Rebel separatists continue to battle Ukrainian troops, but the war can no longer be considered solely a rebellion. As Russian troops have invaded the sovereign territory of Ukraine, the U.S. and its European allies have to ramp up every means short of military intervention to make Russia relent.

In a press conference on Thursday, President Barack Obama repeatedly ruled out an armed intervention by U.S. troops, while decrying Russia's latest aggression against its former satellite state.

Mr. Obama declared that existing economic sanctions are having an effect and that Russia is isolating itself further from its "trading partners" by its military action in Ukraine. Unfortunately, it's not clear that Russian President Vladimir Putin is overly concerned about the West, except to block a pending trade agreement between Ukraine and the European Union.

While Mr. Putin continues to dissemble about Russia's role in the Ukraine, the facts are really no longer in dispute. Satellite images have shown Russian armored troop carriers, tanks and artillery crossing the border into Ukraine. And a leader of the separatists says that 4,000 "off-duty" Russians have joined them in beleaguered eastern Ukraine provinces, where the government has been pushing back the rebels.

"There are active soldiers fighting among us who preferred to spend their vacation not on the beach, but with us, among their brothers, who are fighting for their freedom," Aleksandr Zakharchenko, prime minister of the self-declared Donetsk People's Republic, said in an interview on Russian state-run television.

In a meeting of the United Nations Security Council on Thursday, the U.S. representative, Samantha Power, rightly denounced the double game being played by Russia.

"Instead of listening," she said, "instead of heeding the demands of the international community and the rules of the international order, at every step, Russia has come before this council to say everything except the truth. It has manipulated. It has obfuscated. It has outright lied. So we have learned to measure Russia by its actions and not by its words."

The challenge for U.S. and European officials is to figure out a way short of military involvement to stem Mr. Putin's aggression.

Russia's actions this week suggest that at a minimum it is planning to carve out the coal-rich region known as the Donbas and link it to Crimea, while undermining the economy of the rest of Ukraine. A leading theory is that a successful link-up between Ukraine and the European Union - the clear objective of Ukrainian voters - and a flourishing Ukrainian democracy would, by example, weaken Mr. Putin's autocratic grip on Russia.

Whatever the reason, President Putin's violation of the 1994 Budapest pact in which Russia guaranteed Ukraine's territorial integrity, and his trampling of international law in general make it clear that Ukraine needs help with economic assistance, advanced weaponry and stronger sanctions on Russia. And the U.S. can't do it alone. The European Union should respond to Russia's challenge to its right to strike a trade deal with Ukraine.

So far, the West has seen its warnings to Russia utterly ignored.

Now, the U.S. and its allies must redouble their commitment. They must rally behind the democratically elected government of Ukraine and show Mr. Putin that he cannot succeed in his attempt to carve it up or dominate it by intimidation.