I’m not the huge WWII buff that a lot of people are, but I think I’ve read a dozen or so books so I feel comfortable in making the following assertions:
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WWII was not exclusively the European Theater, the Japanese were also a serious threat and the United States did the vast majority of the work when it came to breaking the Japanese Empire. Thanks go out to the Brits, the Aussies, et al. who helped as best they could, but the British had most of their resources in other areas and the Aussies - while certainly helpful - were not large enough to be able to claim much credit.
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The Eastern front would have been fundamentally different if the Western front did not exist.
If not for the Western front Hitler’s hand would have been strengthened in several ways
i) If there had never been a sustained Western front, basically Hitler never tries to invade Britain and the Western Allies simply say, “sorry” to the occupied nations of Western Europe, then Hitler would have been immeasureably stronger. The Luftwaffe would have been overall untouched.
The German Navy would be in fine shape, the German Merchant Marine would be sailing happily along with impunity.
Furthermore if the rest of the Western Allies hadn’t gotten involved there is a good chance they just simply would not have helped the Soviets vs. Germany. Without Britain herself at threat and with no prospect of warfare against Germany it is hard to say who Britain/the United States would choose to support, Fascists or Communists?
ii) To see the true effect of the Western Allies you need to look at what would have occured without them.
There would be no need for divisions stationed in Italy and Western Europe. There would be a need for a small security force to keep the French and other conquered peoples in line, but nothing like the massed troops looking warily into the Atlantic like we saw.
There would be no need for the Atlantic wall (IIRC the largest construction project in history at that time) which is a huge diversion of resources that would have been available for fighting in the East.
- The Pacific Theater would change dramatically.
Without the United States Japan would be lord of the Pacific in short order. Indochina and China would be Japanese vassals with ease. In this case scenario Japan has nothing to fear from the United States.
Maybe in this case scenario Japan then sees no other valuable targets to gobble up other than the Soviets. Without any American pressure on Japan it is conceivable that the Japanese would have created two fronts for the Soviets to be fighting on.
Completely reversing the situation as it happened historically.
I think many people underestimate just how valuable the American blockade on metals and oil was when used against the Japanese and the Germans. The Axis had built up mighty militaries, with brilliant tactics, strategy, and leaders. They had excellent equipment and training. What both nations lacked were key natural resources that are simply vital to a war machine. That contributed so much to them losing.
To truly say the “Western Allies” did not win WWII, you have to present a scenario where they also were not “passively” fighting the Axis. Which means American oil and scrap metal would be selling regularly to the Japanese and the Germans. This would have tilted the balance of power squarely against the Soviets.