When pro-Russian activists in the eastern Ukraine seized government offices on Sunday they posed the threat of a "serious escalation" of the crisis in Eastern Europe, as the White House said Monday, citing evidence that the agitators were paid agents of Russia. The threat presents a major challenge to the United States, Europe and NATO.

The threat is that Russian President Vladimir Putin will follow through as he did in Crimea, provoking and then taking advantage of unrest to acquire neighboring territory belonging to Ukraine. The industrialized and resource rich east is the most prosperous region of Ukraine.

Mr. Putin might stop short and use his considerable economic and military leverage to force conditions on Ukraine's government that will make it even more dependent on Russia than it already is. That would be a terrible outcome for Ukraine's West-leaning democrats, but there is little that the European Union or the United States could do to stop it. At most they could open their doors to Ukrainian immigrants, giving individuals if not the nation the option of choosing liberty.

But if Russia, through a series of "salami" slices, takes over Ukraine, NATO will find the expansive Russian army on the borders of seven member nations that were once part of the Soviet bloc known as the Warsaw Pact.

That would greatly alarm Poles, Slovaks, Hungarians, Romanians, Latvians, Lithuanians and Estonians who have gained the right through NATO membership to call on the U.S. and other major nations in the alliance for help. The world could see a new costly Iron Curtain being drawn across the center of Europe.

At minimum, economic relations could be heavily and expensively disrupted, something that the EU and President Barack Obama have been careful to avoid so far in the crisis.

The further the Russian land grab goes, the higher the risks for Moscow that the West will be forced to gird itself for a new Cold War. The costs for NATO members would be high, but for Russia they would be disastrous.

But the events over the weekend in Ukraine raise this question: Can President Putin can keep control of events and avoid new provocations that could come back to hurt Russia?

He has deliberately stirred up hopes among Russians living in Ukraine that he will send troops to put them in power. He has exacerbated fears among the Russian public that their compatriots in Ukraine are facing oppressive rule and physical danger. If he now tries to calm things down it could seriously hurt his popularity in Russia.

Mr. Putin has put Russia on a slippery slope that leads to dangerously rising tensions with a Europe that has tried very hard to put its history of national rivalries behind it after two terrible World Wars in the last century.

With nearly 70 years of European integration at stake, the odds are that the nations of NATO, including the U.S., will understand the danger of letting Mr. Putin succeed and find ways to make him pay.

Unless he can pull back soon, Russia would also end up suffering serious economic and diplomatic consequences.