Verified answer

The white gold concorduct that the elves made the imperials sign when they lost the great war, the imperials follow the elves and they told the imperials to stop believing in talos. Talos is a nordic god and it angered them having the imperials taking over their homeland and telling the not to believe in the hero god talos. You will also have to fight the elves (thalmor embassy) in the main storyline.

Their kings were brothers when they were young, but they discovered how one was adopted and the other not. So this petty feud escalated quickly into a great war between their cities.

And that was all crappy babbling.

Okay let's be serious here:

You can read the whol chunk of words here @

http://elderscrolls.wikia.com/wiki/Great_War

But if you simply want a very very very brief summary of it then:

A group of high elves called the Thalmor of the Aldmeri Dominion wants Skyrim.

So they fought a war called the Great War with the Empire of Tamriel.

Then the group won.

And took over Skyrim

And thus the Empire signed a contract called the White-Gold Concordat.

This contract bans Talos worship (a god or sth) and peace if people stop worshipping Talos.

Talos(known as timber Septim in his mortal previous life) is a very strong belief in the Nords of Skyrim.

So these groups(more like an army) of Nords, led by Ulfric Stormcloak, tried to rise and rebel against the Empire for accepting the contract.

Thus was formed the Stormcloaks.

So, in the brief-est summary, this civil war is thus actually Nords angry over their religious suppression. And tadaaa. That's it.

:D

The stormcloaks are rebeling because their god talos(which is kinda the equivilent of Jesus in our time) has been outlawed do to a treaty between the imperials and the thalmor elves because the thalmor don't believe a regular man could become a god. Also thalmor agents snatch up people who worship talks and the stormcloaks are pissed about that too. But the stormcloaks are very racist and only want nords in skyrim.Their leader Ulfric stormcloak kinda uses the talos banning leverage and he is more wanting power. I choose the imperials because my guy is a khajiit and the stormcloaks are racist to him. And also the stormcloaks didn't like the fact the empire signed a treaty with the empire. And don't let ulfric fool you because he fed info to the elves during the war

So basically, after the war, and the White Gold Concordat. The Nords wouldn't be able to pray to Talos, the most favourable and popular Nordic god. That's where the opposition started. After, Ulfric and his fellow soldiers fought back the Forsworn in Markarth since the Empire promised Talos worship if they defeat the Forsworn. Then the Aldmeri Dominion pressured the Empire to capture and place them in the prison. After he was released, Ulfric went to Windhelm and did his Jarl stuff. Later on, Ulfric challenged the High King to a battle for the throne. Ulfric killed the High King but others considered it a murder and they said Ulfric isn't the true High King. And this where the Dragonborn comes in. All this stuff I just said, you start from here, you see Ulfric and blah blah, dragon attacks and all the other stuff, and now what side you take, you end off the storyline :D

The imperials came to Skyrim and tried to take it from the nords. In retaliation, the stormcloak leader, Ulfric Stormcloak, killed the imperial High King of Skyrim.

On the surface the Stormcloaks want to be independent but the Imperials want to unite the continent, which is why the Stormcloaks are often referred to as "rebels".