By 1403, Hotspur was in open rebellion against his King, joining with his kinsman Thomas Percy, Earl of Worcester, whilst Owain Glyndŵr, was undertaking a campaign against English rule in Wales. Hotspur set free his Scots captives and Douglas with his co-prisoners decided to fight alongside their former captors. In the chivalric spirit of the time, Douglas marched with his former enemy Hotspur, and his forces to the meet with King Henry IV at the Battle of Shrewsbury. Fighting on the English king's side was George de Dunbar, 10th Earl of March, then in exile from Scotland. The result of the battle was a decisive Royalist victory, Hotspur being killed by an arrow through the mouth. Douglas was once again captured, and suffered the loss of a testicle after having fought gallantly on the field and personally killing Edmund Stafford, 5th Earl of Stafford and Sir Walter Blount. Douglas had again tasted heavy defeat.