Shortly after his graduation, the United States entered World War I. In June 1917 Bruce joined the United States Army as a second lieutenant in the Infantry Branch. He served in the First Officers Training Camp at Leon Springs, Texas. After completing his training he was sent to combat in France as part of the 2nd Infantry Division's 5th Machine Gun Battalion. He saw action in the trenches of the Western Front in the Troyon Sector near Verdun, in the Aisne Defensive operation near Chateau Thierry, the Aisne-Marne offensive at Soissons, the fighting at St. Mihiel, and the Meuse-Argonne Offensive at Blanc Mont. Following the signing of the Armistice with Germany on November 11, 1918, he moved with his division into Germany to be part of the occupation force. At the relatively young age of 24 Bruce achieved the temporary rank of lieutenant colonel. The government of France awarded him the Legion of Honor, three Croix de Guerre medals, and the fourragère.