After Romania entered World War I on the side of the Entente, Dumitrache took part in the Flămânda Offensive in 1916. He was wounded in the battles of 1916 and 1917, and advanced to the rank of captain in 1917. After the war, he was admitted to the Higher War School in 1919. Promoted to major in 1920, he graduated in 1921 and was assigned as staff officer with the 1st Vânători de munte Division in Arad. In 1924 he was reassigned to the 1st Mountain Division in Sinaia. In May 1929 he was promoted lieutenant colonel and named commanding officer of the 2nd Mountain Battalion in Caransebeș.