With the White House asserting that Russia is stoking instability in eastern Ukraine, President Barack Obama is once again faced with the complicated reality of following through on his tough warnings against overseas provocations.

 Obama has vowed repeatedly to enact biting sanctions against Russia's vital economic sectors if the Kremlin tries to replicate its actions in Crimea, the peninsula it annexed from Ukraine, elsewhere in the former Soviet republic. Despite those warnings, Russian President Vladimir Putin appears to be testing Obama's limits, instigating protests in eastern Ukraine, the White House says, and massing tens of thousands of troops on the border, but so far stopping short of a full-scale military incursion.

"They have been willing to do things to provoke the situation that no one anticipated," Matthew Rojansky, a Russia analyst at the Wilson Center, said of Russia. "It's such a high-stakes, high-risk situation, and here they are right in the middle of it."

For Obama, the U.S. response to the chaos in Ukraine has become more than a test of his ability to stop Russia's advances. It's also being viewed through the prism of his decision last summer to back away from his threat to launch a military strike when Syria crossed his chemical weapons "red line" — a decision that has fed into a narrative pushed by Obama's critics that the president talks tough, but doesn't follow through.