So some critiques on the people defending the empire, you make a lot of assumptions in two directions.
1) That Skyrim couldn't defend itself, with speculation that the Thalmor are stronger than they were when they were fighting with Hammerfell. - Pure speculation.
2) That the Aldmeri Dominion would outright attack Skyrim if it became an independent nation. - Pure speculation, WGC applies to the empire and no one else.
3) That an alliance between Hammerfell, Skyrim, and Cyrodiil would not happen. - Pure speculation.
4) That an alliance between Hammerfell and the Empire would be stronger than an alliance between Hammerfell, Skyrim, and Cyrodiil. Baseless.
Just pointing this out. People are using these non-lore backed assumptions as a means to justify the hunting of talos worshipers and thalmor boot licking.
Discuss.
1) I never said they couldn't defend themselves, but you'd be absurd not to see how weak Skyrim would be after a civil war. The Thalmor are not having a constant battle in their own backyard, they are gaining resources from the Empire the longer they sit on it. That's not exactly speculation. If they weren't gaining resources they wouldn't be there, they'd have no sustenance. That they actually write in their own documents that Ulfric is an asset because of the civil war, speaks volumes about the situation.
2) It would outright attack it just as much as it outright attacked in the first place. The war was brought to Tamriel, with the intent to occupy it. They were fought to a standstill, yet remain their presence and keep documents on progress. They are biding their time, and they say so themselves. A fragmented and weakened Skyrim is an open door.
3) I didn't say it couldn't happen, I just said there's no way to know. It would be speculation to say they would or they wouldn't. And having Hammerfell or not having Hammerfell is a fairly large risk because of the amount of help they would offer. You wouldn't want to base your entire campaign on the hopes that Hammerfell will join you, and then find out for some reason they won't. They very well could, and have no reason not to, as it stands, but that guess is not something a general would base his troops lives on. Assume you'll be doing it alone, if you get allies, great, if not, then you've already planned for that anyway.
4) What I'm saying is that an Alliance between Hammerfell, Skyrim and the Empire would be stronger, than Skyrim, or Skyrim and Hammerfell, and it would be. More troops, more resources, and more battle experience from the start. There's no way you can argue that if Ulfric used his energy and steered the Stormcloaks in an alliance with Hammerfell and the Empire, they wouldn't be much stronger than a solo Skyrim, or a Hammerfell with Stormcloaks. Stormcloaks do not equal the entirety of Skyrim, there are a lot of entire holds there that are Empire loyal. So you wouldn't have Skyrim and Hammerfell, you'd have Hammerfell and some of Skyrim. What you actually need is all of them.
And for some of this there is no lore base as it hasn't happened yet. None of us know what will actually happen with the Thalmor in the end, so it will all be guesswork anyway.