It will start with “NATO” countries stepping in unilaterally, like France for instance. If the tide doesnt turn then, then NATO will step in to ‘save’ its member (s). Id prefer this never happen but given the track record of the west they rarely see a war they dont like.
I just posted below besides France, Poland and Lithuania, both are NATO countries and saying the same things as France about sending troops into Ukraine.
This is insane, at some point you have to ask yourself if NATO is actively trying to pursue a war with Russia by provoking them into taking the first shot so other NATO countries can respond.