I don't know if this has already been discussed in this thread - I admit I haven't read every post - but there are some simple (and not so simple) issues in play here.
In 2014, there was a violent coup in Ukraine. The President of the country fled and a very pro-USA government was installed. Some people say the US was behind it every step of the way, others say it was a popular revolution. I think what you call it is less important than the outcome.
That outcome - and you should read all that you can about it and decide for yourselves - lead to some very important changes: the US got "friends" in Ukraine who would support putting US nuclear missiles right on the Russian border (a 10 minute flight from their biggest population centers), and Russia had to start paying millions (billions?) of dollars a year to transport their gas across Ukraine to their customers in Europe. And not inconsequentially, some US children of important US politicians got some very high-paying jobs with Ukrainian businesses that profited from the new political situation. Does anyone actually think the CIA was NOT involved in this somehow?
To say the least, Russia was highly displeased with this new development. Not to mention a LOT of the Ukrainian people who have strong ties to Russia, family in Russia, speak Russian and don't like the new political leadership. Are they a majority? Do they outnumber the pro-U.S. faction? I don't know, but they exist. And when Russian tanks roll across Ukraine, those people are not going to stand in their way.
Russia gains a LOT from invading Ukraine - access to the Black Sea for shipping, freedom from the threat of US missiles on their border and no more exorbitant payments to move their gas to customers in Europe. Not to mention the opportunity to spit in the eye of that idiot Biden, who publicly declared that if Putin did not invade, it was only because he was afraid of the US president.
I personally don't think Putin will go any farther, but what do I know? I can only say it doesn't appear there is a strong cost/benefit ratio to support further aggression. On the other hand, NATO is certainly a paper tiger, and the US would have a really difficult time justifying a full-scale war against Russia even if they did send their tanks into some small NATO country like Lithuania.
History is unfolding.
