I would expect the USA to hold back and probably not join at all if Trump is in charge.
Trump says he could end the war in 24 hours if elected. I don't really believe that, but I guess we'll see what happens.
If Harris is in charge it could be a different story.. but the European forces are tiny in comparison to Russia's and would probably be overrun very quickly. They have very small stockpiles of ammunition of all kinds. the USA now has very few forces in Europe, it has been pulling out for years.
The combined populations and industrial capacity of just Britain, France, and Germany could field a rather formidable army which would likely outmatch the Russians. Even if their forces are small now, it doesn't mean they can't start a massive build up - if they're really that concerned that something like this could happen.
The Germans never had many forces in Poland until the Russians attacked. The polish Air force personnel retreated to the uk and were the most successful unit in the battle of britain.Their main defence force consisted of mounted cavalry which was obliterated in the first Panza attack.
My point is that if Britain and France had struck at Germany immediately, while they were still bogged down in Poland, the war likely could have been ended much sooner.