King Orys II Baratheon was a man of well noted overindulgence. He often traveled across the Realm speaking and feasting with his Lords, but he ignored the Iron Islands due to his dislike of the ironborn. The prickly Greyjoy lord felt this to be an insult to himself and his House's honor, and his hatred seethed under the surface for years, eventually boiling over when King Orys refused an invitation to Pyke by telling Damron Greyjoy: "Not even to make the eight."
In the months of tension leading up to the war, Tyrius Lannister shipwrecked off the coast of Banefort and was brought to Pyke by Gwynesse Greyjoy. Damron believed that the Lord was conducting espionage and planned to execute him and start his rebellion in earnest, but Gwynesse convinced Lady Alannys Greyjoy to help the Lannister escape.
Greyjoy fleets began reaving and pillaging all along the western coast, going so far as to sail up the Mander in an attempt to breach Highgarden.
Merryk Greyjoy, heir to Pyke, was killed during a raid on Greywater Watch.
Ironically, Orys II Baratheon died before seeing a single battle in the ensuing war, falling to his death in a bay.
Smashing the Shields
In the first major battle of the rebellion, the warships of the newly crowned King Renly Baratheon bore down on the ironborn's position among the Shield Islands. Renly, who had foreseen the signs of the rebellion mounting during his father's reign, sailed the crown's ships to Oldtown where they were quickly able to join the fray.
The ironborn's position was a strong one, and the ensuing battle would have seen more royal bloodshed had it not been for the Harlaws who turned against their liege lord during the initial clash. The ensuing battle was a slaughter, and Durran Harlaw reportedly killed Lord Damron by pushing him into the sea.
The Battle for Lannisport
This battle is also referred to as the Battle of Casterly Rock. With their liege lord dead, the remaining ironborn turned north, sailing along the coastline to solidify their position once more. Instead, they found the Lannister fleet waiting at the Feastfires. Tyrius Lannister's ships, alongside the king's fleet, were able to surround the remaining southern ironborn longships and sink them in the bay. The ironborn who made it to shore were efficiently and mercilessly slaughtered and it was said that bodies washed up on the shores of Lannisport for weeks.
The Battle of Pyke
Renly, Tyrius, and the remaining Harlaw fleet turned north once more, set on crushing the last remaining pockets of resistance. Many were still unaware of their lord's death. Taking place on the shores of Pyke itself, this battle marked the end of the Greyjoy rebellion, and was led by King Renly Baratheon and Tyrius Lannister. The town of Lordsport was burned before the main attack on the keep itself was launched, during which Tyrius received an axe wound to the arm, bleeding to death from the injury. Despite his losses, the King continued on to the castle of Pyke, where he laid siege for five months before the new Greyjoy lord, Aeron Greyjoy, surrendered and once again bent the knee.
“Do you know what caused the Second Greyjoy Rebellion? Egos. Renly’s ego, Damron’s ego, men’s egos.” - Jeyne Lannister