Sending troops in unmarked uniforms with unmarked vehicles to annex the Crimean Peninsula under the pretense of protection from ethnic cleansing, which anyone with access to Facebook and Twitter can easily see is completely manufactured by the Russian propaganda machine, without buy-in from the international community, is an obvious act of aggression. Putin keeps upping the ante because nobody ever bothered to call him on his bluff. The international community consistently let Putin cross boundaries with impunity, and he's been getting more and more insolent with each incident. There comes a time when enough is enough.