The crisis in Ukraine was produced by two sets of blunders, neither emanating from Washington. The European Union’s vacillations and — most significantly, of course — Russia’s aggression created the problem. But it will be up to President Obama to show the strength and skill to resolve it.

For years, the European Union has been ambivalent toward Ukraine, causing instability in that country and opposition from Russia. The union’s greatest source of power is the prospect of it offering membership. This magnet has transformed societies in southern and eastern Europe, creating stability, economic modernization and democracy. For that reason, it is a weapon that should be wielded strategically and seriously. In the case of Ukraine, it was not.

Ukraine is the most important ­post-Soviet country that Russia seeks to dominate politically. If Europe wanted to help Ukraine move west, it should have planned a bold, generous and swift strategy of attraction. Instead, the European Union conducted lengthy, meandering negotiations with Kiev, eventually offering it an association agreement mostly filled with demands that the country make massive economic and political reforms before getting much in the way of access, trade or aid with Europe.

But let’s not persist in believing that Moscow’s moves have been strategically brilliant. Vladimir Putin must have watched with extreme frustration in February as a pro-Russian government was toppled and Ukraine was slipping from his grasp. After the Olympics ended, he acted swiftly, sending his forces into Crimea. It was a blunder. In taking over Crimea, Putin has lost Ukraine.

Since 1991, Russia has influenced Ukraine through pro-Russian politicians who were bribed by Moscow to listen to its diktats. That path is now blocked. Princeton professor Stephen Kotkin points out that in the last elections, in 2010, Viktor Yanu­kovych, representing to some extent the pro-Russian forces, won Crimea by nearly a million votes, which is why he won the election overall. In other words, once you take Crimea out of Ukraine — which Putin has done — it becomes virtually impossible for a pro-Russian Ukrainian ever to win the presidency. Remember, Ukraine is divided but not in half. Without Crimea, only 15 percent of the population will be ethnic Russian.

In fact, the only hope that Russia will reverse course in Crimea comes precisely because Putin might realize that his only chance of maintaining influence in Ukraine is by having Crimea — with its large Russian majority — as part of that country.

Putin has also triggered a deep anti-Russian nationalism around his borders. There are 25 million ethnic Russians living outside Russia. Countries such as Kazakhstan, with significant Russian minorities, must wonder whether Putin could foment secessionist movements in their countries as well — and then use the Russian army to “protect” them. In any case, Russia has had to bribe countries with offers of cheap gas to join its “Eurasian Union.” I suspect the cost to Moscow just went up.

Beyond the near abroad, Russia’s relations with countries such as Poland and Hungary, once warming, are now tense and adversarial. NATO, which has been searching for a role in the post-Cold War era, has been given a new lease on life. Moscow will face some sanctions from Washington and, almost certainly, the European Union as well. In a rare break with Russia on the U.N. Security Council, China refused to condone Russia’s moves in Crimea. Moscow’s annexation of Abkhazia and South Ossetia from Georgia was recognized by Nicaragua, Venezuela and two island nations in the South Pacific. That might be as many as will recognize the annexation of Crimea.

I have generally been wary of the calls for U.S. intervention in any and every conflict around the world. But this is different. The crisis in Ukraine is the most significant geopolitical problem since the Cold War. Unlike many of the tragic ethnic and civil wars that have bubbled up over the past three decades, this one involves a great global power, Russia, and thus can and will have far-reaching consequences. And it involves a great global principle: whether national boundaries can be changed by brute force. If it becomes acceptable to do so, what will happen in Asia, where there are dozens of contested boundaries — and several great powers that want to remake them?

Obama must rally the world, push the Europeans and negotiate with the Russians. In this crisis, the United States truly is the indispensable nation.

Read more from Fareed Zakaria’s archive, follow him on Twitter or subscribe to his updates on Facebook.