This isn't a 1-dimensional scale (NATO > Russia). China can always weigh in. Europe can have mixed reactions. etc. That's how world wars start, which would be catastrophic for everyone. Nukes are also on the table if Putin really feels cornered.
Economic sanctions won't deter Putin. Europe still has to deal with them for energy, and a pro-west Ukraine is an existential threat to Russia. (Imagine if Mexico were to join the Russian Federation.) This is Russia's sphere of influence, which has historical precedent. Stalin's post WWII foreign policy was to maintain a buffer zone in eastern Europe.
Further, Ukraine has little real geopolitical value to the west. They're not a major trade partner, they don't have strategic resources, or control strategic ports. The west only wants Ukraine in order to gain leverage against Russia.
If you're wondering what the catalyst was, this is basically retaliation for 2014. [0] Russia believes the west is responsible for the revolution that ousted their puppet.
February 22nd, 2014 - protesters control Kyiv and Yanukovych has fled to Russia. [1]
Almost exactly 8 years later (Feb 23rd) this happens.
We have to assume Russia has been unable to "counter coup" Ukraine and regain a foothold, thus leading to this drastic action.
Putin has threatened severe consequences if any other nation meddles in the war, but he's not out for conquest. His mentality is simply: "I'd rather wreck Ukraine before letting it fall into the hands of the west." [2]
Putin will likely stop once the message is received.
[0] https://en.wikipedia.org/wiki/Revolution_of_Dignity
[1] https://web.archive.org/web/20140228013838/http://www.boston...