Smallwood joined the Royal Warwickshire Regiment on 8 April 1911 as a Special Reserve second lieutenant on probation,[1] confirmed in April 1912.[2] In December 1912, after passing his examination, he was transferred to be a regular second lieutenant in the 2nd Battalion, East Yorkshire Regiment.[3] In September 1914, after the outbreak of World War I, he was promoted to temporary lieutenant[4] and later that year seconded to the Army Signal Service.[5] He was promoted to captain in 1915.[6] He was twice mentioned in dispatches during the war, and he was awarded the Military Cross (MC) in the King's 1916 Birthday Honours.[7][8]