Stanhope was serving as a diplomat in Spain when the War of the Quadruple Alliance began in 1719 and joined the French army under the Duke of Berwick as a volunteer. He accompanied Berwick's army during its successful Siege of San Sebastian. When the war ended in 1720, Stanhope was appointed British ambassador to Spain and given the Colonelcy of the 13th Light Dragoons, later 13th Hussars; he retained this position until the Anglo-Spanish War began in March 1727, having built up his reputation as a diplomatist during a difficult period.