Over time, my understanding of the importance of the D-Day invasions has shifted. Growing up, I considered these events to be the single key event in defeating the Nazis. Listening to the radio this morning, this still seems to be the common understanding.
Over time, I have had to face the fact that the US (or at least the US Army) was not primarily responsible for defeating Germany -- the Russians defeated Germany, and what's more, would have defeated them whether the Allies had landed in France or not. Check out the casualties by front, from Wikipedia:
The Russians defeated Germany. Period. And I don't think the western allies would ever have had the stomach to inflict the kind of casualties on Germany that were ultimately necessary to defeat her without Russian help. To me, this is the great irony of WWII, that it was not ultimately a victory for democracy. Only totalitarian Russia could defeat totalitarian Germany. This thought often bothers me a lot. It doesn't fit with how we want to view the war.
However, D-Day did have an important effect -- it kept Western Europe out of Soviet hands. We did not know it at the time, but I would argue in retrospect that from mid-1944 on we were competing with Russia to see how Europe would get divided up after the war. D-Day allowed the western allies to overrun most of Western Europe and keep it out of Soviet hands, perhaps an even more important outcome than just speeding the defeat of the Germans. Sure, FDR gets grief for giving the farm away to Russia at Yalta, but what could he do? The Soviet occupation of Eastern Europe at that point was a fait accompli. What would have been FDR & Churchill's negotiation position at Yalta if their armies were not even on the continent (excepting Italy, where we might still be fighting in 2014 and getting nowhere)?