At perhaps the most consequential NATO summit of the post-Cold War era, Western leaders grappled with questions that had not darkened their counsels for decades. They struggled for a response to Russia's forced annexation of Crimea and stealth military invasion of Ukraine, hoping to avoid further escalating a confrontation with a nuclear-weapons heavyweight. They announced the creation of a modest, 4,000-troop rapid-response force in an effort to revitalize the largely forgotten mission of deterrence, hoping to reassure nervous members in Eastern Europe that there is real military force behind NATO's Article V guarantee of collective defense.

And, perhaps most significantly, Western leaders began an overdue discussion about the nature of revisionist Russian leader Vladimir Putin, whose response to the signature Western project of economic and democratic liberalism has channeled former Soviet leader Joseph Stalin's response to the encroachment of Catholicism: "The pope? How many divisions has he got?"

"The Obama administration has criticized Putin for acting like it's still the 19th century, but the truth is Putin never got the word that the use of military force was outdated, and he annexed Crimea by force and sent armored columns into Ukraine to kill Ukrainians because it worked," said retired Gen. Barry McCaffrey, a decorated combat veteran who spent much of his career in Europe deterring the Soviet Union. "Putin's an intelligent and pragmatic leader in the mold of a Russian czar, and he has correctly read the military balance of power in Europe and decided he can reassert Russian dominance in his neighborhood."

As Putin draws red lines in blood around his "near abroad"—first in Georgia in 2008, and recently in Crimea and Ukraine—Western leaders are now being forced to reassess their signature project of the post-Cold War period: securing a Europe that is democratic, whole, and free. And it is in this that they should consider what responsibility they hold in creating the current crisis.

For while Western leaders have viewed as benign the enlargement of NATO and the European Union and the promotion of democracy from a liberal worldview, Russians have continued to view it through the prism of geopolitics where balance-of-power calculations drive threat assessments.

"The pathway to the current Ukraine crisis began with the expansion of NATO, which was part of a broader Western strategy to peel countries like Ukraine and Georgia from Russia's orbit," says John Meersheimer, a political-science expert at the University of Chicago. "Putin has good reason to think that if the West can promote democracy and do social engineering in Ukraine, it can do it in Russia as well in a way that threatens his own rule."

Indeed, Putin and the Russian national security and foreign policy elite have long opposed every element of that strategy, from the expansion of the NATO military alliance and the European Union economic club toward their border, to the promotion by Western nongovernmental organizations of democracy in places like Georgia (Rose Revolution) and Ukraine (Orange Revolution).

The new dividing line that Putin is enforcing is thus not only between West and East, but between two conflicting worldviews. To continue doubling down in this crisis and acting as if that line does not exist risks further escalating the current crisis into an existential confrontation with an aggrieved nuclear-weapons power.

"We got into this trouble because Western leaders thought geopolitics was something that was killed and buried at the end of the Cold War, which allowed them to ignore what the Russians were saying. But if you look at the issue from Russia's realpolitik or realist point of view, this crisis was foreseeable," Meersheimer said. "George Kennan said back in 1998 that this would happen."

When the United States began the project of expanding the NATO military alliance in the 1990s, cashing in the victor's chips at the end of the Cold War, the goal of securing a Europe "whole and free" seemed not only noble, but suddenly attainable. Millions of Eastern Europeans were newly freed from the yoke of communist tyranny, and hungry for Western prosperity and security guarantees. Russia itself seemed set on the path to democracy. Why would the Western alliance not act to secure such potentially historic gains?

And yet a churlish group of dissenters at the time dampened the celebration by asking their own nagging question: Where was the logical end point of NATO expansion? Every member admitted into the alliance would draw a new dividing line in Europe to replace the old "Iron Curtain," and antagonize Moscow in the process.

Push a triumphant military alliance past the detritus of the old Warsaw Pact and right up to the borders of Russia, they warned, and you risked stoking the Russians' historic fear of invasion and provoking a new Cold War. Suggestions of a New World Order in which Russia itself might one day join the West's signature military and economic clubs was a fundamental misreading of the Russian character and realpolitik mind-set, according to the dissenters, who included such Cold War stalwarts as former national security adviser Brent Scowcroft; then chairman of the Senate Armed Services Committee Sam Nunn, D-Ga.; and George Kennan, the chief architect of the successful Cold War strategy of containment of the Soviet Union.

"I think [NATO expansion] is the beginning of a new Cold War," Kennan said in a 1998 interview with New York Times columnist Thomas Friedman. "I think the Russians will gradually react quite adversely, and it will affect their policies. I think it is a tragic mistake ... it shows so little understanding of Russian history and Soviet history."

And yet with an increasingly authoritarian Russia now pushing back against Western encroachment with military force, millions of Eastern Europeans can breathe easier for being protected under the umbrella of NATO membership as a result of three rounds of enlargement that admitted 12 additional countries. For proponents of NATO expansion the genesis of today's crisis was the stubborn unwillingness of Russian leaders to see stable and democratic neighbors as in their own national interest.

Nicholas Burns helped engineer the second round of NATO expansion as the former U.S. ambassador to NATO. "The end of the Cold War left a security vacuum in central Europe, and the expansion of NATO and the European Union helped create a democratic Europe that truly was free and peaceful, which was a strategic goal of the United States going back a century," said Burns, currently the director of the Future of Diplomacy Project at Harvard's Kennedy School of Government. "Just because Putin has now disrupted that democratic peace doesn't mean we should stop standing up for our values, or abandon a strategy that has achieved freedom for millions of people. "

The problem in the eyes of some experts was not NATO's initial expansion to countries such as Poland, Hungary, the Czech Republic, and the Baltics, but rather that NATO's "open door" policy toward membership had no logical end point. Many experts warned that pushing the military alliance to Russia's borders with plans to admit Georgia and Ukraine, as was declared in NATO's 2008 Summit communique, would be a bridge too far.

"I was among a very few Russian specialists who supported the initial round of NATO expansion, on the basis that those countries did not have common borders with Russia and maintained good relations with Moscow," said Dmitri Simes, president of the Center for the National Interest in Washington, D.C. The problem was a "blind momentum" that built behind NATO's expansion, he said, that ignored increasingly strident Russian objections.

"Now we're in a very dangerous period where Russia feels pushed into a corner, and there is a temptation among influential hardliners in Moscow to not only try and subdue Ukraine, but also to teach one of the Balkan states a lesson to show that NATO's Article V guarantee of collective defense is hollow," said Simes. "If cooler heads don't prevail this could start to look a lot like 1914 just before the outbreak of World War I, only with both sides now armed with nuclear weapons."