I think all the trouble in this case really started in April, 2008, at the NATO Summit in Bucharest, where afterward NATO issued a statement that said Ukraine and Georgia would become part of NATO. The Russians made it unequivocally clear at the time that they viewed this as an existential threat, and they drew a line in the sand.
https://www.newyorker.com/news/q-and-a/why-john-mearsheimer-blames-the-us-for-the-crisis-in-ukraine
