The Stormcloaks will prevail as long as there is a Dragonborn who can call upon the help of the Dragons and walks in the shoes of Ysgramor bringing Wuuthrad into battle for the glory of all Nords!
Also take into account that Skyrim is cold very cold indeed now think about the germans/french and Stalingrad/Moscow then think about the Mage's Guild and their power to summon ice storms and blizzards...Think about the Dark Brotherhood and poisons and Thalmor supplies...You get the drill...
Agreed! All excellent points!

I thought about what happened in Russia too. And consider in our current story compared to past human history, there is even less difference in cold adaptation between germans and russians, than there is between cold adaptation between the thalmor and nords. The thalmor will freeze their tails off, maybe even worse than the imperials! hahaha
Besides, those invaders do not really want to live in the cold of Skyrim, they only want it to prevent their other enemies from having it tactically. A free Skyrim might require more expenditure than they are willing to expend just for that, if they knew Skyrim was back under a real Nord leader who would fight like heck against any further attempts at invasion and not the Empire's puppets or anybody else's anymore. The Stormcloaks will beat the Imperials much more easily than people think and the subsequent stronger Skyrim will be united and no longer divided by who is for Imperial Puppets and who is for Skyrim. I think the Civil War will make them stronger than ever, not weakened, as long as the Empire does not kill Ulfric and then put another puppet in as High King somehow. But even if Ulfric lives long enough for the Stormcloaks to free Skyrim and then long enough to make sure the Nord Moot is able to pick true High Kings again instead of puppets for invaders, Skyrim can stay strong.
And i also agree with u that the Dragonborn is a huge asset to the Nords. And since when is somebody racist (as some say of Ulfric instead of the Imperials or others) because the Nords and Ulfric are from the place they live in and resent invaders taking over? U do not think the invaders are racist for having the arrogance to push Nords around in their own homeland and act like it doesn't matter because they deem themselves 'less Barbarian than the Nords who evolved on that land since ancient times and did not turn themselves into something as wretched as some who speak ill of them? (Does that mean the Native Americans were the racist ones because they eventually resented the way they were treated by foreigners coming into their land treating them poorly and taking over the land to their demise? I think not! ) The Nords are no more racist than any of the other races, and probably less than most, but they do not like being stepped on either. So some NPC said this or that against the Nords or Ulfric, yes but some NPC have their own different agenda other than caring about Skyrim's freedom and some really have been Imperial Spies and Thalmor spies, Ulfric has good reason to fear that, so as not to walk into any more traps where everybody there, Stormcloak or not, will still be beheaded! . But If u tell Ulfric u want to fight for Skyrim because it is your home now, he will treat u as an equal to a Nord in his Stormcloaks regarrdless of your race. That's not racist. But if u go around acting like everything is his fault because he wants a free Skyrim, everything from the Thalmor to the Forsworn's wretched behavoir including canabalism and that those supporting their propaganda that they are all just innocent victims of Ulfric or the Nords of Skyrim, are supposedly more reliable witnesses of who he is than the truth, then of course u will get the cold shoulder or be suspected as a spy. LOL So what DOES bring u to Skyrim in that case anyway? Maybe it would be a fair question, and not racist one afterall.

LOL Of course some will say they are just there to be a thief and it does not really matter and others will say they are there to support the Imperials and some will join the Stormcloaks, Some will even play one way, then try another. Maybe which ever side u are on when u play the Dragonborn will always win!

That is probably the real answer. LOL
Can the Stormcloaks defend Skyrim? Possibly. As a the result of being freed can the Nords (now more united) defend Skyrim? Yes, they sure could.