There are several events that might have had the possibility to turn the war:

• Germany doesn’t attack France at all, concentrating their forces in the east which gives the UK fewer reasons to join the war
• Japan doesn’t attack Pearl Harbor so the USA don’t join the war (yet)
• Operation Mincemeat fails and the Axis keeps their troops in Sicily, preventing the Allies from establishing a base in the Mediterranean.
• Axis spies uncover the plans for D-Day before it happens, Germany bombs the landing boats and thousands of Allied soldiers drown before they can reach land
• The Manhattan Project fails to produce a working nuclear bomb. Most of Germany and Italy has already fallen but Japan stays strong and can eventually send troops to Europe.