 During the First World War Collingwood applied for a commission in the army but was rejected because of his poor eyesight. Having been a keen cadet at Rugby School and an enthusiast for the National Service League, this came as a blow. From January 1916 onwards Collingwood spent most of his time living and working in London in the Intelligence Section of the Admiralty; although he continued to see pupils in Oxford weekly, he gave no lectures between 1916 and 1919. He left the Admiralty in June 1919 and resumed lecturing in January 1920. During his time at the Admiralty he was the main author of two book length reports: A Manual of Belgium and the Adjoining Territories (1918); and A Manual of Alsace-Lorraine (1919). A spin off from this work was his entry on Luxemburg in the Encyclopaedia Britannica (12th ed., 1921).