Complex sentence: Field Marshal Günther von Kluge was the German supreme commander in the West, and since Field Marshal Erwin Rommel had been injured by Allied aircraft on July 17, also held direct command of Army Group B, the formation conducting the battle in Normandy.
Split into simpler sentences: Field Marshal Günther von Kluge was the German supreme commander in the West. After Field Marshal Erwin Rommel was injured by Allied aircraft on 17 July, von Kluge also took over direct command of Army Group B, the formation conducting the battle in Normandy.