putin basically has a layup to destroying any confidence of NATO members toward the United States and the organization itself.
1. get trumps assurance that the US wont intervene if Montenegro is occupied.
2 Do that. They could also get Serbia to do it under the pretext of "anti-Serb agression" in Montenegro ala eastern Ukraine.
3. If the US doesnt respond Article 5 is invalidated. That will lead to a huge crisis of faith. I think a lot of eastern bloc countries would leave NATO and seek formal military agreements with Russia to avoid the threat of occupation, knowing the United States will not protect them in that case.