This isn't a "Which one is better" or a "Stormcloak VS Empire" or "Ulfric VS Tullius" thread. But I can it being similar to such threads.
I want to have a calm and reasonable debate about the possibility of a war between the free people of Skyrim and the Aldmeri Dominion. And by that I mean realistic and intelligent points and arguments about why you think what you think.
Now we'll assume that Titus Mede II has been assassinated by the Dark Brotherhood, the Dominion knows this, so the Elder Council recalls Tullius and his men to Cyrodiil, and Ulfric is given free reign over Skyrim.
Personally I find it completely out of the question. Geographically speaking. Skyrim is far north while Alinor is far south. The only way to get there is:
A) By ship, sailing around Highrock and Hammerfell, or
Through Cyrodiil and Valenwood
And because of that I find it highly unlikely. It's as if Ancient China tried to invade Ancient Britain; it's just not reasonable nor is it worth it.
But let's assume they did get into a war, and somehow or another they managed to face one another. Who would win, or who has a better chance of winning? First let me debunk, or attempt to debunk some things.
1. In TES V: Skyrim, Thalmor beat Stormcloak NPC's...
First off I don't think we can go on that; I don't we should base Lore-related issues with in-game stats. Besides, I honestly doubt Skyrim's army is equipped with Stormcloak Cuirass', fur gauntlets/boots, and iron weapons. I am sure they would have steel armor and weapons, possibly some Stahlrim (?). I think Skyrim actually marks the first time Nords got a bonus to Light Armor whereas in the past they was associated with Heavy Armor. So get that out of your heads (unless you can make a reasonable argument about it).
2. If the Stormcloaks can't defeat the Empire, what makes you think they can defeat the Thalmor?
We don't know this. Remember, the Thalmor were pulling the strings in this conflict. For all we know the Stormcloaks may have crushed Tullius and his men, or vice versa, so I don't see this as a reasonable argument. Since the Thalmor were pulling the strings of the war, we can't know for sure.
3. Stormcloaks are a rebellion, not an army.
Assuming Skyrim was a free nation, I am sure more Nords in the land would join the new Skyrim nation. A lot of Nords are not necessarily Stormcloak-sympathizers, but they might be Skyrim-sympathizers. We don't know anythign for sure, so I don't this as a valid argument.
So now let's talk about some things.
The Nords of Skyrim have managed to push into Morrowind, Cyrodiil, and Highrock in the past, and fought Dunmer, Falmer, Breton, Imperials, and probably many more races (consider the Interregnum for example, where they practically fought the Dominion before) so their military capabilities are surely evident.
Next think about Ulfric. Sure, he's a puppet, but he knows how to lead. Ulfric is a capable and battle-tested veteran and he can use the Voice, so I'm pretty sure he'd manage well against the Thalmor - and don't forget he's fought them before.
So how good are the Nords? Well during the Battle of Red Ring:
Source: http://elderscrolls.wikia.com/wiki/Battle_of_the_Red_Ring
So a garrison of mostly Nords held their own against the Thalmor - a garrison from Bravil and Skingrad.
So that's it for me right now. I personally don't see a war ever happening, but if so that's what I think.