A Civil War is raging in Skyrim during the dragon crisis, and the Dragonborn can play an integral part in it. You choose either the Imperial Legion or the Stormcloak army, and fight along with them to liberate Skyrim or keep it under Imperial control. Successful completion of the storyline will mean that your chosen faction has complete control of Skyrim. See the subpages for more details on each story arc (they are similar, but have a few differences).