In November 1811, at the age of 17, Le Couteur was promoted from Ensign in the 96th Regiment (on Jersey) to a Lieutenant in the 104th Regiment of Foot and was ordered to join his regiment in New Brunswick. During the Anglo-American War of 1812, the 104th was ordered to march 700 miles from Fredericton, in the less vulnerable region of New Brunswick, to defend Kingston in the threatened area of Upper Canada. This was necessary because the Commander-in-Chief in Canada, Sir George Prevost, found himself with only 3000 troops to defend 1100 miles of frontier. The march of the 104th, of which Le Couteur's journal provides a detailed account, lasted from 21 February to 12 April 1813. The men marched across difficult terrain in extreme winter conditions and, with temperatures dropping as low as −27 °F (−32 °C), few escaped frostbite.