January 30, 2014 7:27 PM EST
I don't think the Empire can do it with Skyrim and Hammerfell. There are a few simple facts that has to be brought up in a question like this, starting with the Mede's being repeatedly warned that the Aldmeri Dominion potentially posed a threat to the Empire. Both the Blades and the Penitus Oculatus warned the Medes in the course of the 150 years before the war broke out.
Despite this, the Empire was completely unprepared for the Great War, and suffered great losses throughout the entire conflict. Only by, as it does seem from Hammerfell being abandoned and the Forsworn taking control of Markarth again, gathering virtually all forces available and marching them down and surrounding the Imperial City, taking the Dominion invasion army by surprise, was Titus Mede II able to turn a complete loss to just a major loss, with the signing of the WGC. In the process of this, more than half of the Legion was put out of commission.
Turning it around, Summerset Isles is one of the most difficult nations to invade in all of Tamriel, so much so that the only time it has been conquered from the outside is by Tiber Septim when he unleashed a giant time-shattering stompy robot powered by the soul of a uber-powered undead Nord king. A robot the Empire doesn't have control over anymore, mind you.
Best the Empire could do, and even that is questionable, is to drive the Thalmor of the mainland. However, any further effort to attack the Dominion would ultimately be futile. Summerset Isles is as close to an everlasting fortress as you get.
And now, looking at the actual situation the Empire is in, they have lost Hammerfell and might lose Skyrim. And they have the Thalmor being given freedom to operate with official sanctions from the Imperial Government. A government whose history is such that they are only ever able to keep the Empire going efficiently in the times of a strong Emperor. And the Medes don't seem to be very strong Emperors, and their whole ground for being on the throne to begin with was because the High Chancellor of the time thought Titus Mede I to be a fitting puppet.
Add in that the Empire is so set in its ways that it cannot even make itself let go of a province it can't even send a proper Legion to keep when the local Jarls start to openly rebel against them. Instead of cutting their losses and move on, they have to waste what littl resources they might possibly be able to spare on a fruitless stalemate that leads nowhere until a demi-god intervenes and decides who wins.
Add in the most recent development of the leadership of the Empire(Dark Brotherhood ending), and that even generals of the Imperial Legion has to be careful not to offend the Thalmor(Tullius sometimes appear in the quest Diplomatic Immunity), and you really have a problem on your hands.
Then look at the general history of the 4th Era: The human nations become weaker, while the Dominion becomes stronger. The tide has shifted. Without the Covenant with Akatosh, and now the banning of Talos, the Empire is weakening their own mythic strength more and more. And all the while the Thalmor continue to push the Aldmeri forces against them.
In short, the Thalmor beat the Imperial Bureaucracy once already. Allowing more time to pass and giving them more influence and official powers to work with is playing the same game as before. And as someone very smart once made clear to us: The definition of madness is when people continue to do the same thing, over and over, expecting a different outcome.
