Rutherford first volunteered to travel to fight for the Spanish Republican cause in December 1936, crossing via Dover to Dunkirk, with a group of 65 volunteers, including Peter Kerrigan. This group of the International Brigades reached Spain on 22 December 1936, and Rutherford was allocated to join fellow Scot, Harold Fry, a former shoe repairer, who had served as a sergeant in World War One, and led the machine gun company. The two became friends despite their age difference. They were captured at Jarama, two months later (2 February 1937) and Rutherford and Fry were initially sentenced to death, but later deported as part of a prisoner exchange from Spain, on 25 May 1937, on promising not to return.