He left his studies a second time and joined the 25th Foot (originally raised by David Melville, 3rd Earl of Leven in 1689, and later known as the King's Own Scottish Borderers) as an Ensign in 1744 in Flanders, and fought that year at the Battle of Fontenoy, where 1/3 of the regiment was killed. After the Battle of Ath, he returned with the regiment to Scotland to put down the Jacobite rising of 1745, and was besieged by the Jacobites in Blair Castle before fighting at the Battle of Culloden. He continued the war in Flanders at the battles of Roucoux and Lauffeld. He was a Lieutenant by 1748, and was promoted to Captain in 1751.