D-Day was the largest military operation ever in history. Maybe this is not true as regards the sheer number of men involved but more so in terms of the amount of equipment: ships, tanks and aircraft. Not only that but the area into which these forces were operating was a mere 70kms as compared with 1500kms on the Eastern Front.
This was the Second Front that the Russians had been demanding since 1942. Whereas the Americans and the British understood that a naval landing into the heart of German controlled France would result in a titanic struggle which if unsuccessful would set back the Allies plans for the ultimate defeat of the Germans by years, if not forever.