 After training, he was assigned to Battery C, 58th Artillery, of the Coastal Artillery Corps at Fort Schuyler, the retired Civil War coastal battery guarding the junction of Long Island Sound and the East River.Rennie made Private First Class on 1 February 1918.Then, serving overseas between 10 May 1918 to 27 April 1919, he was promoted to Corporal on 1 October 1918 and Sergeant on 24 November 1918.