When the Franco-Dutch War began in 1672, Trelawny joined the Royal English Regiment, recruited as part of the Brigade. The alliance with Catholic France was deeply unpopular; many doubted its reliability against the Protestant Dutch and so it served mainly in the Rhineland. However, some officers took part in the 1673 siege of Maastricht as volunteers, including Trelawny and John Churchill, later Duke of Marlborough.