» Wed Dec 14, 2011 9:12 pm
This is actually pretty complicated, so I'll do my best to hit all the main points.
All of Tamriel was under Cyrodilic control prior to the Oblivion crisis. Think of the Imperials as sort of like the Roman Empire, and Tamriel as most of Europe. The Imperials largely didn't install Imperial governors, however, letting most of the provinces have a large degree of self-rule, as long as they followed Imperial laws, paid Imperial taxes, etc. Thus, the Jarls of Skyrim remained Jarls, just Jarls loyal to the Empire. Being part of the Empire was, by and large, good for everybody - it was peace and prosperity for all, so nobody really had any issues with the status quo.
The Oblivion crisis severely weakened the Empire, and also killed off the "true" line of Emperors, the Septims. Meanwhile, the Oblivion crisis also allowed the Thalmor to come to power on Summerset Isle, home of the High Elves. The Thalmor are, essentially, elven supremacists, and believe Tamriel should (once again) be under elven rule. They were able to annex Valenwood after a pro-Aldmeri coup, and were also able to take Elswyr for reasons I won't go into, that have a lot to do with the Khajit's relationship to Nirn's moons.
Then they essentially strolled up to the Imperials with a list of demands, among them the end of Talos worship throughout the Empire, the ceding of southern Hammerfall (home of the Redguards) to the Aldmeri Dominion, and other things of that sort. They knew the Empire wouldn't agree to the demands, and they knew they could take the Empire in a war.
The Imperials, predictably, did not accept the demands, and so the Aldmeri Dominion invaded Cyrodil and Hammerfall with two separate armies. Hammerfall was the real target, aiming to capture the southern half of the province that they demanded from the Imperials, but when the diversionary army invading Cyrodil was able to make it all the way to the gates of the Imperial City, they concluded that the Empire was weaker than they thought, and that they could actually conquer Cyrodil right off the bat, without needing to take out all of its supporting provinces first. Thus, they shifted their focus to the siege of the Imperial City, concentrating most of their forces in Tamriel there. The Empire did the same, more or less abandoning Hammerfall and the Redguards.
The Aldmeri were successful in capturing the Imperial City, but not the Emperor, who fought his way out with at least one of the Legions. Together with the Legion returning from Hammerfall and a Legion composed mostly of Nords coming down from Skyrim, the Emperor was able to encircle the Aldmeri army and destroy it, wiping out the vast majority of Aldmeri military power in Tamriel. This is what obliged the Aldmeri Dominion to sign the White-Gold Concordat; they got everything they demanded prior to the start of the war, including the ceding of the southern half of Hammerfall and the ban on Talos worship. The Empire signed it because they needed breathing room to recover their strength; they lost over half their Legions in the war, and a fresh Aldmeri invasion - which the elves were fully capable of - would in all likelihood completely overrun Tamriel. The Redguards of Hammerfall outright refused the terms of the Concordat, vowing to keep fighting, which forced the Empire to relinquish its claim on the province; if they kept it, the Redguards' actions would be a violation of the Concordat, giving the Dominion a perfect excuse to invade all over again. The Redguards view this as total abandonment by the Empire, and are understandably upset about it - especially as they fought the Thalmor for five more years afterwards, finally managing to battle them to a standstill and an ultimate truce.
So, that's where we stand as Skyrim starts. If the Empire hopes to win against the all-but-guaranteed next war against the Aldmeri Dominion, it needs the Nords' military strength to do it. The Nords who support the Stormcloaks view the Empire as a dying beast, one which needs the Nords far more than the Nords need it - needing their treasure, their manpower, their trade, but providing little in return anymore, obliging them to give up their traditional religion and allow hated elves to move freely amongst them, kidnapping and torturing their populace. They point to Hammerfall as an example of a province being capable of standing on its own against the Aldmeri. The people who support the Empire, meanwhile, believes that a recovered Empire still provides the best chance to eventually defeat the Thalmor, but that the Empire can't recover if it loses Skyrim.