I have a limited understanding of the grand scale of WWII, but like a lot of people, I am fascinated with that era. What follows, and is the basis of my question, is built off what I've gathered, but it may be wrong on its face, and I apologize for that!

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My understanding is D-Day proved a major turning point for the Allies. It was an enormous invasion that led to, about 10 months later, the end of the war in Europe. But what if the invasion had failed? I know there were substantial casualties as it stands, but D-Day was overall successful. If the Germans had been more prepared, and "held off" the invasion in the sense of delivering greater casualties, what would the Allies have done? Was the United States prepared to drop atomic bombs on Germany? Would Hitler have been successful in taking over Europe, or was his downfall inevitable long before D-Day?

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Thank you in advance!