The Russians, who have been staunch allies of Syrian President Bashar al-Assad, have indicated that the use of chemical weapons would be a step too far.

Ban also set out the U.N. position clearly, saying his announcement of a U.N. investigation “should serve as an unequivocal reminder that the use of chemical weapons is a crime against humanity.”
The relative success of the Chemical Weapons Convention has put their possession and use well outside the norm, experts say. This stigma makes it easier for the United States and others to pressure al-Assad on this point.

However, the muted international response when his forces have used a variety of conventional weapons against his people may have emboldened him, said Esfandiary.

“The best way the West can react is to continue to make their ‘red lines’ absolutely clear,” she said. “The danger is if you get to a situation where Assad really has got nothing to lose, then he really won’t care. To put it simply, he won’t use them until the very, very last minute.”