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What if the United States stays out of the conflict entirely?

I'm going to present my position first at least for Europe. Because America is isolationist in this conflict, there is no Lend Lease support. Because of unobstructed submarine warfare, Britain eventually agrees to an armistice with Germany by 1942 due to starvation and economic destruction. Germany is now free to devote most resources towards the Soviet Union. However, the Red Army is able to resist the German invasion even without American support. Instead of reaching Berlin by summer of 1945 they are able defeat Germany by the end of 1946 instead. Italy and Spain capitulate there shortly after as Russia wants to purge continental Europe of any fascist opposition.

By 1948, almost all of continental Europe perhaps neutral countries in the war like Portugal, Sweden, and Switzerland. All of the colonies belonging to western European nations begin to restructure their governments to All of the others become Soviet satellite states. In the years after, an arms race still begins because Britain fears an invasion by the Russians much as it did during the war or of its possessions in the Middle East and Africa. They rush the existence of atomic warfare in the early to middle 1950's as a means of defense. The Russians acquire this weapon in secret as well from spying on British efforts. After Britain discovers the Soviets have used this weapon, they share the technology with the Americans which are barely coming out of two decades of depression.

In Asia, Japan has successfully encapsulated coastal China and is using it for their economic benefits. Thanks to the Soviet Union which is a super power after winning the European War against the Germans and dominating Europe putting up guerrillas to resist. Free parts of China and also parts of former indochina resist heavily with force by being funded by the Soviet Union. It makes it very hard for Japan to maintain its colonies on mainland Asia with constant fighting. Japan has been in nonstop conflict since the 1930's, which is extremely costly. The development of atomic weaponry in this conflict doesn't make it any easier.
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