Vladimir Putin has done this before.
When he invaded Georgia in August 2008, Western diplomacy and pressure denied him his ultimate goal: marching to Tbilisi and deposing Georgia's democratically elected government. But Putin seized two areas, Abkhazia and South Ossetia, which Russian troops occupy to this day.
The United States and its European allies imposed diplomatic and considered economic sanctions on Russia. The goal was to convince Putin that the strategic costs of his action outweighed the tactical benefits and to deter him from similar actions.
These measures were reversed in the "reset" of relations with Russia that began in 2009. In retrospect, the measures were inadequate and their reversal premature. Putin was not deterred. Crimea is now in the hands of the Russian military, and Putin is projecting military power into the heart of Europe.
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When the Cold War ended and the Soviet empire dissolved, the United States and its allies sought to build a Europe whole, free and at peace -- one with which Russia would find its peaceful place. Europeans increasingly, and rightly, took the lead.
Putin now appears to have had a different agenda: to reconstruct what he could of the former empire but on a Russian model rather than Soviet. He has been cunning and shrewd. His preferred tools of intimidation and blackmail succeeded in keeping Armenia and Belarus in Moscow's orbit.
Where those tools were inadequate, he turned to force, moving incrementally -- first in Georgia, now in Ukraine -- so as not to destroy economic and diplomatic relations with Europe or the United States.
The good news is that his strategy is largely self-defeating. His occupation of Abkhazia and South Ossetia ensured that Georgia became even more determined to withstand Russian domination and pursue integration with Europe. His seizure of Crimea is equally likely to alienate Ukrainians in all parts of that country.
But there is a method to Putin's moves. The territorial disputes his actions create give Europeans pause in considering further integration of those countries into the European Union, NATO and other Western institutions.
This leaves the door open for further Russian pressure to join Putin's Eurasian Union and to accept increased integration into the Russian sphere.
In the short run, the U.S. and European objective will be to prevent further Russian encroachment on Ukrainian territory and to roll back the takeover of Crimea. That would require Putin to engage Ukrainian authorities, return his troops to the status quo ante and let the Ukrainian people sort out their governance of Crimea.
Even as they work to check Russia's move with condemnations and economic sanctions, which must take effect immediately, the United States and Europe must also help the interim government in Kiev survive and lay the groundwork for a more democratic and less corrupt Ukraine. The important signal of support from Secretary of State John Kerry's visit to Kiev should be backed up by significant economic and military aid.
The United States and Europe also need to develop a strategy for reassuring allies in Central and Eastern Europe and for deterring Putin from seeking again to extend Russia's reach by force.
That strategy should begin with a clear U.S. recommitment to NATO and to Europe's security. This could include deploying and exercising NATO forces in Poland, the Baltic states and Romania in the near term while halting the drawdown of U.S. forces and facilities underway across the continent.
President Obama could meet NATO leaders in Brussels this month to demonstrate this resolve and to prepare a more meaningful NATO summit planned for September.
Second, the United States and Europe should demonstrate that Russia's aggression has not undermined their commitment to a Europe whole and free. NATO could invite Montenegro to join this fall, extend a membership action plan to Georgia and restate its commitment to ultimate NATO membership for Ukraine. The European Union could conclude association agreements, advancing integration with Georgia, Moldova and Ukraine.
Finally, Putin must feel the cost of his aggression beyond what was done after his invasion of Georgia. Condemnation by the Group of Seven leaders is only a start. The Obama administration could also reconstitute the Group of Eight without Russia, , ban allied military industrial cooperation or sales and use sanctions to freeze assets and deny visas to Russian leaders and businesses complicit in aggression.
The president should work with Congress to demonstrate bipartisan support for these measures. And he should threaten to deny Russia access to the U.S. and European Union banking systems as the ultimate sanction. If Putin concludes he can get away with occupying Crimea, he won't stop there.
Stephen J. Hadley, was national security adviser from 2005 -2009 and is a member of the executive committee of the Atlantic Council's board of directors. Damon Wilson was senior director for European affairs at the National Security Council from 2007 to 2009 and is executive vice president of the Atlantic Council.