Battle of Stone Mill

The Battle of Stone Mill is a major engagement in the War of the Five Kings. It is recounted by Edmure Tully in his report to King in the North Robb Stark in "Walk of Punishment", the third episode of Season 3. Chronologically, however, it happened before the Battle of the Blackwater, which happened in the ninth episode of Season 2.

Prelude
Anxious about just letting the Lannister forces in the Riverlands - commanded by Ser Gregor Clegane, "The Mountain that Rides" - ravage the countryside with impunity, Edmure impetuously decides to meet them head-on at Stone Mill.

The Battle
Tactically, the battle was an amazing success, inflicting disproportionate losses on the Lannisters and forcing them to retreat from the Red Fork of the Trident. Tywin's young nephews, Willem and Martyn Lannister, are taken hostage in the battle.

Aftermath
Strategically, the battle was a massive blunder, as the overall grand strategy was actually to lure the main Lannister army in the Riverlands back into the Westerlands, away from King's Landing. A direct result was that the Lannister army under Tywin was able to rush to the defense of the capital city in the Battle of the Blackwater.

The combined effect was that the entire past year of campaigning by King Robb was fruitless, and his invasion of the Westerlands was abandoned, as now if Tywin moved his army into the Westerlands to counter him, the Baratheons were no longer in a position to threaten the unguarded capita city.

In the books
In the A Song of Ice and Fire novels, the Lannister attempt to force a crossing of the Red Fork of the Trident River is actually a series of numerous simultaneous battles, which might more aptly be called a campaign across the entire "front" of the river. Ser Jason Mallister commanded the defense of four fords to the south of Riverrun, and Karyl Vance commanded the defense of fords to the north, while Edmure Tully commanded the defense of the fords closest to Riverrun itself. The fiercest assault came at Stone Mill in the center, where the forces commanded by Ser Gregor Clegane himself managed to gain a foothold on the west bank of the river despite suffering terrible losses, but were then driven back when Edmure hit them with his reserves.

The entire campaign was masterminded by Tywin, while Gregor Clegane personally commanded the main thrust at Stone Mill. The entire series of battles/campaign later becomes known simply as the "Battle of the Fords". The TV series simplified events, so that it was one big battle at Stone Mill - which is where the main thrust happened in the books - and it is officially named the "Battle of Stone Mill". Ser Gregor Clegane is stated to have been in direct command, though he was a prominent general in Tywin's overall Lannister army-group in the Riverlands. Also, in the books, the Battle of the Fords was an assault by Tywin's armies to attempt to cross the Red Fork at multiple points, thus Edmure fought defensively from a strong tactical position on the other side of the river. Though he was actually only supposed to give token defense and then let the Lannisters cross the river, he ended up repulsing them. In the TV series, Edmure says he seized the initiative to attack Gregor's army at Stone Mill, winning a decisive tactical victory. Either way, this made the Lannisters withdraw from their offensive posture and attempts to cross the Red Fork to halt Robb's invasion of the Westerlands. Unfortunately, this also meant that they were able to make a forced march to reach King's Landing in time to crush the Baratheon assault in the Battle of the Blackwater.