The theory in advanced in my idea for ES VI is that it's set in Summerset, and no one's really quite sure who won (the treaty, reunification, and liberation are all theories). And it doesn't matter.
If Ulfric won, then after a period of mutual sulking in which Skyrim went hungry and Cyrodiil went without timber and fur, the two provinces reestablished trade. Skyrim managed its own affairs, Cyrodiil became complacent and corrupt, and eventually fell into near-anarchy. The Aldmeri ultimately took it over.
If the Empire won, or if a treaty was made, incompetent leadership caused Cyrodiil to descend into turmoil. Skyrim and High Rock had to manage their own affairs as best they could. The Aldmeri ultimately took over Cyrodiil.
Either way, they overran Morrowind and High Rock easily, separating Hammerfell, Skyrim, and Black Marsh. Hammerfell fought them for a long time, but without a source of food (you can't grow crops in a desert), they were overcome by famine and had to surrender. Black Marsh fared better, proving a logistical nightmare for the Aldmeri. And the Skyrim-Orsinum alliance was the worst of all. However, the Dominion ultimately owns Tamriel. And that's where ES VI begins...