I would say there where 3, rather than one turning point.
1. Pearl Harbor: This brought a heavily industrialized, geographically isolated, warlike country, rich in fossil fuels, into the mix. Bad news.
2. Hitler's attack on Russia. Whether he could avoid it or not, attacking the Russians opened up a second front, one which swallowed up troops and material like a black hole.
3. The loss in North Africa. This denied the Germans any chance at the rich oil deposits in the middle east. The lack of fuel was crucial in the remaining years of the war.
Sure, DDay was an incredible achievement, but it never would have been possible if the germans had been able to focus all their forces on the western front, if they had had enough fuel to move their armor/infantry effectively, and if the American air, land, and navy (and FACTORIES) hadn't had a good excuse to get involved.
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