Campbell succeeded his father as Duke of Argyll and Chief of Clan Campbell and also became colonel of the 4th Troop of Horse Guards in 1703. For the help he gave the King persuading the Parliament of Scotland to support the Act of Union, he was created Earl of Greenwich and Baron Chatham in 1705. He then returned to the continent and, having been promoted to major-general early in 1706, served as a brigade commander under Marlborough at the Battle of Ramillies in May 1706 and at the Siege of Ostend in June 1706. After being appointed colonel of Prince George of Denmark's Regiment in 1707, he went on to command a brigade at the Battle of Oudenarde in July 1708 and at the Siege of Lille in Autumn 1708. Promoted to lieutenant general in April 1709, he also took part in the Siege of Tournai in June 1709 and the Battle of Malplaquet in September 1709.