 For example, when the German attack began on 16 December, the U.S. 7th Armored Division was assigned to XIII Corps, U.S. Ninth Army, 12th Army Group.Later that day, its alignment became VIII Corps, U.S. First Army, 12th Army Group.On 20 December, the alignment switched to XVIII Corps, U.S. First Army, 12th Army Group — and later that day to XVIII Corps, U.S. First Army, 21st Army Group.