People need to realize that the Dominion is crap and always was crap. They'll never be able to conquer anything again, and worst case scenario they manage to defeat the Legion in Cyrodiil. They wouldn't be able to touch Skyrim if they tried.
People also need to realize that the Dominions fighting force purely comes out of Alinor and those few spots in Valenwood that actually and truly support the Thalmor. Elsweyr isn't going to go to war for them, Valenwood is on the brink of civil war, and Alinor couldn't hope to generate a properly sized army to take on the rest of Tamriel, very much less Skyrim itself, without several hundred years to do so. What this means is that the Dominion's army is and always was one of the worst on the continent. It only ever saw its initial victory in the Great War because it attacked before the Legion could know it was at a war, and split it six different ways, engaging in an indirect war that relied heavily on shock and awe. And while that may have been effective, that same army went on to be destroyed in its near entirety, and then sit in a direct ground war for five years against a single province that had no support beyond what was in its borders.
Without the support of other nations, skyrim would be lost to the thalmor. The only reason the empire signed the treaty was so that the thalmor did not destroy the empire completely, which would include all of skyrim , without the treaty, the thalmor would have marched on a weakened cyrodill and skyrim with ease.
Get a clue. The Dominions army was virtually wiped out in Cyrodiil, the rest of it was sitting in Hammerfel, where it got fought to a standstill within months and was kept there for 5 years afterwords until the Dominion gave up. They couldn't have marched on Cyrodiil if they tried. And Skyrim? HAHAHA. As if the Dominion could ever hope to successfully invade Skyrim.