IT TOOK Vladimir Putin less than a day to trample on President Obama’s warning against a Russian military intervention in Ukraine. On Saturday, the Russian president orchestrated a unanimous vote by his rubber-stamp upper house of parliament authorizing invasion not just of the Crimean peninsula — where thousands of Russian troops are already deployed — but of all of Ukraine.

The United States now faces a naked act of armed aggression in the center of Europe by a Russian regime that is signaling its intent to steamroller this U.S. president and his allies. Mr. Obama must demonstrate that can’t be done.

Mr. Obama’s first response on Saturday was to spend 90 minutes on the phone with Mr. Putin. According to the White House, the president offered the Russian ruler peaceful ways to pursue his interests in Ukraine, including the dispatch of international observers to monitor treatment of Russian speakers and internationally-mediated talks with the Ukrainian government.

It’s reasonable to offer Mr. Putin a face-saving way out of the crisis he has triggered. But Mr. Obama needs to be prepared for the very real possibility that, as in his previous invasion of Georgia, Mr. Putin will ignore international condemnations and press ahead.

Mr. Obama has been vague about the consequences of continued Russian aggression. Saturday’s statement spoke only of the suspension of U.S. participation in preparatory meetings for an upcoming G-8 summit in Sochi. Mr. Obama should make clear that its aggression will comprehensively damage all aspects of Russia’s relations with the United States.

Such a policy would start with cancellation of Mr. Obama’s attendance at the Sochi summit and suspension of talks with Russia on trade issues. Mr. Obama should convoke a meeting of the seven industrial democracies that excludes Russia. Mr. Putin should get the message that he will henceforth be a pariah to the democratic world.

Mr. Obama should make clear that he will no longer shrink from applying sanctions to Russian leaders and businesses complicit in aggression or human rights violations. An expanded list of Russian officials subject to visa denials and asset freezes that was drawn up by the State Department late last year should be immediately approved by the White House. Russian officials in the chain of command of the Ukraine invasion, as well as Russian companies and banks operating in Crimea, should be the next targets of financial sanctions.

The most powerful non-military tool the United States possesses is exclusion from its banking system. Mr. Obama should make clear that if Russia does not retreat from Ukraine, it will expose itself to this sanction, which could sink its financial system. Russia’s economy, unlike that of the Soviet Union, is heavily dependent on Western trade and investment. It must be made clear to the Kremlin that the Ukraine invasion will put that at risk.

Many in the West did not believe Mr. Putin would dare attempt a military intervention in Ukraine because of the steep potential consequences. That the Russian ruler plunged ahead shows that he doubts Western leaders will respond forcefully. If he does not quickly retreat, the United States must prove him wrong.