These events are what lead to the civil war. The rebels consider Ulfric the winner of a duel and rightful king. Ulfric killed the High King in combat by using shouts and was branded a murderer by the Empire. The Hing King of Skyrim, appointed by the Empire to oversee Skyrim, was challenged in combat for position by Ulfric, Jarl of Windhelm and leader of the Stormcloak Rebellion.
This lead to a lot of resentment towards the Empire from the Nords who wanted Skyrim to be free from the Empire's control and free from the restrictions imposed by their surrender to the Thalmor.
This lead to the ban of worship of Talos, the God that the Nords worship. The Empire lost a war against the Thalmor (Elves) and as part of their surrender the Empire had to agree to Thalmor demands, the White-Gold Concordat. Skyrim was a Nordic region of Tamriel under the control of the Empire. There is a Civil War waging in Skyrim between the Imperials and the Stormcloaks. You're thrust in to the middle of a civil war waging in Skyrim between the Empire and the Stormcloak rebellion. If you've ever played Skyrim then you've woken up bound in the back of an Imperial cart on the way to the executioners block alongside Stormcloak soldiers.
