The outbreak of the Spanish Civil War in 1936 resulted in many international communist parties sending manpower and financial support to Republican Spain. Garland, being a staunch anti-fascist and member of the American communist movement, volunteered for service in Spain. He sailed for France aboard the "SS" "Champlain" on 5 January 1937 and from there made his way to Republican lines. Once in Spain he joined what would become the Abraham Lincoln Battalion, in which he was appointed section commander on account of his previous military experience. Garland was wounded in battle on 27 February during the advance of the 17th International Column on Jarama. During his convalescence, Garland was selected to attend officer training school, and he returned to the front as the commander of a machine gun battalion (dubbed the Washington Battalion.) Garland was wounded for a second time in July 1937 during the Republican defeat at the Battle of Brunete. Following his second wounding, Garland was promoted to captain and given command of the Canadian Mackenzie–Papineau Battalion while the formation was in training. In August he collaborated with Langston Hughes and Harry Haywood to broadcast a radio message to the United States live from Spain with the intent to drum up support for the Republican cause. Garland was ordered to return to the United States in October to raise funding and manpower for the International Brigades. Once he had returned to the United States, Garland began to lobby Congress to lift the armaments ban on Republican Spain. In 1938 he ran as the Communist Party's candidate for the New York 17th Assembly District, an office he would run for again in 1940. Garland continued to be active with the Friends of the Abraham Lincoln Brigade and the American Medical Bureau after the defeat of the Republican cause in 1939.