Japan attacks the USA on 7 Dec 1941. For the next 6 months, they continue to have successful attacks. However, by the end of the battle for Guadalcanal in Feb 1943, they are completely on the defensive.
Allies turn the tide in the Atlantic, where initially German U-Boats harass freely convoys to supply Britain. The Kriegsmarine calls off their submarines. The route to bring American soldiers who will land on D-Day is clear.
Their attack on the Soviet Union having failed, the German Army suffers a series of counter-offensives on the Eastern Front. Hitler's constant orders to stand ground backfire, and the Wehrmacht's losses are huge as the Soviets advance West.