http://www.vox.com/2014/8/27/6069415/lets-be-clear-about-this-russia-is-invading-ukraine-right-now

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.