This interpretive frame may be hard to understand, but some things are not wrong just because Russians happen to believe them. Russian news crews were covering a real story in Ukraine: the chaotic dismantling of a legally sanctioned government, the quick breakdown of an agreed framework for new elections, and the creeping transformation of political disputes into ethnic ones.

All of this points to the chief opportunity as well as the chief danger for Mr. Putin. The Crimean affair is a grand experiment in Mr. Putin’s strategy of equivalence: countering every criticism of his government’s behavior with a page from the West’s own playbook. If his government has a guiding ideology, it is not the concept of restoring the old Soviet Union. It is rather his commitment to exposing what Russian politicians routinely call the “double standards” of American and European foreign policy and revealing the hidden workings of raison d’état — the hardnosed and pragmatic calculation of interests that average citizens from Moscow to Beijing to New Delhi actually believe drives the policies of all great powers.

The United States typically interprets its own actions through the lens of its principles. It reads the principles of other countries from their behavior. In most instances that leads to precisely the hypocrisies that Russia, China and other countries find so easy to condemn.

But Crimea is different, and the results are potentially disastrous. With rival militias now forming on the peninsula and the Russian flag flying over government buildings in parts of southeastern Ukraine, the immediate task of diplomacy is to rescue Ukraine from the consequences of its accidental revolution.

First, the European Union, the United States, and Russia must all agree that the principal goal is to prevent greater violence. The Ukrainian government must block armed groups from traveling to Crimea from other parts of Ukraine, just as Russia must halt the formation of pro-Russian militias in Crimea and beyond. Rather than returning to base, as President Obama has demanded, Russia must use the current disposition of Russian forces to police the very situation they have created. Any confirmed transfer of arms to civilians or non-uniformed soldiers from Russian stores should be treated as a violation of the United Nations charter.

Second, European and American officials must be clear on the reasons why the international community should band together to condemn Russian actions. It is not because of the violation of national sovereignty — a concept imperfectly defended by Americans and Europeans in recent years — but because Mr. Putin’s reserving the right to protect the “Russian-speaking population” of Ukraine is an affront to the basis of international order. Not even the alleged ultranationalists who Mr. Putin claims now control the Ukrainian government have tried to export their uprising to Ukrainian speakers in Poland, Moldova, or Romania, or indeed Russia itself. It is Mr. Putin who has made ethnic nationalism a defining element of foreign policy.

Russia was in fact a pioneer of the idea that, in the jargon of international affairs, is now called R2P: the responsibility to protect. Under Czar Nicholas I, Russia asserted its right to guarantee the lives and fortunes of Orthodox Christians inside the territory of its chief strategic rival, the Ottoman Empire. In 1853 Russia launched a preemptive attack on the Ottomans, sending its fleet out of Sevastopol harbor to sink Ottoman ships across the Black Sea. Britain, France and other allies stepped in to respond to the unprovoked attack. The result was called the Crimean War, a conflict that, as every Russian schoolchild knows, Russia lost.

The future of Ukraine is now no longer about Kiev’s Independence Square, democracy in Ukraine or European integration. It is about how to preserve a vision of Europe — and, indeed, of the world — where countries give up the idea that people who speak a language we understand are the only ones worth protecting.