President Barack Obama's stern warning to Ukrainian officials Wednesday 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 deployed chemical weapons.

But Obama's admonition to not "step over the line" during a crackdown on mass protests has raised questions about whether he would be any more effective at matching his words with deeds than he has been in Syria's 3-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.