WITH RUSSIAN troops still massed near Ukraine’s borders, some in Washington are debating whether the United States and NATO should rush weapons or even troops to bolster the fragile interim government in Kiev. It’s the wrong question. While U.S. and European leaders should not be ruling out arms for Ukraine’s military, Western supplies could no more stop a Russian incursion than they prevented the 2008 invasion of Georgia.

Russia’s seizure of Crimea and Vladimir Putin’s dangerous revanchism will require an overhaul of NATO’s deployments and strategy in Europe. That should mean more forward deployments in front-line states such as Poland, Estonia and Latvia and better preparation for Moscow-initiated cyber­attacks.

The most important means of containing the threat, however, will be economic and political. That includes blocking a potential invasion of eastern Ukraine: The best way to stop one is for President Obama, German Chancellor Angela Merkel and other leaders to spell out, more forcefully than they have, what economic sanctions will be leveled against Russia in the event of an incursion.

Mr. Obama has laid the groundwork for measures against energy companies, banks and other sectors of the Russian economy. But he could be clearer about how far the United States is prepared to go. In a major speech in Brussels on Wednesday, the president was vague, saying that “the toll on Russia’s economy and its standing in the world will only increase” if it “stays on its current course.” Does that mean steps to exclude the largest Russian banks from the global dollar economy and to target the foreign assets of energy companies such as Gazprom and Rosneft? Are European states ready to stop arms sales to Russia, seize the bank accounts and homes of Russian oligarchs or suspend imports of Russian energy? Better to summon the resolve for such measures now — and let the Kremlin know.

The West also must prepare for the possibility that Mr. Putin will eschew further military action in favor of a more patient and insidious strategy. He could seek to destabilize Ukraine by cutting off trade, suspending deliveries of energy or encouraging Russia’s allies inside the country to disrupt planned elections.

While promising to act in the event of an invasion, Western governments have been less clear about how they would answer such a non-military offensive. They should declare that any Russian attempt to disrupt the Ukrainian economy or political system is unacceptable and will prompt sanctions as surely as would a military attack.

In his Brussels speech, Mr. Obama forcefully described Mr. Putin’s assault on the liberal international order while saying that “this is not another Cold War that we are entering into” and that there is no “military solution” in Ukraine. That is true; but it is also the case that, as during the Cold War, the United States and its allies must be ready to sustain a resolute policy for an extended period, to accept economic sacrifices and to forgo easy compromises that would sacrifice the self-determination of Ukraine and its neighbors, democracy or human rights. Mr. Putin’s Russia may not be equal to the Soviet Union, but the defeat of his aggression is anything but a foregone conclusion.