There are times when a president can't be specific, times when specificity would commit him to a course that later proves untenable. Obama, somewhat of a master at saying something without saying too much, has found the skill very useful in diplomacy. His comments served as a warning to Syrian President Bashar al-Assad without binding the United States to actions that might lead to entanglement in yet another unpopular Mideast war.
Obama could just as easily argue that U.S. intervention is warranted without any additional redlines being crossed. According to the Syrian opposition, Assad is responsible for the deaths of more than 17,000 civilians since the beginning of the uprising in March 2011. And he seems willing to sacrifice many more lives to hang onto power for as long as he can. But this country has learned from the wars in Iraq and Afghanistan how difficult it is to walk into another cultural sphere and try to install a democratic system, even if U.S. troops are capable of winning the war.
Of course, there will be conflicts in the future in which the United States will want to intervene militarily, if taking such action is both necessary and reasonable. But whoever is president must be sure he, or she, is picking the right battles to fight.
It's hard to know exactly who the driving forces in the Syrian opposition are, or what the rebels would do with the country if they managed to defeat Assad. In fact, one of the few things that is clear is that the opposition is splintered and disorganized. That, and that the rebels' fight has attracted foreign jihadists, including members of al-Qaeda.
After hunting down Osama bin Laden, Obama has demonstrated that he is willing to take quick and determined military action whenever it is necessary for the good of the country. However, he is also cognizant that the most important fight for the United States now is to get its economy back on track. He's right to be cautious about Syria.