 Following the outbreak of the First World War, Jeffries was commissioned as a second lieutenant in the Citizens Military Force on 22 August 1914. By this time he was in charge of the survey department at the Abermain Collieries, but was mobilised for home defence duties and the instruction of volunteers for the newly raised Australian Imperial Force at Newcastle and Liverpool camps. Promoted lieutenant in the Citizens Military Force in July 1915, he transferred to the Australian Imperial Force on 1 February 1916 with the substantive rank of second lieutenant, and was placed in command of C Company of the 34th Battalion. In May, the battalion embarked from Sydney for the United Kingdom, with Jeffries aboard HMAT Hororata. Arriving in late June, the battalion spent the next five months training in England, during which time Jeffries was promoted to lieutenant.
