The long period of peace from 1713 to 1739 meant limited opportunities for promotion, while the commission purchase system worked against those like Forbes with little money. It was not until April 1742 he was promoted lieutenant, shortly before the regiment was posted to the Austrian Netherlands to fight in the War of the Austrian Succession. Forbes became aide-de-camp to James Campbell of Lawers, (1690-1745), colonel of the Scots Greys and commander of the Allied cavalry. He fought at Dettingen in June 1743 and in September 1744, purchased a commission as captain.