Folks I feel like we are beginning to get into a little war here with one another ourselves, soon we'll need some troops in this forums for peacekeeping...
If I may I just want to express some opinions (my own) and facts regarding the situation given I'm well versed in the history of this part of the world , I also live here (Latvia) and it's rather hard to not know ones history when you live in eastern Europe...
First and foremost, Russia has a long long history of imperialism, the same can be said about Germany and other European countries , we Latvians were under German rule in the Middle ages for centuries, then we were under Russian empire aka the Czar up until 1917, when ofcourse the October revolution happened and the world changed forever.
There is one major difference that helps the Baltic states that doesn't help Ukraine. We were independent established countries before the 1940 Soviet invasion during WW2 and the subsequent Soviet rule from 1945 to 1991.
Latvia as well as other Baltic states established it's freedom in 1918, right after the end of WW1 and while Lenin was busy fighting the opposition within the newly formed USSR.
We here had our own civil war from 1918 to 1921, our newly established government had to fight against both German troops retreating and Soviet staged coup attempts, the Soviets actually set up a government that tried to incorporate us into the USSR as long back as 1919 and 1920, luckily our freedom forces won and the Soviet staged government was sent in exile.
This is just scratching the surface, the real history is much more complex and longer and would require a book.
One of the reasons Putin is so assured of his actions is that Ukraine first emerged as a country only after the fall of he USSR in 1991.
Second reason is that Ukraine and Russia have deep roots , basically they are both ethnically similar as well as historically share much in common. I in no way agree with this as being a basis for invasion but it seems this is partly what Putin bases his ideas on.
https://en.wikipedia.org/wiki/History_of_Ukraine
Third of all, Putin was really angered back in 2014 when he saw that the Ukrainians ousted his loyal president and government. I think he saw that as Ukraine's move towards the west , which by the way Ukraine itself (their government after 2014) said they would want to do.
Ukraine being a strategic point from a geopolitical view, I think Putin decided that he will take it back and reverse the European (possible NATO?) course on Ukraine.
I don't agree with the assessments some here have made that Putin shares some psychological problems, If there is one thing about the KGB , especially back at their peak of being the worlds most feared and effective intelligence agency, then it's safe to say they did not recruit mentally ill people or psychologically weak minds to run their business.
This is all just a cold blooded calculation and chess. Human lives mean nothing to the security forces both of the historical Czar regime as well as the following NKVD, Stalin and later KGB people.
I know this history well, I have read books and even talked to former agents, around here every family had at least someone who was an agent in the KGB.