There was a lot to criticize about the way President Viktor Yanukovych’s government was thrown out in Ukraine and hurriedly replaced with an interim team. The victorious opposition should have known how critical it was to reassure all groups in that country that their rights would be respected in any new order; instead, one of the Parliament’s first actions was to abolish a law that ensured a legal status for Russian and other minority languages, thus raising fears among Russian speakers that Ukrainian nationalists were taking over.

Yet none of this justifies Vladimir Putin’s cynical and outrageous exploitation of the Ukrainian crisis to seize control of Crimea, nor any other power grab he may be hatching. The United States and the European Union have few effective levers short of military force, which is not an option, to compel President Putin of Russia to back down, but they must make clear to him that he has stepped far outside the bounds of civilized behavior, and that this carries a steep price in international standing and in economic relations. Whatever else they do, the Western powers must provide prompt and substantial assistance to the Kiev government, whose treasury was left bare by Mr. Yanukovych.

Mr. Putin’s claim of an immediate threat to Ukrainian Russians is empty. There were some scuffles in the industrial cities where Russians predominate, but nowhere were Russian speakers or Russian interests seriously threatened — certainly not in Crimea, where Russians are the majority and the Russian Federation has military bases. If anything, Ukrainians there were in danger. And if the Parliament in Kiev was for the moment on a nationalist high, new presidential elections are not far off, and there are plenty of peaceful ways for Mr. Putin to make Russia’s legitimate concerns and national interests clear to the interim rulers.

Mr. Yanukovych fled knowing full well that he would not last long given the public fury over the killings in Kiev’s Independence Square and the shock that would follow once the full scope of his thievery became public. If he thought he had a shred of credibility left, he should have stayed and faced the music. Mr. Putin knows this; his defense of the ousted government is a pretext to tighten Russian control over Crimea, buttress his claims to special rights over what he calls Russia’s “near abroad,” and to humiliate Ukraine, the way he humiliated Georgia in 2008, for looking wistfully westward.