By Matt Spetalnick
WASHINGTON, Feb 20 (Reuters) - U.S. President Barack Obama's stern warning this week to Ukrainian officials was the closest thing to a "red line" moment he has had since his threat in 2012 to act against the Syrian government if it used chemical weapons.
But Obama's admonition on Wednesday to not "step over the line" in cracking down on mass protests rocking the Ukraine raised questions on whether he would be any more effective at matching words with deeds than he has been in Syria's three-year-old civil war.
His decision to lay down another rhetorical "line" in a geopolitical crisis left many foreign policy experts puzzled, especially given the limited options he has at his disposal for dealing with the Ukraine's spiraling conflict.
"Hasn't he learned his redline lesson?" tweeted Robert Danin, a senior fellow at the Council on Foreign Relations in Washington.
Obama's choice of words evoked comparisons to the chemical weapons "red line" he established for Syrian President Bashar al-Assad and then failed to enforce with military action last year, something critics say undermined U.S. credibility.
Obama warned only vaguely at a summit in Mexico of unspecified "consequences" in the event of escalating violence in Ukraine.
But despite the rhetoric, the White House made clear on Thursday that there were limits to how far the United States would go in punishing Ukrainian officials.
The U.S. president, mindful of Americans' weariness of war in Iraq and Afghanistan, has no intention of even considering the possibility of military intervention in Ukraine.
The reasons are clear: While Syria is a Russian ally long shielded by Moscow, Ukraine is a former Soviet republic that Russia still sees as part of its sphere of influence.
"Right now," White House spokesman Josh Earnest assured reporters, "we're talking about sanctions."
LIMITS OF U.S. LEVERAGE
The limits of U.S. leverage in Ukraine are sure to hamper Washington's ability to quell the country's bloodiest crisis since it won independence from the Soviet Union. At least 75 people have been killed since Tuesday afternoon.
"Syria's really a different story," said Matthew Rojansky, an expert on the former Soviet Union at the Wilson Center in Washington, D.C. But one point in common, he said, is that "the declaration of red lines and consequences has no guarantees."
Obama's conservative critics were quick to deride his threats, including a warning for Ukraine's military to stay out of the crisis, as meaningless.
"Obama says there 'will be consequences' if people 'step over the line' in Kiev. Bashar al-Assad doubled over in laughter," tweeted Denver radio talk-show host Michael Brown, a former Bush administration official who resigned after his controversial handling of Hurricane Katrina in 2005.
But Earnest insisted that Obama had a "range of options ... to hold accountable "those who perpetrate violence against peaceful protesters."
Obama put himself in a tough spot with his "red line" ultimatum to Syria, a phrase he first used at a news conference in August 2012 and repeated thereafter.
Following U.S. findings that Assad's forces killed hundreds of people in a gas attack last August, Obama vowed a military response. But he backtracked in the face of congressional opposition and then agreed to a Russian-brokered deal for international disposal of Assad's chemical stockpile.
Obama's failure to enforce his "red line" remark militarily raised questions among friends and foes in the Middle East on whether he would someday be prepared to make good on his threat of military action against Iran as a last resort to prevent it from developing nuclear weapons.
As a result of what many see as a blurry "red line" on Syria, one Middle East diplomat said of Obama's Ukraine threat: "No one is impressed." (Editing by Alistair Bell, Bernard Orr)