Russia may deny it, but it has apparently moved quickly to seize control of the Crimea, an "autonomous" province of Ukraine and long-time base of the Russian Black Sea fleet that pays rent to the Ukrainian government for its use. This marks a new and very dangerous phase of the Ukrainian crisis.

Masked armed men in unmarked military uniforms swarmed into the Crimea's parliament building on Thursday, raised the Russian flag and took control of the Crimea's main airports and airspace Friday, blocking flights from Ukraine. Reports said Russian helicopters and aircraft then landed, disgorging numerous personnel.

Meanwhile, Russia put 150,000 troops on Ukraine's borders on alert and took other military preparations.

U.S. Secretary of State John Kerry said Russia had informed him it had no intention of violating Ukrainian sovereignty, and the State Department said it was seeking to establish the identity of the armed units that have seized control of the Crimea. The Russian fleet denied being involved. But reporters on the scene say it is an open secret that the troops are Russian.

Ukrainian Interior Minister Arssen Avakov accused Russia of supplying the troops and said, "I consider what has happened to be an armed invasion and occupation in violation of all international agreements and norms."

It seems pretty clear that he is right. If the takeover persists - and it has the clear support of the Crimea's inhabitants - Ukraine, already $35 billion in the hole, will lose an important part of its tax base and Russia's lease payments for the naval base. On the positive side, it would also lose its most vocal pro-Russian voters, making an eventual association with the European Union a less divisive issue.

Meanwhile, former President Viktor Yanukovich, who fled his office last weekend, turned up in Russia and urged it to use force to restore him to the presidency. And Russia's Foreign Minister Sergei Lavrov called on the international community to enforce an earlier agreement that would have kept Mr. Yanukovich in office through this year.

Instead, the international community should do all it can to persuade Russia of the folly of open intervention in response to pro-Russian minorities in the rest of Ukraine. But the seizure of the Crimea is probably an irreversible fait accompli.

It is unlikely that the inevitable outrage about this coup in Kiev, the European Union, the United States and the United Nations will suffice to persuade Russia to undo its mischief, which is likely to take the form of a nominally independent Crimean republic allied to Russia. But Russia might be made to pay a price through trade sanctions.

Perhaps the fairest deal, from Ukraine's perspective, would be an international finding that Russia had illegally intervened in the Crimea and owed Ukraine a very large payment for damages. International economic pressure might then be sufficient to make Russia pay.

That's a rosy scenario.

The darker ones may see the tanks roll.