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.