it was not a true neccesity, we would not have lost without it but it was the smartest move at the time, like they said we already had italy and we could have moved through the alps but we would have encountered fierce resistance. in fact most of the german panzers were waiting for an attack on the alps led by general patton. what d-day did was for all intensive purposes was a very big flanking menuever to get around the brunt of the nazi's forces.