As with so many others of his generation, the outbreak of war in August 1914 saw Christison volunteering for service with the British Army. Subsequently, he was commissioned as a temporary second lieutenant into the 6th (Service) Battalion of the Queen's Own Cameron Highlanders on 5 September 1914. The battalion, a Kitchener's Army unit created from volunteers, formed part of the 45th Brigade of the 15th (Scottish) Division and, after training in the United Kingdom, departed for the Western Front in July 1915. He saw action in the battles of Loos (where he was wounded and awarded the Military Cross), the Somme and Arras. Promoted to lieutenant on 11 February 1917, in July 1917 he was awarded a Bar to his Military Cross. The citation for this award reads: