> "The Other Side" was fighting on four fronts at their borders, and had supply lines a thousand miles long on ground
Do remember that it's basically fighting the same enemy on all the fronts (who have the same this-is-not-close issue as Germany). In the one case where it's different, keep in mind that Soviet factories were moved east of the Urals to places like Novosibirsk, which is further from Moscow than Berlin is to Moscow.
> the US against the Axis in Europe, the US was not a major or deciding force
The US was about ½-⅔ the total force in Western Front. While the Western Front was smaller than the Eastern Front, the US also played a major role in the supply chain of the USSR--virtually all of the trucks and trains that kept the USSR's war economy running were supplied by the US, which was also supplying Chang Kai-Shek via The Hump (although Britain was responsible for those logistics), while meanwhile reconquering the Philippines and other Pacific islands from the Japanese. And don't forget, most of the US manufacturing is several hundred miles from the coasts (the Great Lakes don't count for transoceanic shipping purposes).
If you know anything about logistics you'd conclude that the latter had a much harder logistical challenge, especially since the Axis didn't have much of an Atlantic Navy.
The issue of a US v China confrontation is that this won't repeat. The US will not be able to go unopposed in the seas, and it will have to supply with non-naval means.
Also, it's worth remembering that in the case you're talking about, the US against the Axis in Europe, the US was not a major or deciding force, which is a major part of why it was able to afford such extended supply lines. In an alternative scenario where the rest of the Allies were so weak as that the US was the dominant power in the conflict, maintaining those supply lines would have been incredibly difficult.