Having determined that the government of Syrian President Bashar Assad has used chemical weapons against his opponents in the country's bloody two-year civil war, the Obama administration is now reportedly preparing to send lethal military aid to rebel forces battling the regime. Mr. Obama said earlier this year that any use of chemical weapons by the Syrian military would cross a "red line" that invited a U.S. response. Now that American intelligence has confirmed Syria has crossed that line, the U.S. response must be measured but leave no doubt that the use of such weapons will not be tolerated.

Reports today indicated the U.S. is considering sending shipments of small arms and ammunition to moderate rebel groups, who are increasingly desperate after having suffered a string of military setbacks in recent weeks. But some opposition leaders are concerned that the aid being contemplated is too little, too late. The rebels say they need heavy weapons, including anti-tank and anti-aircraft missiles, to blunt the government's attacks and alter the military balance on the ground. But so far the U.S. has been reluctant to supply such weapons (particularly anti-aircraft missiles), fearing they could fall into the hands of Islamic extremists who eventually might use them against America's allies in the region.


That's left the U.S. and European nations such as Britain and France scrambling to rethink their policies toward Syria at a time when the international community, including Russia, is attempting to organize peace talks in Geneva next month in hopes of finding a negotiated settlement of the conflict. Complicating matters, both Mr. Assad and his Russian backers deny he has used chemical weapons, and they have denounced U.S. and European plans to arm the rebels as an unwarranted foreign intervention.

Republican Sens. John McCain and Lindsey Graham have been pressing the administration to impose a no-fly zone in Syria and to create safe corridors along its borders where rebel forces can train and regroup. They argue the U.S. military could disable the Syrian air force relatively easily by cratering its runways with unmanned cruise missiles that don't put the lives of American pilots at risk.

Attacking Syria's air force would demonstrate American resolve and also prevent Mr. Assad from using his helicopters and fighter jets to launch chemical attacks or move troops and military materiel around the country. But President Obama so far has understandably shown little enthusiasm for getting more deeply involved in Syria's conflict beyond sending light arms to the rebels.

We share the president's reluctance to take steps that could put America and its allies in the middle of a civil war that has already claimed more than 90,000 lives and forced millions more to flee their homes, with many ending up in squalid refugee camps in neighboring countries. Sending small arms to the rebels is an appropriate step, but there are two reasons to believe that the president now needs to go at least a step further.

First, the Asssad regime's use of chemical weapons, which the Obama administration estimates have killed at least 150 people, carries broader implications beyond this conflict. It is not without reason that such munitions have been shunned by the international community since the end of the First World War. The response by the United States and its allies must be sufficient to dissuade Mr. Assad — and any other leaders who are watching — from using them again. Anything less would demonstrate a dangerous lack of American resolve and send a terrible signal to Iran and North Korea.

And second, Mr. Assad's forces have been gaining the advantage on the battlefield, and he will likely feel little pressure to engage seriously in next month's talks if he believes he is on the verge of winning militarily. That's a recipe for more killing, more suffering and more potential for this conflict to spread.

It is time for the president to listen to those in his government who have been advocating a limited set of airstrikes to cripple the facilities the Syrian military has used to launch chemical weapons attacks — an action that would be narrowly tailored as a response to the use of such arms but which would also hinder Mr. Assad's ability to move troops around the country or receive arms shipments from Iran. Such an action need not and should not lead to deeper military intervention by the United States and its allies, but it could help shape the conditions on the ground to make a negotiated settlement possible.