Before the Ukraine crisis erupted, the alliance’s current chief, Anders Fogh Rasmussen, had invested much time in trying to allay Russian suspicions of NATO. But Poland and the Baltic states were skeptical about the idea of NATO and Russia establishing a durable partnership. They were also worried about NATO going so far out of area to Afghanistan because that mission reinforced a view that Europe and its neighbors were safe and secure. Russia was not a threat, it was believed, so old-fashioned territorial defense was no longer vital.

Of course, historical experiences of Russia conditioned the views of Poland and the Baltic states toward Moscow. At the same time, many Western European NATO countries downplayed their Eastern neighbors’ concerns, even treating them as a second-class tier inside the alliance. Crimea has changed all that—for the moment.

For Poland, Russia’s annexation of Crimea means that the United States will go ahead with its missile defense system, however scaled down it may be. NATO also intends to hold more military exercises in Eastern Europe to show its commitment to territorial defense.

As for non-NATO countries such as Finland and Sweden, they are already taking a hard look at their own defense capabilities and considering closer ties to the alliance. In 2012, Finland acquired a standoff cruise missile from the United States. That bilateral deal confirmed that Finland needed a strong deterrent weapons system—clearly motivated by the threat from Russia as well as the growing geosecurity importance of the Arctic.

But is beefing up territorial defense enough to deter further Russian advances into Ukraine or to bolster the security of other countries sandwiched between NATO and Russia?

Russia took over Crimea without any attempt by Ukraine’s armed forces to prevent it. Perhaps that explains why Ukraine’s interim defense minister, Ihor Tenyukh, was recently forced to resign. The ease with which Putin annexed Crimea may tempt him to invade parts of Eastern Ukraine, which would surely destabilize a country now embarking on a presidential election campaign. As it is, Russian troops are amassing on Ukraine’s borders.

It is hard to find a Western European government or public willing to contemplate the use of force, either by Ukraine’s own military or by some sort of NATO mission. Yet most NATO countries, especially the Eastern members, know what is at stake: the post–Cold War consensus on borders, territorial integrity, and international rules. But it stops there. Even if the United States has rediscovered its interest in NATO, U.S. President Barack Obama has no appetite for resorting to force either.

Instead, the West believes and hopes that its panoply of soft power tools, including tougher sanctions, will stop Putin. But as long as Putin can bank on soft power, there is little reason to believe he will stop meddling in Ukraine or Moldova.

In short, with NATO now returning home to Europe, its rediscovery of territorial defense will reassure Poland and the Baltic states. But that will hardly comfort its increasingly vulnerable non-NATO Eastern neighbors. They are becoming Russia’s new playground, where the West’s values and resolve are being sorely tested.