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.
FIRST MAJOR DEFEATS
El Alamein #2 for Germany
LAND: Kokoda-Milne Bay and SEA: the Battle of the Coral Sea for Japan.
These we way before Iwo Jima and Midway.
If the Battle of Britain had been lost by the RAF in 1940 and Britain invaded, it would have been game over. I am therefore, firmly convinced that the Battle of Britain was the turning point.
Studies at Sandhurst say other wise, it is their opinion that even had the Germans landed in strength neither the Kriegsmarine nor the Luftwaffe could have stopped the Royal Navy from breaking into the invasion lanes and destroying supply barges and reinforcements.
In the end the Germans may have made it as far as London but starved of supplies the invasion would have failed.
Operation Sealion was always a non-event.
Why? It had very little effect on the eventual dismantling of German military power. It was hardly vital for Germany to capture the UK - there were no scarce reserves in the UK that the Germans weren't able to get from elsewhere. There was no chance for Great Britain to threaten the European mainland in any meaningful way whilst the bulk of the Wehrmacht was in place. The turning point of the war in Europe was always going to be what happened when Germany clashed with the Soviet Union, which was almost inevitable. The Germans had one chance to KO the Russians during Operation Barbarossa and then Operation Typhoon. For various reasons they blew it. The Soviets won. Moscow was the true turning point of the European theatre of World War 2.I still firmly believe that the Battle of Britain was the turning point of WW2, in Europe at least.
SL again :the old myths never die
It was a non sequitur,from the beginning .
For SL to have a small chance,a lot of things were needed,and ,if ONE of them failed,it was over
1)The LW needed an air superiority in South East England=preventing the intervention of FC and BC.This failed : NO SL possible
2)The WM needed a transport fleet to transport men and material :there was no such fleet:NO SL possible
3)This transport fleet needed the protection from the KM;this was not possible :no SL possible
4)The WM needed on the first day to capture at least one intact harbour :this was not possible :no SL possible
5)The WM needed several weeks of good weather for the build up :as this was not possible in september,NO SL.