It was a success in the sense that it got the Western allies ashore, leading to the liberation of France a few months later and the end of the war in Europe the

It was a success in the sense that it got the Western allies ashore, leading to the liberation of France a few months later and the end of the war in Europe the following spring. More importantly than that, it meant that Nazi Germany's eventual defeat, which was probably inevitable at that point anyway, did not come solely at the hands of the Red Army. I imagine the scenario that really kept Roosevelt and Churchill up at night was one where the Soviets defeated Germany and then kept going until they reached the Atlantic, with France and the Low Countries falling under Soviet domination the way that the countries of Eastern Europe ultimately did. The success of the D-Day landings meant that the iron curtain ran down the middle of Europe and not down its Western shore.