The Greyjoy Rebellion was a brief civil war in the history of the Seven Kingdoms. It took place nine years before the beginning of the series and was an attempt by House Greyjoy of the Iron Islands to break away and secure independence from the King on the Iron Throne. Balon Greyjoy declared himself King of the Iron Islands and led the rebellion, supported by his vassals.
During the Greyjoy Rebellion, House Greyjoy secured several early victories, including the burning of the Lannister fleet at anchor in Lannisport in a raid led by Euron and Victarion Greyjoy. However, the superiority of numbers and resources saw the Iron Throne crush the rebellion. Theon Greyjoy remarked that the Iron Islands were outnumbered ten to one. During the final assault on the Greyjoy stronghold of Pyke, a battle-crazed warrior priest of R'hllor named Thoros of Myr led the way through a breach in the wall with his flaming sword, Ser Jorah Mormont of Bear Island not far behind him. Balon's two eldest sons, Rodrik and Maron, were both killed during the rebellion.
The rebellion was crushed and Balon Greyjoy was forced to surrender. Balon's two eldest sons died in the war. He was accepted back into the king's peace, but only on the condition that his last surviving son Theon be made a ward of House Stark as hostage for his good behavior. Robert's victory cemented his hold on the throne, after having overthrown the Targaryens a few years before.
Lord Eddard Stark, King Robert Baratheon, Lord Stannis Baratheon, Lord Tywin Lannister, Ser Jorah Mormont, Ser Jaime Lannister and Jory Cassel all fought in the Greyjoy Rebellion.
In the books
In the Song of Ice and Fire novels, the Greyjoy Rebellion took place five years after Robert Baratheon seized the Iron Throne and nine years before the events of the books. Lord Balon Greyjoy of the Iron Islands calculated that Robert's rule was as yet insecure and that many families would not go to war for a usurper. Balon declared independence from the control of the King on the Iron Throne. When Robert mobilized his armies, the ironborn launched several preemptive strikes, destroying the Lannister navy at anchor in the harbor of Lannisport and raiding the castle of Seagard, as well as many smaller attacks along the coast. However, Stannis Baratheon and Paxter Redwyne, with their fleets from Dragonstone, Storm's End and the Arbor, defeated the ironborn fleet in a significant battle off Fair Isle. A large army assaulted the Greyjoy castle of Pyke, with the red priest Thoros of Myr and a brave northman knight named Ser Jorah Mormont the first through the breach, both surviving to win high honor. Pyke was severely damaged and Lord Greyjoy's eldest two sons were killed before he finally surrendered and bent the knee. King Robert agreed to forgive Balon and restore him to the king's peace as long as his surviving son and heir Theon was given up to Lord Eddard Stark as hostage and ward.