During the First World War Thomson first served at the British Expeditionary Force Headquarters and was Chief Military Interpreter between Sir John French and General Joffre. In 1915 he was sent to Bucharest as military attaché on Kitchener initiative to bring Romania into the war. But when there he quickly formed the view that an unprepared and ill-armed Romania facing a war on three fronts against Austria-Hungary, Turkey and Bulgaria would be a liability rather than an asset to the allies. This view was brushed aside by Whitehall, and he signed a Military Convention with Romania on 13 August 1916. By the end of 1916 he had to alleviate the consequences of Romania's capitulation, and he supervised the destruction of the Romanian oil wells to deny them to Germany.