He served in the Lincoln-Washington Brigade, rising to the rank of Brigadier. He was attached to the staff of the 15th International Brigade in charge of publicity – writing, broadcasting and visiting the front line to see conditions first-hand. He fought in a number of engagements – at the Battle of Jarama (February 1937) he took over command of the British Battalion (the Irish were split between this and the Lincoln Battalion) after it suffered heavy losses. He was seriously wounded in March 1937, and returned to Ireland to recover. He took advantage of his return to launch another left-republican newspaper, entitled "The Irish Democrat". On his return to Spain, he again served in the war until he was captured in March 1938 by Italian troops fighting for the Nationalists. He was accused of murder, court-martialled, and sentenced to death before being incarcerated in Burgos Prison in 1938. He was under the death sentence for 16 months. During this time he expressed his disagreement with the IRA bombing campaign in England. His sentence was later commuted to thirty years hard labour in January 1940.