As a general response to the initial question and a specific response to the recent post by
@CWSooner about the "decisiveness" of D-Day and Midway, I think that ultimately neither were individually "decisive".
I'll submit the chart near the top of this link:
http://www.combinedfleet.com/economic.htmPer the link, the total economic power of each country (as a percentage of the global total) as of 1937 was:
- 41.7% USA
- 14.4% Germany
- 14.0% USSR
- 10.2% UK
- 4.2% France
- 3.5% Japan
- 2.5% Italy
- 9.5% all other countries in the world combined
Pre-nuclear modern industrial warfare is, IMHO, generally a simple equation:
stuff you have + stuff you can make - stuff your enemy can destroy <=> stuff they have + stuff they can make - stuff of theirs that you can destroy
Meaning that if you can make 41.7% of the world's stuff you are awfully tough to beat. Note also that the US/UK total is just over 50% meaning that even a global alliance of EVERYONE against the US/UK would be unlikely to win.
I'll take Midway first (it was only mentioned in passing, but it is a major interest of mine):
We are getting way off topic here but I want to make this point first: Most everyone with even a passing interest in WWII history is aware that Japanese Admiral Yamamoto was opposed to attacking the US and is credited with the quote about "awakening a sleeping giant" after Pearl Harbor (even though he probably didn't say it). What a lot of people do NOT know is that Yamamoto was far from alone. Nearly the entire Japanese naval high command was opposed to war with the US.
The Japanese Admirals were not idiots. They were smart enough to know that they simply couldn't sink ships as fast as American shipyards could launch them. In 1940 Congress passed and FDR signed what is known as The Two-Ocean Navy Act. The act authorized construction of:
- 18 Aircraft carriers
- 2 Iowa Class Battleships
- 5 Montana Class Battleships
- 6 Alaska Class cruisers (these were really Battlecruisers or even arguably small battleships)
- 27 Cruisers
- 115 Destroyers
- 43 submarines
- 15k aircraft
- and a lot more
In short, the Two Ocean Navy Act authorized the construction of a collection of ships that alone would have been the most powerful navy in the world and arguably more powerful than all other navies (including the UK) combined. This act was not secret. It was a publicly debated and approved piece of US Congressional legislation. The Japanese were aware of it and they KNEW that they couldn't possibly hope to face down this massive armada once it was constructed.
Even ignoring the Brits, the French, the Dutch, the Chinese, the Australians, and all the other US allies in the Pacific, Japanese production was less than 10% of US production.
That is why I believe that neither Midway nor any other Pacific Battle can be viewed as a "turning point". There simply never was any plausible chance that the Japanese could win militarily. Their only hope was to be part of a broader winning side.
That brings us back to D-Day and the German war situation in general:
The Germans and the Soviets had roughly equivalent productive capacity. If you add some of the areas captured by the Germans to their tally (France, Czechoslovakia, etc) then Germany held a slight productive advantage over the USSR. This, however, was not enough to give the Nazi's an advantage over the USSR and UK combined.
As a practical matter, if there had been some way to keep the US neutral (actually neutral, not the undeclared enemy that we actually were in 1940 and 1941) then Germany may have been able to hold off a UK/USSR alliance indefinitely due to other advantages like interior lines. However, once the US entered the war with nearly half of global economic capacity it was not a question of if but rather when the Nazi's would lose.
The US, UK, and USSR combined controlled around two-thirds of the total productive capacity in the World. Even if everyone else in the world had rallied to the Nazi side they still would have been facing a 2:1 production disadvantage. Superior strategy can make up for some deficiencies but not THAT big.