He was notable for his outspoken opposition to the First World War, which caused his arrest under the Defence of the Realm Act and loss of his teaching post, after which he became a full-time Marxist lecturer and organiser. In April 1918 he was arrested for sedition, and his 75-minute speech from the dock became a celebrated text for Scottish left-wingers. He was sentenced to five years' penal servitude, but was released after the November armistice.