Complex sentence: The German offensive was code named Operation Citadel ("Unternehmen Zitadelle") and would lead to both one of the largest armored clashes and the costliest single day of aerial warfare in history.
Split into simpler sentences: The German offensive was code named Operation Citadel ("Unternehmen Zitadelle"). It would lead to both one of the largest armored clashes and the costliest single day of aerial warfare in history.