To Ellis' excellent unit breakdown, I would only add that there is also an Army Group, comprised of two or more armies. Not seen in the Pacific Theater during WWII, but did exist in the MTO in the form of the 15th Army Group under Gen. Mark Clark (U.S.). Also in ETO there was the 6th Army group under Gen. Jacob Devers and of course the 12th Army Group under Gen. Omar Bradley (U.S.). Rick