The Siege of Whiterun was a Stormcloak attack on the city of Whiterun.
A few weeks before the battle, both an Imperial and Stormcloak messenger arrived at Dragonsreach. The Imperial came with a letter from General Tullius, while the Stormcloak came with Ulfric Stormcloak's axe. If Jarl Balgruuf kept Ulfric's axe, it meant that he had sided with the Stormcloaks; however, if he returned the weapon, it meant that he had turned it down. Balgruuf decided he would tell them soon. Two days later, the decision was made: the Jarl told the Stormcloak courier to return Ulfric's "damned axe", and sent the Imperial messenger to Tullius, accepting his offer.
After word reached Tullius, Legate Silian Secunia was sent to Whiterun with his centuria and to inform Balgruuf that General Tullius had made Silian the captain of the guard, and to discharge Caius immediately.
In the early nightfall, Stormcloak catapults launched projectiles over Whiterun's walls, causing panic and confusion. Legate Secunia rallied his men and the Whiterun Guard outside of the city, but within the walls, where they attempted to defend the gates.
The fighting continued until halfway through the night, while the Stormcloaks were run through by the dozens. Finally, a group of rebels fought their way through Imperial defenses, and opened the drawbridge. Fighting continued for hours in the streets leading to Dragonsreach. The Dragonborn busted in the doors of Dragonsreach with Ralof and Galmar Stone-Fist, and became engaged in a fight with the remaining city guards, Irileth the Housecarl, and Balgruuf himself.