At the beginning of the First English Civil War in August 1642, like many other young lawyers who afterwards distinguished themselves in the field, he joined Essex’s life-guard, was wounded at the first battle of Newbury (1643), obtained a regiment in 1644 and fought at Naseby. He had already been appointed receiver of the court of wards, and in 1646 became member of parliament for Marlborough. In the dispute between the army and parliament he played a chief part, and was said to have been the principal author of the plot to seize King Charles at Holmby, but he did not participate in the king’s trial. In 1649 he was appointed a governor of the Isle of Wight, and in 1650, as lieutenant-general of the horse, took part in Cromwell’s campaign in Scotland and assisted in the victory of Dunbar. The next year he was elected a member of the Council of State, and being recalled from Scotland was entrusted with the command of the forces in England, and played a principal part in gaining the final triumph at Worcester (3 September 1651).