Following an unchallenged invasion of Scotland by Æthelstan in 934, possibly launched because Constantine had violated a peace treaty, it became apparent that Æthelstan could be defeated only by an alliance of his enemies. Olaf led Constantine and Owen in the alliance. In August 937 Olaf and his army sailed from Dublin to join forces with Constantine and Owen, but the invaders were routed in the battle against Æthelstan. The poem "Battle of Brunanburh" in the "Anglo-Saxon Chronicle" recounts that there were "never yet as many people killed before this with sword's edge ... since the east Angles and Saxons came up over the broad sea".