12 former USSR countries are now in NATO and Putin would not dare to attack a NATO country. With hindsight, not fast tracking Ukraine into NATO was our error, but Ukraine itself was confused on the issue.
We also did too little after Russia annexed Crimea in 2014. They got away with it then, and that encouraged them to attack Ukraine itself.
Trump was a big Putin fanboy (and still is) and did not mention Crimea in their frequent calls and meetings. Trump tried to unfund NATO, lobbied to readmit Russia into the G8, and heaped praise where ever he could on Putin. The tinpot dictator must have been been kicking himself he didn’t take the whole country in 2014.