Despite their brutal effect, poison gases turned out not to be particularly effective in winning battles, because of their unpredictability.
"During the Great War, they didn't have reliable meteorology," explained John Pike, director of Globalsecurity.org, an Alexandria, Va.-based think tank on military and security issues. "They couldn't necessarily predict that the wind was going to blow the gas in the right or the wrong direction. Temperature and humidity also can change how chemical weapons work. In general, you'd like to be using weapons in a war that are not dependent upon guesses about weather."
The trauma of that war led nations to agree to forgo their use in warfare, but nevertheless, both the United States and the Soviet Union hedged their bets by building vast secret chemical arsenals, possibly as a deterrent against one another. In 1992, the Chemical Weapons Convention, another international treaty, banned nations who signed it from possessing such weapons at all.
The United States, which ratified the treaty in 1997, has since destroyed more than 80 percent of the nearly 30,000 tons of chemical warfare agents from its Cold War arsenal. U.S. forces, however, remain equipped with protective gear against chemical, biological and radioactive threats.
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."
Blair also points out that if Syrian use of chemical weapons is ignored, that may help to legitimize their use and encourage escalation to banned biological and radiological weapons as well. But he also worries that a U.S. air strike against the Syrian regime might cause a breakdown of the command-and-control structure and free local commanders to unleash more chemical attacks on their own.
"There might even be a system in place in Damascus, where subordinates have been told that if they lose contact with the high command, it's like a trip signal, for them to do as they see fit," he worries.
Here's a detailed guide to chemical weapons prepared by the Federation of American Scientists.