after the american entry into world war i, in april 1917, the 8th illinois, still in national service, was renamed the 370th infantry regiment. it was the only african-american regiment to go into the american expeditionary forces (a.e.f.) while retaining most of its african-american command structure. as the infantry took ship for the western front, duncan became the field commander of the regiment's 3rd battalion. still in this capacity, he was promoted to lieutenant colonel on april 3, 1918. after the regimental commander, colonel franklin a. denison, was replaced by a white officer, lieutenant colonel duncan became the highest ranking african-american officer in the a.e.f. this was a significant achievement due to the segregationist attitudes of president woodrow wilson and his secretary of war, newton d. baker. while serving on the western front against the german army, duncan was awarded the purple heart and the french croix de guerre for gallantry in action.