The ghastly images reveal rows of the dead, many of them children, wrapped in white burial shrouds.
Survivors are gasping for air, their bodies twitching, and foam oozing from mouths.
This was unlike any other scene in Syria's brutal civil war, where bombs and bullets have killed and maimed tens of thousands over the past 2½ years.
The U.S. says the Aug. 21 attack on the rebel-held suburbs of Damascus was carried out with chemical weapons — and that more than 1,400 died.
President Barack Obama says it demands a military response against the Syrian government.
But in a war where only a fraction of more than 100,000 Syrian deaths have come from poison gas, what is it about chemical weapons that set them apart in policy and perception?