The US is a signatory of the Budapest Memorandum, this is where US (with others) offered Security for Nuclear Arms...
Russia doesn't have the real capability to have a World War unless they use nuclear weapons... Putin does that, Russia would be destroyed. I think that might be unpopular in Russia too...
Ukraine can win this war, just give them Air Superiority with a Mercenary Airforce and increase the Mercenary Army too.. Bit of Anti Drone Tech and Russia will be retreating... Russia are pretty shit all things considering, this war has shown us how weak they really are.
I wouldn't take all the land back... Would take all the Crimea and most of the Donbas except for last 100 - 200 miles corridor... Use that for land swaps for Ukrainians and Russians to swap and live in country of choice. This also make Ukraine more Ukkrainian and gets rid of Russians who placed in Ukraine to make Ukraine more Russian... This will make joining EU and NATO easier as well, there would be a larger majority of Ukrainians voting...