NATO has no mandate as yet and no means to counter this. Nowhere to deploy naval power, no means to achieve air superiority and no time or territory to amass the necessary manpower and logistics for adequate land forces. That only leaves a non-tactical nuclear response which won't happen as it wil only be considered as retaliatory and will assure mutual destruction. Short of supplying a limited amount of equipment there's nothing that can really be done in terms of a military response. NATO, and the UN are outflanked. Russia is a nation of chess players and this is check. Ukraine is a tough, brave nation but it's only a matter of time until check mate. As far as I can tell the only possible way they can stop Russia is to draw them into the cities and have them destroyed around them, fighting from solid defence and grinding down the attacking force. Stop, but not defeat, at the expense of their population. Gidt forbid we see another Stalingrad in Ukraine.
All outcomes are bleak, as are the events of this day, but there is no 'best case' scenario for Ukraine and only worst case scenarios for the rest of Europe.