The Siege of King's Landing (300 AC) is an event that happened during the War of the Five Kings, and was a deciding victory in the conflict.
King Renly I Baratheon and his host advanced up the Kingsroad toward the capital city. Tywin Lannister and his army were encamped at Harrenhal, intent on moving west to drive the Rivermen from the Westerlands, when word arrived of the imminent siege. He immediately began to march eastward, hoping to relieve it. The Lannister soldiers currently engaged as garrison troops in taken Riverland castles abandoned them, returning to Lord Tywin for the final, major assault.
Mace Tyrell is left in charge of the siege, while Randyll Tarly, Renly Baratheon, Garlan Tyrell, and Loras Tyrell take fifty thousand men to destroy the Lannister army at Harrenhal. Overconfident in his own abilities Renly underestimates his foe, and sends Randyll Tarly after him with twenty thousand men while he and his companions wait with the thirty thousand strong reserve. The Battle of the God's Eye follows, and Lord Tywin routs the Baratheon-Tyrell host, then continues to follow them down the Kingsroad toward King's Landing, harrying them all the way.
Another Westerland host from the south, gathered at Deep Den, marched up the Kingsroad toward the capital, intent on joining with Tywin. When word arrives of the Battle of the God's Eye, their pace quickens, and they arrive just in time to join the battle.
The Battle for King's Landing
The Battle for King's Landing is often labeled as the bloodiest and largest in Westerosi history. Renly's shattered host rejoined the fifty thousand outside the gates of King's Landing, and they made their preparations to face Lord Tywin. The Lord of the Rock, seeing this as his best opportunity to break the siege, Tyrells, and Renly in one stroke, wagered upon his men's experience, discipline, and recent morale boost to carry the day. In addition, noted Reach commanders such as Randyll Tarly were absent from the field - most of the forces gathered were nominally under the control of Mace Tyrell, though his son Garlan took the lead.
When battle was joined, Tywin committed all he had into that first charge, hoping to break the inexperienced Reachmen early. In addition, he hoped for the garrison inside King's Landing to rally out, striking the Tyrells in the rear, while the ten thousand fresh troops from Deep Den struck them on the opposite flank. King's Landing, however, was weak from the siege, and dared not open the gates. Riots had broken out, and the Gold Cloaks were weakened, and the household guards of the Lannisters were sore set trying to defend the Red Keep from the desperate. Tywin's reinforcements from the south were cut apart by a cavalry charge led by Renly Baratheon himself, with Loras Tyrell on his right, and Lord Mathis Rowan on his left. The final blow came when Lord Tywin Lannister himself was felled, taking a crossbow bolt to the chest at close range.
The Westerland army broke and fled, much of the Reach giving chase. Inside the city, many of the defenders wept openly, some going as far as to throw themselves off the walls of the city in their despair. The Mountain Clansmen of Tyrion Lannister, however, were not so inclined. They forced their way into the gatehouse of the Lion's Gate, slaying the guards there and throwing it open for the Baratheon-Tyrell army. And so it was that the city fell, with half the army giving chase to the routed Lannisters, and the other pouring into the starving city, claiming it in the name of King Renly Baratheon.
After the prolonged siege and the terrible battle, the sack of King's Landing was inevitable. Though Renly did his best to curb the worst of it, all agreed the men could not be denied this. The first few nights were a terrible affront to gods and men - but then the food poured in, and the soldiers calmed and resumed their duty, and order returned to the city. King Renly was well loved by many already - and his public execution of Cersei Lannister only aided his image. Joffery "Baratheon" was already dead - found in his chambers, slain of unknown cause. Tommen and Myrcella were spared by the will of the King, but sent to live out their lives in exile in Essos, alongside their uncle, Tyrion.
With the Lannisters defeated and King's Landing taken, Renly Baratheon was the uncontested ruler of Westeros - below the Trident, at least. The Kingdom of the North was still independent, but the combination of the terrible loss they took fighting the West, and the predations of the Ironborn in the Reach, stripped him of much of his momentum. Hoping to consolidate his southern territories before pressing on the North, he allowed Mace Tyrell and his forces to return to the Reach, and end the threat of the Ironborn - perhaps for good.