On 13 May 1939 he was commissioned into the
King's Own Scottish Borderers of the
Territorial Army. He served with the 4th Battalion of the regiment in the
Battle of France. Between 1942 and 1945 Swan was seconded to the
British Indian Army and served with the No. 1 Mule Training Regiment in Jullundu. Following the end of the Second World War, Swan returned to farming in Scotland. He served as
Army Cadet Force County Commandant for Roxburgh, Berwick and Selkirk from 1955 to 1973 and was granted the rank of lieutenant-colonel. He was a colonel in the Lowlands Territorial Army from 1983 to 1986. ==Agriculture==