For Vladimir Putin the break-up of the Soviet Union was the “greatest geopolitical catastrophe” of the 20th century. Everyone has something that makes them tick. Putin’s obsession is the restoration of Russia’s pride through the restoration of its imperium.

The Russian seizure of control of the Crimean Peninsula, a clear violation of the very international law Putin likes to invoke, has turned Ukraine into a European tinderbox. Sarajevo and the Sudetenland: Europe’s ghosts hover. Putin argues he is protecting Russian-speakers from the usurpers of Kiev, a pro-European government seen in Moscow as the undercover agents of a predatory West whose talk of liberty is mere camouflage for the advance of its interests.

This is baloney, a “trumped-up” Russian case, in Secretary of State John Kerry’s phrase.

It is worth recalling that the catalyst for this crisis was not proposed Ukrainian membership in the European Union. It was not proposed Ukrainian membership in NATO. It was not some threat to Russia’s Black Sea Fleet in Crimea. It was, in its infinite banality, a planned trade agreement between Kiev and the European Union.

This was the minnow Putin inflated into a whale through his attempt to strong-arm Ukraine into rejection of the deal, a course the Russian president had followed with equal imperial vehemence elsewhere in Russia’s near-abroad. On this occasion, however, the people rose up, forcing Ukraine’s bungling, sybaritic, trigger-happy president, Viktor Yanukovych, into flight and the arms of his Russian patron.

Putin’s Crimean message to President Obama and the West is clear: Not one inch further. After NATO’s expansion into the Baltic states (and how critical NATO’s protection looks now to Lithuania, Estonia and Latvia), after the European Union’s embrace of the likes of Poland and Romania (freed, like the Baltic states, from the Soviet empire), after the humbling by NATO of Serbia (Russia’s Orthodox ally), after the West’s perceived manipulation of a United Nations mandate to have its way in Libya — after all this the Russian president, as he has already made clear in Syria, is saying: “Game over.”

But this is no game. Putin’s obsession with a 20th-century order, with turning back the clock to before the “catastrophe,” blinds him to the passionate attachment to their nationhood of states liberated from stifling Soviet subjection. There is a grotesque amnesia to Russia’s Ukrainian gambit.

It was in Ukraine, beginning in the 1930s, that Stalin chose to conduct his first experiment in agrarian “utopia,” collectivizing the land, declaring war on the kulaks for grain, and ultimately annihilating the livelihood of the Ukrainian peasant farmer. The result, in 1933, was famine; several million died. The Nazis later did their worst in Ukraine with similar contempt for the very idea of its independence. More millions died.

To imagine Germany today (unthinkable notion) moving into western Poland with a claim of protecting ethnic Germans there conveys some idea of the historical offense Putin has given to many Ukrainians — and of the fear he strikes into other nations with Russian minorities and dire memories of Moscow, like Lithuania.

Obama has said Putin will pay a price. Kerry has spoken of a “huge price.” But the administration’s Syrian equivocations underwrote Putin’s assertiveness and sense of impunity. Options are now limited. This is the Age of Reluctance, a time when American power is dominant but no longer determinant. Americans have turned inward.

The president must lead. Since 1945 America’s security and prosperity have been tied to the steady spread of liberty in a Europe made whole and free. There is a vital U.S. interest in not seeing this process reversed — not in the land of Yalta and the corpse-filled ravine of Babi Yar.

If Ukraine were subjugated to Moscow once more, or dismembered through a Russian annexation of Crimea in flagrant violation of Russia’s own commitments in 1994, Obama would become the president who presided over a watershed diminishment of the trans-Atlantic bond.

Pivot to Asia cannot mean abandonment of Ukraine. Every form of diplomatic, trade and economic pressure should now be mustered by Obama to isolate Putin (China may be ready to help, given its commitment to noninterference); every political means used to buttress the Kiev government; and NATO’s readiness to defend its members should be ostentatiously underscored.

President Obama might say this: “We need to use the United Nations Security Council and believe that preserving law and order in today’s complex and turbulent world is one of the few ways to keep international relations from sliding into chaos. The law is still the law, and we must follow it whether we like it or not. Under current international law, force is permitted only in self-defense or by the decision of the Security Council. Anything else is unacceptable under the United Nations Charter and would constitute an act of aggression.”

The words, of course, are Putin’s. He used them about Syria — a real catastrophe.