The bigger question is "why now". Ukraine is not joining NATO or the EU any time soon. Russia has de facto annexed Crimea and its ongoing support for separatists in Donbas keeps the region unstable to its advantage. I suspect two reasons: Putin's flagging domestic support and Russia's (and to be honest, the world's) perception that America is currently so divided and weak it will be unable to offer much resistance th…
Difference, at least with regard to China/Taiwan, is timeline. Russia could likely steamroll much of Ukraine in a short period of time, and the US/NATO has few assets available for immediate response to hundreds of thousands of Russian troops. The conflict could potentially be over before any meaningful military response could be mounted outside of what Ukraine can muster (which compared to the Russians only goes so…
It takes, what, 12-20 hours to get a bunch of planes in the air - at that point the Russian soldiers will be smashed unless they can keep control of the air. You can't win a air power immediately, but if it comes to a NATO fight Poland is right next to Ukraine and has some pretty nice equipment and no love for Russia.