In command of a newly raised corps of Irish foot, Dormer went to Spain, and took part in the Battle of Saragossa. He was taken prisoner with General James Stanhope at Brihuega in December 1710, and was sent home on parole. On the death of Charles Mohun, 4th Baron Mohun in a noted duel with the Duke of Hamilton in 1712, Dormer, who had been exchanged, was appointed colonel of Mohun's regiment, which was disbanded the year after.