WASHINGTON – Growing fear that civil war in Syria could unleash the world’s first use of chemical weapons in nearly three decades is based on two grim scenarios – neither considered likely but both carrying risks of civilian massacre and a major escalation of violence.

The first is that President Bashar Assad, in a last-ditch effort to save his regime, would order chemical attacks – either as a limited demonstration to the rebels of his willingness to use the internationally banned weapons, or in a large-scale offensive designed to turn the tide of a conflict that already has killed an estimated 40,000.

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.