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 than token sanctions (nor will the EU, given Germany's self-flaggelating energy policy). The danger is that this might snowball into a larger conflict - if Russia thinks the US is no longer a threat, what's stopping the Chinese from going into Taiwan, or the North Koreans causing mischief?
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 than token sanctions (nor will the EU, given Germany's self-flaggelating energy policy). The danger is that this might snowball into a larger conflict - if Russia thinks the US is no longer a threat, what's stopping the Chinese from going into Taiwan, or the North Koreans causing mischief?