When the USSR fell, Reagan promised Gorbechev that NATO would not expand. What's the point of NATO when there was no more USSR/Warsaw pact, it was "The End of History" after all.... Well NATO kept expanding East, closer and closer to Russian borders. Western propaganda will tell you NATO is a defensive alliance, but it's totally an offensive alliance to project US-Western power, just look at how they destroyed Libya.

With the NED/CIA maidan coup in Ukraine, Ukraine turned into a hotbed for Russophobia. The Ukraine military was getting trained by NATO, and there were talk of Ukraine joining NATO. That was the red line for Russia.