Wullu Wullu:
There are many points that could be logicaly argued as to what the real turning point was that lead to an Allied victory.
In my own view the war was far to complex and all encompassing event to be able to say that "event a" was the point at which the nazis could no longer win.
There are so many factors to consider, including the ones that never took place in a battle. Did the lend/lease program allow the commonwealth to stay in the fight and even allow them in a small way to take the fight to the nazis? How much of factor was the installation of Churchill as PM? Why did the Japanese miscalculate so badly in determining US reaction? Did the Allies ability to read virtually all of the axis mail all but guarantee the outcome? How about the fact that by 42 or so the fact that MI-5 controlled the vast majority of nazi spys in the UK?
All these points were extremely important factors when considering the conduct of the war.
I think it has to be Stalingrad. After that, it was all downhill for the Nazis. Other factors did contribute, but the Eastern Front meat grinder was what ultimately determined who would win the war.
I agree that the lend-lease act helped the UK stay in the fight, but they were already outproducing the Germans by the Battle of Britain in almost every major weapons category. It wasn't until 1942 when Albert Speer reorganized production that German weapons factories really began turning them out.
The fact that Britain is an island saved their bacon too, because the German navy was almost totally annhilated after the Norwegian campaign in 1940. There was no way that the German Navy had any ability to launch an amphibious invasion without the complete destruction of the RAF.
Churchill, I would argue, was Britain's greatest wartime leader in the past two or three hundred years. And he had no moral qualms doing what had to be done (like bombing civilian targets).
Breaking Germany's Enigma machine was a very lucky break, but until the war was well in hand could not be used with regularity, as Churchill sacrificed Coventry to safeguard that source of intel.
As for Japan's surprise at American resolve, it probably had something to do with their own previous successes. When they crushed the Chinese in 1898 and the Russians in 1905, both sides capitulated quickly. They likely thought that the US would react in a similar way, but the US took the attack personally and fought them tooth and nail for the Pacific.