In a column from 2000, I examined what military historians suggest might have happened if the D-Day landings had been repulsed. Or what if they had taken place in 1943 instead of 1944? The short answers are that if D-Day had failed, Stalin would have ended up occupying almost all of German, which would have significantly changed the balance of power in the Cold War. Had the Allies invaded France in 1943, rather than invading Sicily, they probably would have made faster progress than they did in 1944. VE Day would have come a year earlier, with the Allies capturing most of Germany.
In 1994, Dan Gifford and I wrote that “D-Day was almost a German holiday.” That is, in the darkest days of the war, defending U.S. coastal areas was a crucial concern. Fortunately, the states were able to call forth their self-armed citizen militias for coastal defense, while the U.S. Army and National Guard were busy elsewhere.