That's the tricky part. If Russia decides to use one or several tactical nukes on Ukrainian military targets, does that mean we're going to respond with nuclear armageddon? Probably not. But the west would likely respond with striking the launch positions (small nukes = shorter range). And if it gets to a full nuclear showdown, I wouldn't expect it to be anything like their ground game, except the fact that Russians have no moral hangups about hitting major population centers (with the dirtiest bombs they have). Keep in mind its all just a matter of escalation. The more Russia loses, the more frustrated and desperate they'll become. Don't pretend Russians aren't sore losers.