To be fair, the US came into both wars late. If anything, America is the guy that comes in when the fight is nearly over and finishes off people who were already bled down.

US war weariness has always been worse than most other first-tier military states. I don't think the population of the US have the appetite for conducting a war from start to finish against a major power -- back then or now! Look at wars like Iraq, Vietnam and Afghanistan for examples where America led an invasion and war weariness took over quickly. During WWI and WWII, the US played the isolationist card and there was tons of support for it.

In any conflict against Russia + allies (even China), I'm certain that the US + allies dominates militarily but I definitely think that this time there will be no way to avoid casualties on the home front. That's something the US has NEVER had to worry about in the first two world wars. The reason the US was the [b]winner[/b] of the second world war was because their infrastructure and economy remained in tact -- and in many ways strengthened. It wasn't completely crushed and left to rebuild with a scarred population like many other countries (current allies) in WWI or WWII.