"Iron Duke" served as the flagship of the Grand Fleet during the First World War, including at the Battle of Jutland. There, she inflicted significant damage on the German battleship early in the main fleet action. In January 1917, she was relieved as fleet flagship. After the war, "Iron Duke" operated in the Mediterranean as the flagship of the Mediterranean Fleet. She participated in both the Allied intervention in the Russian Civil War in the Black Sea and the Greco-Turkish War. She also assisted in the evacuation of refugees from Smyrna. In 1926, she was assigned to the Atlantic Fleet, where she served as a training ship.