Was Waterloo important? Yeah, I guess so, reluctantly. Really more like Napoleon equivalent of Eagles doing one more farewell tour, but still? I guess it was a major battle of Western Civ (why do you even use that term anymore?) just cause it was the total, decisive end for him.
Most important battles in Europe in the past thousand years. Battle of Hastings, Siege of Vienna, sure Waterloo, and D-day.
So why D-day Dave? All those other battles were decisive, while the 1944 Normandy invasion was hardly the end of a war. Well? It ain't like if D-day had failed we'd have gone home and let Hitler have his way. Probable that Berlin would have been nuked? Yep, likely.
What our boys managed on June 6, 1944 by succeeding in taking and holding that narrow strip of beachfront? They determined the future boundaries of the Iron Curtain. Had the invasion been another Dunkirk, would've taken at least six months to try again. Aside from the fact the A-bomb would've been about ready by then, and powerful constituencies would've been lobbying for its use? The Red Army was steadily marching west.
USSR only particpated in the European war, but they lost 7 soldiers for every US war death. They had a handful of really brilliant generals, I'd put in the same class as Lee, Rommel and Grant. But those guys, for all their brilliance, they saw soldiers mostly as cannon fodder. They didn't have the least qualms. So if D-day had failed, the Iron Curtain's boundary might have been Paris.
Stalin's guys were moving west. We had to move east because the smartest of our generals knew Hitler's end was a done deal, and Stalin was the next foe.
The reason Lee had his way with every Union General before Grant, is cause he always picked where to fight. Only mistake he ever made was Gettysburg, a place to fight that he didn't choose. In the same way, US generals involved in the European war were mindful where the next war would be fought.
That's why D-day was a pivotal battle. Success there determined the battle lines in the next war.