Ukraine is not a member of NATO, so not subject to mutual-defence treaties and agreements. However, if Putin gets too big for his boots, he may well look at the Baltic states - which are in NATO, and which used to be under the aegis of the old USSR. And which are now covered by defence agreements with NATO. And does anyone think Putin is not too big for his boots ? He now seems to think he is a Tsar, and he knows that Russians ( in spite of their communist history) have always loved a Tsar.
I should not be remotely surprised if the Russians brought about the whole Ukraine conflict from the start by deliberately fomenting internal tensions, just so they could provoke this situation exactly the way it has happened. It would be child's play to do the same with Latvia, Estonia and Lithuania, if he wanted to push the Western Powers to their limits. These states' governments may not be as corrupt as Ukraine's was, but we have to wonder just how reckless and power-hungry Putin is. He has clearly read the history of the USSR and the Bolshevik revolution. He knows that the old Russian Empire broke up after the Revolution, just as it did after Communism, but the Communist government got all 17 states back in the fold by setting up puppet governments in each one and ( probably composed of ethnic Russians) and getting them to send for the Red Army. And there are an awful lot of ethnic Russians in the Baltic states. Planted there long ago for exactly this reason.