be a fool to think Tulius leaving Skyrim "weakened" means anything positive for the Thalmor, because they can't do anything to Skyrim anyway. Thus Tulius will never be an asset for the Thalmor but Ulfric definitely is. Tulius is not helping their ultimate goal of defeating the continental humans but Ulfric definitely is.
You may think their goal is hopelessly unrealistic, which is arguable but still not something that has slowed them down thus far, but you have to agree that wrecking the biggest obstacle in their way is not something that makes their plans more unrealistic than it currently is.