Chemical warfare is again underway in Syria. This time the weapon of choice is chlorine, which may be harmless in a swimming pool but is lethal in concentrated form. Human Rights Watch says there is “strong evidence” of multiple chlorine attacks in Syria last month, with clouds of yellow gas killing 11 people and sickening another 500 with symptoms that can include vomiting, splitting headaches and uncontrollable coughing. Syrian activists say another chlorine attack earlier this week killed a disabled teenager. France’s foreign minister says Syrian forces may have used chlorine 14 times in recent months.

For a president who said he “will not tolerate” the use of chemical weapons, that’s a major problem, one that puts his credibility on the line and again raises the specter of military force.

It’s true that chlorine is far less deadly than nerve gas—but it does kill, and has done so from the battlefields of World War I to Al-Qaeda attacks in Iraq. More to the point, its use is prohibited by the international Chemical Weapons Convention, which Syrian dictator Bashar Assad signed last fall to avert U.S.-led airstrikes. (Syria did not declare chlorine, which has numerous commercial and industrial uses, as part of its chemical arsenal for destruction.)

Washington is still largely in denial about this dilemma. But it won’t be long beore it becomes unavoidable.