AFAIK, their war lay primarily in the Pacific, and beyond supporting the Brits and Russians materially, I’m not really sure why the US would want to involve themselves physically in the European theatre. I do feel fear of Germans beating them to the bomb might have something to do with it, but that’s just conjecture.
Basically the prevailing theory here and among left wing academics is that the US didn't want the Soviets to dictate the terms of the peace with Germany. Most of the area taken from the Nazi's by the USSR was left with socialist governments, while the areas occupied by the US and UK had the same system of government that existed prior (and sometimes during) the Nazi occupation restored post war.
A similar thing actually played out in the Pacific. Stalin had promised to invade Japan 3 months after the end of the war in Europe, which is precisely what he did - two days AFTER the US dropped the first nuke. It is theorized that one of the motivators of using the bomb was to draw the war to a close before the USSR launched a land invasion of Japan. In the end USSR occupied all of Manchuria, the North island of Japan and the Northern part of Korea. The partition of Korea was actually very hastily drawn up by the US, they were worried the Soviets would hold the entire peninsular before they had drawn up a proposal. The 38th parallel is actually far further North than the US believed they would have been capable of meeting the Soviets.
That sounds plausible but I’m not entirely convinced that the Soviets in the 40s could spook the US into mobilising at such scale.
Nazi Germany was the logical endpoint of US ideology. Hitler explicitly stated he was inspired by the US's birth via genocide and expansionist colonialism, and the whole fascist project in general is a last-ditch effort to save capitalism from the threat of a socialist revolution. The US is evil but it's not stupid, it knew enough to step in before it got to the point of explicit fascism on their own part.
don't underestimate anticommunism
Is this a serious answer?
The whole reason they gave Czechoslovakia to Nazi Germany and Poland was to give them a clear path towards invading the USSR.
Stalin’s diplomats engineered the Molotov-Ribbentrop pact at the last minute to drive a wedge between Germany and Poland, forcing the Nazi Germany invasion of Poland, and the Brits and the French to respond in kind.
The Brits already signed the Dusseldorf Agreement just a few months before Germany invaded Poland, which encompassed industrial cooperation between German and British industries. They fully expected Nazi Germany to take the USSR out, and the forging of close economic ties with Germany down the road.
The fact that Hilter turned against them was a genuine surprise (most of the Western imperialist powers refused Stalin’s plea to form a coalition against the rise of Nazism in Germany, and in fact most of them already signed military and economic cooperation pacts with Germany since).
The US definitely did not want Stalin to take over the whole of Europe.
You have to take into account that to industrial capitalism, communism is not just a threat, it is THE existential threat.
The first Red Scare had already taken place, the US was scared of labour organization, which was at an all time high inter-war. The depression had just ended, returned soldiers from WWI had marched on Washington, Russian immigrants expelled on espionage charges, anarchists bombed wall street and harsh laws were put in place during and immediately after WWI to curtail the protection of free speech when invoked by dissidents.
This is only in the US, in Europe, South America and Asia communism was getting widespread public support. El Salvador, China, Finland, Bulgaria and Spain had all had communist uprisings or civil wars by the start of WWII.
The Soviets had also resisted the onslaught of Nazi Germany, the combined weight of all of continental Europe's industrial might pressed on them and they broke through, they had twice the number of men the US did. There would have been no illusion that the Soviets were not capable of taking all of Europe.
Both sides would have seen early that, if the Nazi's were beaten, the division of their territory would set the stage for the conflict between the US and the USSR to follow.