Stannis Baratheon came within a wall's breadth of the Iron Throne, and the river itself caught fire. The wildfire trap, the chain across the Blackwater, the sortie that saved the city, and the relief that stole the credit.
What the realm knows
Of the great battles of the War of the Five Kings, none is remembered so vividly as the fight for King's Landing upon the Blackwater Rush. The realm knows its broad course: Stannis Baratheon, the dead king's brother and the sternest claimant to the throne, brought a great host and a greater fleet up the river to take the capital from the boy Joffrey. The city seemed all but lost. Then the river itself caught fire, a Lannister host fell upon Stannis's rear, and the storm king's cause was broken in a single night.
This the chronicle may frame as any war is framed, in the manner of a battle set down after the fact. Yet the particulars belong to the pages of the second book, and are spoiler-heavy for those who walk the tale by the screen's slower road — the trap, the chain, the sortie, and the treachery that scarred the man who saved the city. These the chronicle veils, and tells below as the war-history they are.
The full account
The wildfire trap
The city's chief defender was not a soldier but the least likely of the Lannisters: Tyrion the Imp, serving as Hand of the King in his father's absence, who understood that King's Landing could not be held by walls and swords alone against Stannis's numbers. His answer was fire. The Alchemists' Guild, keepers of the old pyromancers' art, had been set for months to brewing wildfire — the green substance that burns hotter than any natural flame and cannot easily be quenched — until thousands of clay jars of it stood ready in the vaults.
When Stannis's war-fleet crowded into the mouth of the Blackwater, a single ship crewed by none but a condemned man was sent drifting among them, streaming wildfire into the water; a fire arrow set the whole river ablaze. The green flame leapt from hull to hull and turned the crowded estuary into an inferno. Whole squadrons burned at their oars, and the screams and the eerie green light of that burning were seen and heard across the whole city, so that the smallfolk thought the world itself had caught fire.
The chain
The wildfire alone would not have been enough, for a burning fleet may still land its men and flee its ruined ships. The second half of Tyrion's trap was a great chain, forged link by link and strung in secret across the mouth of the Blackwater beneath the waterline. Stannis's fleet was suffered to sail up the river over the slack chain unopposed; only when the ships were well past was the far end winched up, sealing the river's mouth behind them.
So the fleet was caught in a killing-box of its own making — fire before and above, a chain and the open sea behind, and the burning wrecks of its own vanguard fouling any retreat. Ships that fled the flames ran onto the chain; ships that turned collided in the press. It was a stratagem worthy of the histories, and it cost Stannis the better part of his strength on the water before a single wall had been assailed. The river ran with wreckage and drowned men, as such rivers do.
Tyrion's sortie
Fire and chain broke the fleet, but Stannis had landed men enough to storm the walls, and for a long hour the city's fate hung on the courage of its defenders. That courage nearly failed. The boy king Joffrey, having played at valour, was withdrawn from the walls at his mother's command, and the garrison's heart went with him; the men began to waver as the battering rams bit at the Mud Gate.
It was the Imp who held them. Tyrion Lannister, no warrior by build or temper, put himself at the head of a sally and led the wavering defenders out a postern to fall upon the attackers from the flank, buying with his own body the time the city needed. In the confusion of that fight he was struck down by treachery from his own side — a knife meant to end him under cover of the battle, his face laid open near to the bone — and lived only by narrow chance. The city's salvation and its saviour's ruin were struck in the same hour.
The relief, and the ledger
The blow that broke Stannis came not from the city but from behind it. Tywin Lannister had marched hard from the west, and had made a secret pact with House Tyrell of Highgarden, whose strength had until then sat idle. Onto the field at the crucial hour came the combined host of Lannister and Tyrell — with, by a stroke of grim theatre, a knight arrayed in the fallen King Renly's own armour at its head, to put fear into the storm king's men who had once cheered that armour's true wearer. Caught between the walls and this fresh host, Stannis's army broke and fled to the ships that fire had left them.
The victory belonged, in the ledger, to those who arrived last. Tywin took the glory and the office of Hand; the Tyrells took the marriage that bound Highgarden to the throne, having spent their swords only once the winning side was plain. Tyrion, who had conceived the wildfire, forged the chain, and bled in the sortie, was quietly set aside, his part diminished, his wound his only reward. A maester notes the shape of it: battles are won by those who fight them and remembered for those who claim them, and the Blackwater is a fair schooling in the difference.
The Blackwater saved the boy Joffrey's throne and remade the war. Stannis Baratheon, who had come within a wall's breadth of the Iron Throne, fled broken to Dragonstone with a shadow of his host, and turned thereafter from open war toward the red priestess at his side and, in time, toward the Wall and a colder war entirely. The Lannister-Tyrell alliance sealed on the battlefield was crowned with a wedding — Joffrey to Margaery Tyrell — and the crown that had been all but toppled stood, for a season, secure.
For the man who had saved the city, the reckoning was bitter: cast aside, disfigured, and denied his due, Tyrion emerged from his triumph with less than he began. And the wildfire that won the day left its own long shadow, for a city that keeps caches of green flame in its cellars has armed its future as surely as its present. The smallfolk who watched the river burn green did not soon forget it; nor, the later pages suggest, should they have.
The World of Ice & Fire — The War of the Five Kings
Follow the threads
What was the Battle of the Blackwater?
It was the great battle of the War of the Five Kings for King's Landing, fought on and beside the Blackwater Rush. Stannis Baratheon brought a huge host and fleet to take the capital from the boy king Joffrey; the Lannister defence, directed by Tyrion as acting Hand, destroyed the fleet with wildfire and a chain, and a Lannister-Tyrell host broke Stannis's army.
How did the wildfire and the chain work?
Tyrion had the Alchemists' Guild brew thousands of jars of wildfire — green flame that burns hotter than any natural fire. When Stannis's fleet crowded into the river mouth, a fire ship set the water ablaze. A great chain, strung in secret beneath the waterline, was then raised behind the fleet, sealing it between fire and the open sea.
Who won the Battle of the Blackwater?
The crown held the city, but the victory belonged in the ledger to those who arrived last. Tyrion conceived the wildfire, forged the chain, and led the sortie that held the walls, only to be struck down by treachery from his own side. Tywin Lannister and the Tyrells arrived at the crucial hour, broke Stannis, and took the glory — and the marriage that bound Highgarden to the throne.
How does the book differ from the show?
The books make the great chain a full half of Tyrion's trap — the fleet lured past it, then sealed in — while the screen leaned almost wholly on the wildfire and let the chain fall away. Readers should credit the Imp with both halves; it is the marriage of fire and chain, not the fire alone, that turns a desperate defence into a slaughter.