The shelling of suburban Damascus with a suspected nerve agent last week was potentially the third large-scale use of a chemical weapon in the Middle East and may have broken the longest period in history without such an attack.

If confirmed, the attack, which U.S. officials say warrants a decisive military response from the West, would dash hopes that the world would never again see the large-scale use of chemical weapons, a prospect that had appeared increasingly realistic in recent years as all but a handful of nations signed a treaty agreeing to destroy their stockpiles.

If the weapons were deployed by the Syrian government, as Western officials allege, it would represent the first major chemical weapons attack by a nation against its own citizens since President Saddam Hussein gassed Kurds in northern Iraq in 1988, an act so barbaric it galvanized the movement for a world free of chemical weapons.