The Obama Administration, which last year warned the Syrian regime that using chemical weapons would cross a "red line" for unacceptable behavior, now seems to be preparing to inflict punishment upon Assad, probably in the form of an air strike.

But many people are wondering why Assad’s alleged use of chemical weapons is being seen by the United States as the last straw. After all, the Syrian civil war has been raging for more than two years, and 100,000 people already have died, many of them in atrocities that shock the conscience.

VIDEO: Flying Car to Help Soldiers in War

Last December, for example, a Syrian air force plane reportedly dropped eight cluster bombs on civilians waiting outside a bakery to buy bread in the small town of Halfaya, slaughtering 68 people.

"Anyone who has actually seen wounds from conventional artillery -- or badly treated body wounds from small arms -- realizes that chemical weapons do not cause more horrible wounds," Anthony Cordesman, an analyst for the Center for Strategic and International Studies, writes in a blog post. He argues that "the case for intervening cannot be based on chemical weapons."

But other experts say that chemical weapons, whose use was banned in warfare by a 1925 international treaty, do indeed cross a line, into a sort of brutality so extreme that the civilized world cannot afford to tolerate it. Because modern armies are equipped with protective gear, chemical weapons tend to be effective only for committing atrocities against helpless civilian populations. And in addition to inflicting excruciating pain upon victims who often die a lingering death, their use has an even more widespread, devastating psychological effect.

NEWS: Why A Chemical Threat in Syria Is Terrifying

"Our minds are hard-wired to be afraid of poisons," explains Charles Blair, a senior fellow with the Federation of American Scientists who teaches graduate courses in biodefense at George Mason University and Johns Hopkins University. "Things that you can’t see or smell or taste are very frightening to people."

The psychological trauma inflicted by chemical warfare was documented in a 2006 study of survivors of the 1980-1988 Iran-Iraq war, one of the few modern conflicts in which militaries used such weapons. Researchers found that nearly 60 percent of subjects who'd been exposed to chemical weapons during the war suffered from post-traumatic stress disorder, about twice the rate of those who had been in the war but avoided chemicals. About 40 percent of the subjects who’d been exposed to chemicals suffered from severe depression, compared to only 12 percent of those without chemical exposure.

Chemicals have been used in warfare since ancient times, when Spartan soldiers burned wood dipped in a mixture of tar and sulfur to create noxious fumes on the battlefield. But it wasn't World War I that armies on both sides began deploying industrially-manufactured poison gases -- such as lung-searing phosgene and chlorine, and mustard gas, which causes blistering when it comes into contact with skin and mucous membranes -- as weapons.

An estimated 90,000 soldiers were killed by gas attacks during the war, and about a million soldiers suffered injuries that often plagued them for the rest of their lives, according to the Organization for the Prohibition of Chemical Weapons, an international monitoring group. In a secret report prepared at the war's end, U.S. Army Lt. Col. C.G. Douglas, a physiologist, concluded that "The particular value of the poison is found in its remarkable casualty-producing power as opposed to its killing power."

Chemical weapons are most effective at committing atrocities against helpless civilians.iStockPhoto

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, 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.

PHOTOS: Deadliest Poisons Known to Man

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.