Following the invasion of Scotland, it became apparent that Æthelstan could only be defeated by an allied force of his enemies. The leader of the alliance was Olaf Guthfrithson, King of Dublin, joined by Constantine II, King of Scotland and Owen, King of Strathclyde. (According to John of Worcester, Constantine was Olaf's father-in-law.) Though they had all been enemies in living memory, historian Michael Livingston points out that "they had agreed to set aside whatever political, cultural, historical, and even religious differences they might have had in order to achieve one common purpose: to destroy Æthelstan".