re: WW2: What if D-Day failed?

Only two things would've led to a defeat at Normandy:
• Air SUPERIORITY by the German Luftwaffe
• A strong, threatening presence of the German U-boat fleet
If these threats had existed prior to the invasion, I doubt the operation would've been carried out. In it place perhaps, there might have been an invasion of Norway, albeit on a much smaller scale than Normandy, or a stepped up effort to dislodge the Germans from Italy. I imagine too that every effort would've been made to draw out and decimate the Luftwaffe.

Allied air dominance over Europe was the key to victory on that continent. I could see that being a primary Allied strategy had Normandy failed or not been attempted.