After the outbreak of the First World War, Davidson joined the III Corps as a staff officer and participated in the First Battle of the Marne, First Battle of the Aisne and the First Battle of Ypres. After the formation of the First Army in 1915 he became the operations officer for Douglas Haig, the Army commander. As operations officer Davidson was the principal organiser of the Battle of Neuve Chapelle, Battle of Loos and the Second Battle of Artois. After Haig became Commander-in-Chief of the British Expeditionary Force on the Western Front Davidson's star was in the ascendant as a part of Haig's inner circle and he was appointed to the post of Director of Military Operations at General Headquarters, one of the key posts controlling activities on the Western Front during the war. In 1916 he was made a Chevalier of the Legion of Honour and in 1917 he was appointed a Companion of the Order of the Bath. During the planning of the initial attack at Third Ypres, he urged more limited advances with regard to tactical objectives so as to increase the concentration of British artillery fire and leave the British attacking forces less vulnerable to German counter-attack, but his advice was not followed.