December 7, 1941
Japan bombing Pearl Harbor also proved to be the Death sentence for Nazi Germany as well.
Hitler had no reason to support Japan in their war against the United States, and got nothing out of it, not even a deal with Japan for an invasion of Siberia, which would have helped him.
Before the U.S entered the war, the Soviets probably could have pushed Germany out of their territory, but actually making it to Berlin would have been difficult, if not impossible, even with all the Lend-Lease equipment.
With the U.S in the war, a two front (North Africa/Italy) and eventually three-front (France/Western Europe) became plausible, and that sealed Hitler's fate.