PRESIDENT OBAMA made an unscheduled appearance before the press at the White House Friday to warn Russia against a military intervention in Ukraine, which he said would be a “clear violation” of Russia’s commitments to respect Ukraine’s territorial integrity as well as of international law. But the president made no mention of consequences other than international “condemnation” and unspecified “costs” — and Russian President Vladi­mir Putin is unlikely to be deterred by that.

Mr. Obama also spoke in the conditional, referring to “reports of military movements.” But by late Friday it was clear that Russian forces were operating across Crimea, a southern region of Ukraine made up mostly of Russian speakers and that hosts Russia’s Black Sea fleet. Journalists broadcast images of Russian transport planes, helicopters and columns of armored personnel carriers penetrating the region. Ukraine’s acting president decried an “invasion.”

The West responded with phone calls. The Kremlin said Mr. Putin spoke to British Prime Minister David Cameron and German Chancellor Angela Merkel, and Secretary of State John F. Kerry called Russian Foreign Minister Sergei Lavrov. Mr. Lavrov protested that Russia was not violating its commitment to Ukraine’s territorial integrity, and Mr. Kerry was not ready to contradict him. “The question is whether or not what is happening now might be crossing a line in any way, and we’re going to be very careful in making our judgments about that,” he told reporters. Again, hardly likely to alter Mr. Putin’s calculations.

Perhaps it was difficult for U.S. intelligence to discern Friday exactly what the Russian forces — some of which wore no identification — were doing. But Mr. Putin’s likely objective was not difficult to figure. He appears to be responding to Ukraine’s overthrow of its pro-Kremlin government last week with an old and ugly Russian tactic: provoking a separatist rebellion in a neighboring state, using its own troops when necessary.

Frozen conflicts” created by Moscow in the territory of the former Soviet Union already include the Transnistria region of Moldova and the Georgian provinces of South Ossetia and Abkhazia, which Russia invaded in 2008. For Moscow, the incursions have the benefit of establishing dominion over strategic strips of territory while destabilizing the countries to which they nominally belong. Stripping Ukraine of Crimea would be Mr. Putin’s way of putting his foot on the neck of the country’s fragile interim government, which is already facing a severe economic crisis.

As Mr. Obama noted, it would also be a blatant violation of Russia’s commitments, including the 1994 Budapest memorandum, in which Moscow pledged to the United States and Britain that it would respect Ukraine’s independence and borders in exchange for the removal of its nuclear weapons. It could create another chronic trouble spot in Europe — particularly as Crimea’s population includes a large minority of Ukrainian speakers as well as Crimean Tatars, a Muslim group that rejects Russian rule.

Mr. Obama and European leaders must act quickly to prevent Ukraine’s dismemberment. Missing from the president’s statement was a necessary first step: a demand that all Russian forces — regular and irregular — be withdrawn from all parts of Crimea outside the Sevastopol naval base and that Moscow recognize the authority of the Kiev government in the region. If Mr. Putin does not comply, Western leaders should make clear that Russia will pay a heavy price — not just in “condemnation,” but in economic and diplomatic sanctions.