From Max Fisher:
…There’s more behind this confusion than just careful diplomacy. Russian President Vladimir Putin learned a crucial lesson from Syrian leader Bashar al-Assad last year, when the latter got away with using chemical weapons against his own people.
That lesson is this: the Western world can set all the red lines it wants — don’t use chemical weapons, don’t invade sovereign countries — but if you cross that red line just a little bit at a time, inching across over weeks and months, rather than crossing it all at once, then Western publics and politicians will get red-line fatigue and lose interest by the time you’re across.
Read more at Let's be clear about this: Russia is invading Ukraine right now – Vox.