What is the significance of D-Day? June 6, 1944

I think a
case can be made for D-Day being the moment where the German loss became only a matter of time. 
At this precise instant, the war for the Germans became a two front affair, with the Russians in
the Eastern theatre and the Americans and British on the Western side.  Hitler and the Nazis had
to fight two fronts, with equal magnitude and intensity.  There was little way that Germany
could possess the sustainable and replenish-able resources to fight both for an extended period
of time.  The Allied invasion of the beaches at Normandy also constituted the first time that
Hitler was repelled from advance, and like all of those who bank of the air of invincibility,
the first time that there is exposure of weakness is a significant instant, a confirmation that
defeat is not that far off from being recognized.