During World War I, Ernst refused all honorary appointments at the Kaiser's headquarters, which would have been considerably safer than other areas. Resigning from his courtesy rank of Generalleutnant, he requested and was granted the colonelcy and the command of his duchy's regiment, the 153rd (8th Thuringian) Infantry. Quickly promoted to General der Infanterie, he led several brigades on the western front. In 1915, he was awarded the Pour le Mérite award and was given command of the 8th Infantry Division, further distinguishing himself in the Battle of the Somme. In late 1916, he relinquished his field command because of illness and returned to Altenburg for the remainder of the war.