While the young wolf carried his war into the westerlands, Lord Tywin Lannister broke camp at Harrenhal and marched west to bring him to battle, only to find the swollen Red Fork of the Trident barring his path. Ser Edmure Tully, left to hold Riverrun and the river line, gathered the strength of the riverlords and set them to guard every ford and crossing along the Red Fork. Time and again the Lannister van sought to force a passage; time and again the rivermen threw them back into the water. The fiercest fighting fell at the stone mill of the fords, where the monstrous Ser Gregor Clegane hurled his riders across the shallows three times and was three times driven off with grievous loss, so that some name the whole affair the Battle of the Stone Mill.
Edmure won a true victory upon the river, and the riverlords hailed him for it, for they had shielded their lands and their smallfolk from the Mountain's burning. Yet the triumph undid a subtler design. King Robb had meant to draw Tywin deep into the west, away from the Trident, and trusted that the old lion would sweep past a beaten Edmure to give chase. Instead the fords held, Tywin's advance was blunted and turned back east, and the great host that Robb had hoped to lead astray was free to march for King's Landing and the Blackwater. When the king learned of it he was sorely wroth with his uncle, for the battle had been bravely fought and yet had cost him the very stroke he had schemed to land.