Complex sentence: German forces in the area had been further split up, as German general Erich von Manstein was given command of the newly created Army Group Don, comprising the German Fourth Panzer and Sixth Armies, and the Third and Fourth Romanian Armies.
Split into simpler sentences: German forces in the area had been further split up, as German general Erich von Manstein was given command of the newly created Army Group Don. This was composed of the German Fourth Panzer and Sixth Armies, as well as the Third and Fourth Romanian Armies.