 With his final exams over two years away, he enjoyed an idyllic Trinity (summer) term at Oxford, just before the outbreak of the First World War.Volunteering immediately for active service in the War, Macmillan joined the British Army and was commissioned as a temporary second lieutenant in the King's Royal Rifle Corps on 19 November 1914.Promoted to lieutenant on 30 January 1915, he soon transferred to the Grenadier Guards.