The Battle of the Blackwater was the greatest clash of the War of the Five Kings, and the nearest King's Landing came to falling. Lord Stannis Baratheon, having crushed his brother Renly by shadow and sorcery and won the greater part of the Baratheon and Stormlands strength to his banners, sailed up the Blackwater Rush with a mighty host and a fleet to match, meaning to break the city walls before Lord Tywin Lannister could return from the west. Against him stood only the boy king Joffrey's household and the defense contrived by the acting Hand, Tyrion Lannister, who had labored in secret with the pyromancers of the Guild of Alchemists and the smiths of the city to ready a trap upon the water.
As Stannis's warships crowded into the mouth of the river, Tyrion loosed a single ship laden with wildfire and put the whole fleet to the torch; the jade-green flames leapt from hull to hull until the Blackwater itself seemed to burn, drowning and burning thousands, among them the ships and men of Ser Imry Florent. A great chain, raised across the river's mouth once the enemy had passed, sealed the survivors within the inferno so they could neither advance nor flee. Yet Stannis had landed the bulk of his army on the north bank, and these pressed the assault upon the walls with ladders and rams, Stannis himself in the thick of it; the Hound Sandor Clegane, unmanned by the fires, quit the field, and even Tyrion's own sally through a postern was cut down until he fell wounded. The city all but broke.
The day was saved not upon the walls but behind them. Lord Tywin Lannister had made common cause with House Tyrell of Highgarden, wedding their strength to his by promise of marriage between King Joffrey and the Lady Margaery, and the two hosts marched hard from the west. They fell upon Stannis's landed army from the rear at the height of the storming, the van led by Ser Garlan Tyrell arrayed in the fallen Renly's green armor, so that panicked men swore the dead king had risen to smite them. Caught between the burning river and the fresh host at their backs, Stannis's soldiers broke and were slaughtered or scattered; the pretender himself was carried from the field against his will. King's Landing held, the Lannister and Tyrell alliance was forged in blood and fire, and Lord Tywin rode into the city as its savior and Hand of the King.