You might argue that NATO hems him in to the West. I can’t say this is any consolation, but if you want a hard trip wire, it will be if he starts this activity with a NATO member, not that that makes what he is doing to a sovereign nation just because he wants it, wants more territory to be acceptable. In the meantime, NATO imo should fast track new member memberships for those countries deemed worthy and say fuck him.
It’s being reported that Putin made an announcement that he is conducting a special military operation to “de-militarize“ Ukraine just because…he does not like having an armed country next to him, not under his influence, really? 
So because this is not NATO, he thinks he has cart blanche to cause mayhem Because the NATO states won’t risk a major war over Ukraine.
From a strategic standpoint I would love to be privy to a couple of NATO meetings to see that the considerations were. My guess is illegal action without any reasonable justification vs the economic impact of a war with Russia, a nuclear armed State.
Russia is a menace. We can either bolt our shutters and hope he keeps looking the other way at the vulnerable countries that make up for former USSR, and allow him to absorb them. Ukraine seems to be quite a prize. If he actually stops in the “breakaway” provinces, I’m looking to see how NATO treats Ukraine- you’re on your own kid or meaningful assistance including NATO membership.
Yeah I know, that’s not going to happen, and as I understand it, now that Russia has drawn this line in the sand, they will easily say, if NATO Forces cross into Ukraine we consider this an act of war. Because to make Ukraine a NATO state, most likely NATO would have to assemble their own large force, because my guess is that the Russian forces if they don’t take Ukraine now, will remain in the vicinity. It’s also possible that now that they seem to be having their way with Ukraine militarily, they might actually make ovations to them to try to lure them back into the fold while they undermine Ukraine every way they can, including assassinations and placing their own pro-Russian stooges in key positions.
However, it you want to be scared, be scared of the nukes and if it comes to a point where someone under stress calculates a well placed nuke might help their cause where as it might spark a general nuclear exchange.
