Following the outbreak of World War I, Robertson served as Quartermaster General and then as Chief of Staff of the British Expeditionary Force (under John French) before being promoted to Chief of the Imperial General Staff in December 1915.
Following the outbreak of World War I, Robertson served as Quartermaster General and then as Chief of Staff of the British Expeditionary Force (under John French). Promoted to lieutenant general on 28 October 1915, he became Chief of the Imperial General Staff in December 1915.