Growing fear that war in Syria could bring the world's first use of chemical weapons in nearly three decades is based on two scenarios. Neither is considered likely, but both carry risk of a civilian massacre and an escalation of violence.

The first is that President Bashar Assad, in a last-ditch effort to save his regime, would order chemical attacks. The second is that some portion of Assad's arsenal could be moved to Iran or Lebanon or fall into the hands of foreign fighters with ties to terrorist groups who are helping Syrian rebels.

Syria is believed to possess substantial stockpiles of mustard gas and a range of nerve agents, including sarin, a highly toxic substance that can suffocate its victims by paralyzing muscles around their lungs.