"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. M. 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.