Some food for thoughts on the great war/civil war;
1st High Rock and Skyrim would be very hard to invade, their geography and placement make them inconvenient targets for the Aldmeri Dominion, The Aldmeri Dominion seemed to do very well in fields and jungle, but would not have the alpine training for invading those provinces, Skyrim would only be accessible via either the Sea of Ghost or a number of mountain passes and High Rock is the most fortified province in the empire, they be able to hold Daggerfall, Anticlere, or Waywest, but the hilly interior with their liberally place strongholds and fortresses would be a costly nut to crack, the Thalmor may have ambitions in Skyrim and High Rock, but selling that to the Merific public would be difficult to say the least.
2nd Altmer Armies employ goblins as frontline cannon fodder and shock infantry, but these regiments are heavily reliant on Altmer officers that have expertise in handling the little mongrels', Khajiit and Bosmer tend to make for poor field soldiers and would be undermotivated even beyond their natural disposition due to thalmor political police interfering in their affairs. Skyrim would probably do better to engage in guerilla tactics in the western holds with their more flat temperate climates( Holding Markarth would actually just be a waste of men), while holding the elves up in the Pale and Mountains separating the Old Holds (funnily enough the Old Holds are the Holds that sided with Ulfric). The Dominion forces would have to either land in Morthal/Solitude or march up through Falkreath. Both routes make winter reinforcements impossible and are quite dangerous, Windhelms coast would be unreachable via the west and would have to be approached from the East, Morrowind, although I do not know if Morrowind (who many people in this thread seem to forget are not part of the Empire, right now it is just Cyrodiil, High Rock, and Skyrim) would give them port access meaning they would not even attempt it if not as Senchal Elsweyr would be the closest port, literally on the exact opposite side of the continent. (cut there for sake of brevity)
3rd The imperial Holds seem unenthusiastic about the (civil) war, Solitude included, with it being the norm that only the legionaries actually fight beyond their borders or in the field, comparing the various Holds Markarth is more interested in dealing with the uppity Bretons of the Reach the fighting the war with locals actually hold pro-Ulfric sympathies, Morthal/WinterHold (Much more Winterhold) is broke and lacking in both men and motivation, with both really just being part of the spheres of the leading Holds (Winterhold has literally nothing in it, and Morthal's occupied mine is owned by a Solitude Thane), Falkreath/Riften are uninterested in the war in general and just send enough to be at the table when the treaty is signed, Dawnstar/Windhelm have leadership that have completely desolated their holds with the amount of man power and gold poured into the war, they are the only mobilized holds, Solitude is actually a strange case, with Captain Advar training men, and them fighting on the front, but it seems Tulias is undermining Firebeard causing some tension and the hold largely just seems to be the central place the empire organizes their forces from, Whiterun will join the war and will largely spend its time in the war either rebuilding under Greymane or half assing it like Riften and Falkreath, given the circumstances, the stand still makes sense, though eventually moral issues for the imperials or exhaustion on the other side will bring the war to an end by 206 4E.
I am working on a script for a Skyrim video right now and it is already 20 pages long and I have barely scratched the surface.