David I had a personal stake in the English civil war that historians now call the Anarchy. His niece, the Empress Matilda, had been promised the throne of England by her father Henry I. When Henry died in 1135, his nephew Stephen of Blois seized it instead. David, who was Matilda's maternal uncle and one of her most powerful supporters, responded by invading northern England. The first campaign in 1136 secured Cumberland for Scotland. A treaty followed, then broken peace, then more raids. By 1138 David was crossing the Tweed yet again, this time coordinating his invasion with rebellions in southern England by Matilda's other allies. Wark Castle, perched on its motte above the river, lay directly in his path and had already been besieged once that January. The garrison there knew David would be back.

What turned a passing nuisance into a six-month siege was a raid out of Wark itself. As David's army moved through Northumberland in May 1138, the English garrison slipped out behind enemy lines, ambushed a Scottish supply column, and took hostages for ransom. The chronicler Richard of Hexham, writing closest to the events, called it the only such raid by a castle garrison during the entire Scottish invasion. David turned his army around. The first assault was sharp and bloody, with battering rams and siege engines hammering the walls and high Scottish casualties for little gain. Frustrated, David left two barons in command of a blockade and marched south to seek a decisive battle. He found one at Northallerton on 22 August, where Archbishop Thurstan of York rallied an English army and stopped him cold. The Battle of the Standard was a Scottish defeat, but Wark kept holding.

By late September, while David regrouped at Carlisle, a papal legate named Alberic of Ostia arrived to broker peace. Both sides agreed: the Scots would return the women they had carried off in the raiding, and there would be no more attacks on England before 11 November 1139. There was just one exception. Wark Castle was specifically written out of the truce. David could go on starving it. He did. When he returned to the siege, the townspeople of Wark joined the defenders in a desperate sortie that destroyed much of the Scottish siege equipment. But siege engines could be replaced. Food could not. By November the abbot of Rievaulx negotiated the surrender. The defenders walked out with weapons and honour. David promptly ordered the castle pulled down.

Defeated in battle but holding the chess pieces of the borderlands, David walked away from 1138 with a strong hand. Wark, Norham, and Alnwick castles were all his. When the Treaty of Durham was signed on 9 April 1139, Stephen ceded Northumberland to David outright. For the next fourteen years David ruled both Northumberland and Cumberland as part of Scotland, governing from Carlisle, minting Scotland's first native silver coinage from the mines at Alston, and using the new revenue to underwrite his famous programme of monastery foundations. He died at Carlisle in May 1153. Within four years of his death, with a twelve-year-old grandson on the throne in Scotland, both provinces were lost again, beginning a century of Anglo-Scottish struggle to reclaim them.

The castle David ordered destroyed in 1138 was rebuilt, fought over, and destroyed again across the centuries that followed. What survives at Wark on Tweed today is a low mound and scattered masonry on the south bank of the river, with England on one side and Scotland on the other. The Tweed still marks the border here, much as it did when starving Englishmen surrendered to a Scottish king who had decided their fate from a hundred miles away.