Ukraine's President Viktor Yanukovich abruptly abandoned his post last weekend, leaving Ukraine's protest movement momentarily victorious. It was an ignominious defeat for a corrupt leader who has sought to counter at every turn the popular forces opposed to his misrule.

But virtually at the moment of victory, the Ukraine faces the strong possibility that the nation's eastern Russian-speaking provinces will secede and call in Russian troops to defend them, opening up a new chapter in Ukrainian turmoil.

Avoiding this potentially chaotic split and turning the victory for the Ukraine's pro-democracy movement into a peaceful and stable government will call for wise decisions by the Ukraine's new leaders, firm support for a peaceful outcome by Europe and the United States, and a Russian decision to stop meddling in the Ukraine. Good luck on that last condition. Time Magazine reported that Russian diplomats have already signed a declaration with pro-Russian Ukrainian politicians calling for resistance to the new government and the formation of militias "in cooperation with regional security structures," an apparent reference to Russian forces.

The threat of a Russian military move into the Ukraine led U.S. National Security Adviser Susan Rice and British Foreign Minister William Hague to issue warnings Sunday. Rice said such a move would be a "grave mistake."

Secession sentiment apparently is strongest in the Russian-speaking Crimea, an isolated peninsula that already considers itself an "autonomous republic" within the Ukraine. A major Russian naval base at Sevastapol has nearly 30 years to run on a lease from the Ukrainian government, so there are already 25,000 Russian military personnel in the Crimea, giving Russia strong leverage.

Russian President Putin would like to undo the breakup of the Soviet empire that led to Ukrainian independence and create a "Eurasian Customs Union" trading bloc to compete with the European Union. He used a combination of trade sanction sticks and economic support carrots last year to persuade the Ukraine's Yanukovich to turn down an association with the European Union and create closer economic ties to Russia. It was that decision that triggered the student protests in November that culminated in the collapse of Mr. Yanukovich's authority on Saturday.

The Ukraine also has another problem to deal with - a plummeting economy. Russia is contributing to the problem by putting on hold the economic support it offered former president Mr. Yanukovich.

The interim Ukraine government must take steps to shore up national support in the eastern provinces that lean to Russia. Meanwhile, Europe and the United States should move swiftly to prop up the Ukrainian economy, and counter Mr. Putin's effort to undermine it.

EU and U.S. support is essential to bolster the Ukraine's democratic forces and possibly avoid a civil war in Eastern Europe. Mr. Putin shouldn't be allowed to seize this moment to realize his imperial dreams.