Devereux served in the army of King Charles I during the first Scottish Bishops War in 1639 as Lieutenant General of the army in the north of England. He became increasingly sympathetic to the critics of Charles over the following three years and by the start of the English Civil War he was in opposition to the monarch as the first Captain General and Chief Commander of the Parliamentary army. Essex resigned his commission in 1645 when the war against Charles adopted more aggressive policies under Cromwell’s leadership.