2. D-Day was Made Possible by Allied Efforts Elsewhere
Allied efforts were not just centralized at the Normandy beach. They were spread out across various fronts before and after June 1944. The Allies conducted a strategic bombing campaign that started in 1942. This destabilized the German establishment and forced the Nazi to commit manpower and resources to home defense away from Normandy.
Allied control of the Atlantic achieved through the 1943 victory in the Atlantic Battle also determined the success of the D-Day invasion by directing away the German troops. So did Operation ‘Bagration’, the Soviet Belorussian offensive launched soon after ‘Overlord’. It smashed the entire German Army Group Centre and kept German forces knotted down in the east.