Mr. Putin’s assault on the idea of Europe is three-pronged. First, as he told his country’s parliament in a March speech justifying the takeover of Crimea, he is waging this war in the name of ethnic nationalism – he is doing so, he said, in order to “defend the interests” of “millions of [ethnic] Russians and Russian-speaking people.”

Second, Mr. Putin is doing this in the name of something very similar to imperialism, albeit without the means or ability to really carry it out: An expressed desire to control any territories where Russian is spoken and secession can be engineered (including not just Crimea and eastern Ukraine but also the periphery of Georgia). And third, as a leader who has effectively ended democracy in his own country, he is attacking Ukraine in opposition to the democratic desires expressed freely and fairly by its people.

In other words, Mr. Putin is challenging the three core ideas of the postwar peace. The Brussels-based institutions of modern Europe were built in order to prevent authoritarianism, imperialism and ethnic nationalism from ever again taking root in the continent and leading it to war. It has worked well.