If Trump got what he wanted by getting European commitment then why suddenly cut off Ukraine? That makes no sense at all.
And you have to say that troops in is possible, or are you expecting America to only? It has to be credible to work, which funnily enough I don't consider nuclear weapons to be.
Europe out number and out gun Russia in regular forces without the US, and with far better tech. The only thing Europe doesn't beat Russia on is ICMBs, but since we're on the same continent doesn't really matter.
China isn't Russia's mate, the moment Russia is balls deep (they kind of are) then China will be making incursions into their former old territories out there I reckon.