The execution part in the beginning is all because of just one person really, so do not place your judgement of the Imperial Legion so quickly.

Ulfric cares about humans. He doesn't care about other races being in Skyrim, merely that Nords deserve Skyrim land. How can you say that isn't true? Skyrim is originally Nord land and does anybody have the right to take that away from them. That's like saying Germans deserve to control Germany. Or French people deserve to control France. Well of course! Although Skyrim was conquered during Tiber Septim reign, TALOS HIMSELF WAS A NORD.
Although the Empire afterwards was ruled by Imperials, it was always an Emperor with some Dragon blood in him to control it. But after Uriel Septim, Mede controls the Empire! It would be fine but he signed over the Empire to the Thalmor! The Battle of Red Ring should've ended with a simple cease fire. Titus Mede II destroyed the Thalmor and although he suffered heavy losses, it was enough for a long cease fire.
And what about after the Stormcloaks win? The cold environment alone would provide hard for the Thalmor who have been in Summerset Isles or in Elsweyr and they'd have to go to Skyrim after the Skyrim branches are killed, which would happen very easily. Even after the Thalmor succeed in conquering the cold environment, Skyrim's landscape is a death trap for those who are foreign to the land.
Caves are everywhere and each city is built to stand against a siege. Will they stand? Perhaps not as demonstrated by the battle of Whiterun but to get one city, Thalmor would lose at least a hundred men to the guards and soldiers alone. There's no mention needed about the humans who will definitely raise arms against this foreign invader. Although the Stormcloak and Empire war divided the population, nobody is with the Thalmor.
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Really, the Empire's recovery will be slow and the Thalmor will thwart many attempts of a war as they're too involved in Empire politics. Will the Empire recover enough to kill the Thalmor. Yes. Will they do so quick enough? Perhaps not, perhaps so. Remember that Stormcloaks are hell bent on killing the Thalmor while the Empire...well they just want to recover from the Great War.


