After the outbreak of the Great War, Thommée joined the 276th Infantry Regiment and served with distinction as a commanding officer of a company and then battalion. In 1916 he was promoted to the rank of lieutenant colonel and became an adjutant at the staff of the 48th Corps in the area of the Romanian front. After the Russian Revolution of 1917, he quit the army and on 25 September 1918 he joined the Polish Army in the area of Kuban. There, on 9 November he became the "de facto" chief of staff of Lucjan Żeligowski's Polish 4th Rifle Division, formally a part of the Polish Blue Army allied to France, United Kingdom, U.S. and Imperial Russia. After the division returned to Poland and was reformed into the 10th Infantry Division, Thommée served as the chief of its staff during the opening stages of the Polish-Bolshevik War until August 1919. On 22 August that year he became the head of the "Third department of the staff" (Offensive intelligence "B"), controlling the intelligence net in the European part of Russia, at the North-Western Front, Masovian Front and then the Polish 1st Army under Gen. Franciszek Latinik.