Should Russia's invasion and looming annexation of Crimea be blamed on President Barack Obama? Of course not, just as it should not be blamed on NATO expansion, the Iraq war or Western interventions to stop ma** atrocities in the Balkans and Libya. The blame lies squarely with Vladimir V. Putin, an unreconstructed Russian imperialist and K.G.B. apparatchik. But in a broader sense, Crimea has exposed the disturbing lack of realism that has characterized our foreign policy under President Obama. It is this worldview, or lack of one, that must change. For five years, Americans have been told that “the tide of war is receding,” that we can pull back from the world at little cost to our interests and values. This has fed a perception that the United States is weak, and to people like Mr. Putin, weakness is provocative. That is how Mr. Putin viewed the “reset” policy. United States missile defense plans were scaled back. Allies in Eastern Europe and Georgia were undercut. NATO enlargement was tabled. A new strategic arms reduction treaty required significant cuts by America, but not Russia. Mr. Putin gave little. Mr. Obama promised “more flexibility.” Mr. Putin also saw a lack of resolve in President Obama's actions beyond Europe. In Afghanistan and Iraq, military decisions have appeared driven more by a desire to withdraw than to succeed. Defense budgets have been slashed based on hope, not strategy. Iran and China have bullied America's allies at no discernible cost. Perhaps worst of all, Bashar al-Assad crossed President Obama's “red line” by using chemical weapons in Syria, and nothing happened to him. For Mr. Putin, vacillation invites aggression. His world is a brutish, cynical place, where power is worshiped, weakness is despised, and all rivalries are zero-sum. He sees the fall of the Soviet Union as the “greatest geopolitical catastrophe of the century.” He does not accept that Russia's neighbors, least of all Ukraine, are independent countries. To him, they are Russia's “near abroad” and must be brought back under Moscow's dominion by any means necessary. What is most troubling about Mr. Putin's aggression in Crimea is that it reflects a growing disregard for America's credibility in the world. That has emboldened other aggressive actors — from Chinese nationalists to Al Qaeda terrorists and Iranian theocrats. Crimea must be the place where President Obama recognizes this reality and begins to restore the credibility of the United States as a world leader. This will require two different kinds of responses. The first, and most urgent, is crisis management. We need to work with our allies to shore up Ukraine, rea**ure shaken friends in Eastern Europe and the Baltic States, show Mr. Putin a strong, united front, and prevent the crisis from getting worse. This does not mean military action against Russia. But it should mean sanctioning Russian officials, isolating Russia internationally, and increasing NATO's military presence and exercises on its eastern frontier. It should mean boycotting the Group of 8 summit meeting in Sochi and convening the Group of 7 elsewhere. It should also mean making every effort to support and resupply Ukrainian patriots, both soldiers and civilians, who are standing their ground in government facilities across Crimea. They refuse to accept the dismemberment of their country. So should we. Crimea may be falling under Russian control, but Ukraine has another chance for freedom, rule of law and a European future. To seize that opportunity, Ukrainian leaders must unify the nation and commit to reform, and the West must provide significant financial and other a**istance. Bipartisan legislation now before Congress would contribute to this effort. More broadly, we must rearm ourselves morally and intellectually to prevent the darkness of Mr. Putin's world from befalling more of humanity. We may wish to believe, as President Obama has said, that we are not “in competition with Russia.” But Mr. Putin believes Russia is in competition with us, and pretending otherwise is an unrealistic basis for a great nation's foreign policy. Three American presidents have sought to cooperate with Mr. Putin where our interests converge. What should be clear now, and should have been clear the last time he tore apart a country, is that our interests do not converge much. He will always insist on being our rival. The United States must look beyond Mr. Putin. His regime may appear imposing, but it is rotting inside. His Russia is not a great power on par with America. It is a gas station run by a corrupt, autocratic regime. And eventually, Russians will come for Mr. Putin in the same way and for the same reasons that Ukrainians came for Viktor F. Yanukovych. We must prepare for that day now. We should show the Russian people that we support their human rights by expanding the Magnitsky Act to impose more sanctions on those who abuse them. We should stop allowing their country's most corrupt officials to park ill-gotten proceeds in Western economies. We should prove that countries like Ukraine, Georgia and Moldova have a future in the Euro-Atlantic community, and Russia can, too. We must do all we can to demonstrate that the tide of history is with Ukraine — that the political values of the West, and not those of an imperial kleptocracy, are the hope of all nations. If Ukraine can emerge from this crisis independent, prosperous and anchored firmly in Europe, how long before Russians begin to ask, “Why not us?” That would not just spell the end of Mr. Putin's imperial dreams; it would strip away the lies that sustain his rule over Russia itself. America's greatest strength has always been its hopeful vision of human progress. But hopes do not advance themselves, and the darkness that threatens them will not be checked by an America in denial about the world as it is. It requires realism, strength and leadership. If Crimea does not awaken us to this fact, I am afraid to think what will.