I would add this here - Russia, having previously delivered ultimatums a couple of times, and having received what it wanted, decided that they would continue to calmly achieve their goals by the method of threats, blackmail, ultimatums, and their fifth column in the EU (Merkel, Macron and many others) . They were sure that the EU, the USA, Britain would allow Ukraine to be swallowed up, once again closing their eyes or "expressing concern."
The sad thing is, nobody could stop them and they realized this long time ago. I mean what could the world do about it? The power of nuclear they have, and the willingness to use it if they are not given what they want to creates the biggest issue right now.
What we are looking at right now is the fact that Russia could do whatever it wants, and if you do not do just sanctions, if you do something militarily, like send soldiers, then they will send nukes and that's it, there is no discussion after that and everyone is afraid of that. I do not think that Russia will stop here, they will do whatever they want to do, because nobody else could top them.
If I'm not mistaken, many has tried though, but it's that Putin has enjoy the support of the people, maybe by intimidation or they simply love Putin. But it has reach it's pinnacle by being authoritarian regime that he is not unstoppable as a President and we could even call him a dictator. And they have the bargaining power, with all their oil against the EU countries. So the only way to stop him is to removed him right now but it is going to be very hard without any repercussions inside Russia.