Gentlemen,
You are all smart guys, but sorry you have no idea what is going on in Ukraine (and Russia).
Briefly.
1. Putin is a new czar of Russia. Right now Russia = Putin. His power is absolute. There is no any feedback from people.
2. Putin does not want to annex any part of Ukraine other than Crimea.
3. Putin does not want to start a new world war (he has no resources, his money is in the Western banks or real estate).
4. Putin wants to remain a ruler of Russia until his last day. He is ready to sacrifice millions of lives to achieve his goal.
5. Putin is not Hitler. Putin is much worse.
6. Modern Russia is not a new Soviet Union, it is much weaker.
7. There is no any significant separatist movement in mainland Ukraine. This is a war between Russia and Ukraine.
That's it.
It is very difficult to deal with Putin. However, it is still possible. If the West wants to bring Russia to some controllable condition, IMHO, two things are required:
1. Do not allow Putin to crush Ukraine (by providing Ukraine with modern weapons, recce technology and communications, economical support).
2. Mount economical and political pressure on Russia.
Sooner or later, Russia will follow the path of the Soviet Union.
No NATO troops are required.