The Syrian government's use of nerve gas on rebel-controlled Damascus neighborhoods this summer focused renewed attention on the threat posed by chemical and biological weapons. The attacks, which killed up to about 1,400, led President Obama to threaten military retaliation. Syrian President Bashar al-Assad responded by agreeing to destroy his chemical arsenal. Chemical weapons have been outlawed since 1928, after the world saw the horrors of their effect in World War I. After Iraq used chemical weapons to kill tens of thousands of Iranians and Iraqi Kurds in the 1980s, a 1993 international accord strengthened enforcement of the ban. The Syrian gas attacks have spurred debate over whether chemical weapons are worse than conventional arms. Meanwhile, biological weapons also are outlawed, but some experts fear they could be used by terrorists.