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 assistance. 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.