The siege of Dumbarton was a successful four-month siege of the Brittonic fortress at Dumbarton Rock in 870, initiated by the Viking leaders Amlaíb, King of Dublin, and Ímar. Dumbarton was capital of the Kingdom of Alt Clut, the only surviving Brittonic kingdom outside of Wales. It represented a valuable target for the Viking invaders, who were likely motivated by strategic considerations, as well as loot. The attackers may have wished to remove Alt Clut as a maritime power, and the location and defensiveness of Dumbarton itself was of major value.