After 20 months and tens of thousands of deaths, the civil war in Syria may be reaching a turning point. The ragtag group of rebels that took up arms during the Arab Spring has advanced to the outskirts of Damascus and has a credible chance, if all goes well, of ousting the Assad dynasty that has ruled the country for more than 40 years.

Should the United States, which mostly has confined its involvement to fruitless efforts to oust President Bashar Assad through economic sanctions, belatedly get involved militarily? It's a tough call, especially as the humanitarian crisis deepens, but we remain unpersuaded.

War-weary after prolonged conflict in Iraq and Afghanistan, the Obama administration and its allies early on ruled out direct military intervention in Syria, even the sort of no-fly zone that served as a cover for air attacks on the regime of Moammar Gadhafi in Libya. That was and remains the correct course, with the important exception President Obama again hinted at this week: the possible use of U.S. forces if necessary to prevent Assad from using chemical weapons against his own people.

The harder question has been whether the United States should provide arms directly to the rebels, as some Arab nations have done. During the presidential campaign, Mitt Romney pledged to ensure that the rebels "obtain the arms they need to defeat Assad's tanks, helicopters and fighter jets" -- but only after identifying anti-Assad forces "who share our values."

That has been precisely the rub for the Obama administration, which wants to be sure that a new infusion of arms wouldn't end up in the hands of Islamic fundamentalists, as weapons from other sources apparently have.