Have you read his recent announcement? He warned every foreign nation not to meddle or else:
> To anyone who would consider interfering from the outside - if you do, you will face consequences greater than any you have faced in history,’ he said on a television broadcast around 6am Moscow time.
How popular do you think Putin will be at home when he announces he's going to full war with NATO? How would he even do that? You think he's going to launch ICBMs?
Putin is not elected by a popular vote, the voice of the people does not matter there.
Estonia is a NATO member already, the current conflict has been escalated because of the previously announced red line demands (Ukraine never getting into NATO) that the US diplomats discarded. It is their job to negotiate peace in the region too. Negotiations is a nasty business and the outcomes are nasty too. Those who seek easy and pretty solutions that satisfy all parties in one step are destined to escalate the situation further up.
The alternative is to let him repeat 1939.
The lesson learned should be that if you don't stop the thug with force, then he will eventually become strong enough to stop you.
this is a lesson of a past war, the world has moved, this is no longer a WW2 era, if you try to apply a similar foreign intervention now the chances are you'll get a nuke on your lawn.
You don't get to put that one back in the bottle, and you're going to have to have more faith than the Pope to think that no one will respond.
Nuclear War isn't like Civilization. It's a momentary bang followed by a lot of whimper.