Chemical weapons became obsolete on the battlefield, but repressive regimes have turned to them for a different purpose -- attacks against civilians who lack the training and equipment to protect themselves. In 1988, for example, Iraqi dictator Saddam Hussein unleashed mustard gas and nerve agents sarin, tabun and VX against the Kurdish inhabitants of Halabja, a city in northern Iraq.
As many as 5,000 people died, and grisly photographs of the corpse-strewn city shocked the world.
"They're more of a taboo, because they've been effective against civilians," Pike explains. "And because they've been used against civilians horrifically."
Blair notes that different sorts of chemical weapons can be used for different purposes. Some poison gases quickly dissipate, allowing government troops to move quickly into an area and seize it. But more persistent gases can be used to keep adversaries out of an area completely.
"The high persistency agents -- remember the creature's blood in the movie "Alien," how it would go through floors?" he said. "Some of these substances stick around like that."