WASHINGTON (AP) -- For President Barack Obama, Russia's aggressive annexation of Crimea is testing central tenets of his foreign policy philosophy: his belief in the power of direct diplomacy, his preference for using economic sanctions as punishment and his inclination to proceed cautiously in order to avoid creating larger long-term problems.
The question facing the White House now is whether actions that have done little to stop Russia from claiming Crimea are tough enough to stop further escalations by Moscow. And if they continue to prove insufficient, what else is Obama willing to do to change Vladimir Putin's calculus?
The menu of additional options appears limited. The White House says a military response is not being considered, and officials have so far resisted calls to supply Ukraine's fledgling government with military equipment. Instead, the U.S. is likely to focus on financial assistance to Ukraine and deepening economic sanctions against Russian officials whom the White House deems responsible for the crisis.
White House spokesman Jay Carney vowed Wednesday that "more action will be taken." He indicated that financial penalties could spread to the Russian arms sector, wealthy oligarchs and additional Kremlin officials.
And Vice President Joe Biden, trying to soothe concerns in nations on Russia's borders, said in Lithuania that the U.S. will respond to any aggression against its NATO allies. He declared, "We're in this together with you."
But thus far, sanctions levied by both the U.S. and the European Union have done little to deter Russian President Putin. Nor have Obama's direct appeals to Putin in four lengthy phone calls or his efforts to isolate Russia internationally by rallying allies to suspend preparations for the economic summit Putin was scheduled to host this summer.
"We have gotten ourselves backed into a pretty bad corner," says Rosa Brooks, an international law professor at Georgetown University who served in the Pentagon during Obama's first term. "Putin quite correctly calculated that there's really not much we can do."
Almost every punishment or warning from the U.S. has been followed by defiance from the Russian leader. Hours after the U.S. and EU imposed their first round of asset freezes and other sanctions against Russian and Ukrainian officials, Putin formally recognized Crimea's independence from Ukraine. The following day, he signed a treaty making Crimea Russian territory.
"If you push a spring too hard, at some point it will spring back," the Russian leader said in a fiery speech Tuesday. "You always need to remember this."
The crisis in Crimea has become a flashpoint in a new dispute between East and West. Russia moved troops into the peninsula after Ukraine's Kremlin-backed president fled the capital of Kiev amid rallies protesting his decision to abandon plans for deepening ties with Europe. On Sunday, voters in Crimea overwhelmingly cast ballots in favor of joining Russia. On Wednesday, Russian forces seized military installations across Crimea.
The White House has decried Russia's maneuvers as a violation of international law and does not recognize Moscow's annexation of Crimea.
Putin's actions have opened Obama to fresh criticism from Republicans, who argue that the second-term president, already politically weakened at home, now looks wobbly on the world stage.
Republican Sens. John McCain of Arizona and Lindsey Graham of South Carolina have called on Obama to provide military assistance to Ukraine in the form of small arms and ammunition, as well as non-lethal assistance to the government in Kiev.
"The West must impose real costs on Russia for its aggression in Ukraine. By failing to do so, we only invite further aggression elsewhere," the two senators said in a statement.
Other lawmakers, including Virginia Rep. Eric Cantor have called on the U.S. and its international partners to revoke Russia's membership in the Group of Eight. Cantor, the No. 2 Republican in the House, also urged the administration to increase energy exports to weaken what he called Russia's "stranglehold" on oil and gas supplies to Ukraine and much of Europe.
Administration officials privately acknowledge there is little chance Putin will give up Crimea, a strategically important peninsula that has long housed a Russian military base. Instead, the most pressing U.S. concerns are now cooling tensions in Crimea, where both Ukraine and Russia have troops, and preventing Putin from pushing into areas of eastern Ukraine that have similarly pro-Russian populations.
Secretary of State John Kerry said any further Russian incursion into eastern Ukraine would be a "major breach." But he declined to give specifics on how the U.S. would respond.
Even as the U.S. and Europe talk tough, there are practical concerns on both sides of the Atlantic that are likely to factor into future decisions about punishing Russia.
European nations, including powerful Germany, have deep economic ties to Russia and fear Putin could retaliate financially if the EU ordered tougher sanctions. The U.S. is also dependent on Russia keeping open supply routes the Pentagon is using to withdraw from Afghanistan, and relies on Putin's cooperation on an agreement to strip Syria of its chemical weapons stockpiles.
And perhaps most pressing for Obama is Russia's partnership in tense international negotiations with Iran, which are aimed at blunting the Islamic republic's nuclear program.
The Russians aren't interested in easing those concerns. A top Russian diplomat told the Interfax news agency Wednesday that Moscow may revise its stance in the nuclear talks in response to actions taken by the U.S. and Europe.
EDITOR'S NOTE: Julie Pace has covered the White House for The Associated Press since 2009. Follow her at http://twitter.com/jpaceDC