It disables, but it doesn't kill.
And in wartime, tear gas has been used to incapacitate the enemy from Bosnia to Vietnam.
But as U.S. troops prepare for possible battle in Baghdad, home to enemy forces and civilians alike, using tear gas to separate the two could be against the rules of war.
"It is a very awkward situation," Defense Secretary Donald Rumsfeld said recently.
Rumsfeld recently reminded the House Armed Services Committee that in 1997, the United States signed an international treaty banning wartime use of chemical weapons, including tear gas.
"Absent a presidential waiver, in many instances our forces are allowed to shoot somebody and kill them, but they're not allowed to use a non-lethal riot-control agent under the law," Rumsfeld said.
As of this month, 150 countries belong to the United Nations' Chemical Weapons Convention but not all have signed on to the treaty banning chemical weapons. The treaty went into force in 1997.
The Organization for the Prohibition of Chemical Weapons oversees countries' moves to stop developing, stockpiling, transferring and using chemical weapons. The treaty even bans using these harmful agents during military operations. It specifies: "Each state party undertakes not to use riot-control agents as a method of warfare."
That provision came under hot debate during the 15 years it took to craft the treaty. It arose as an objection to the United States' reliance on tear gas to flush out Viet Cong fighters and kill them during the Vietnam War.
The convention does, however, allow riot-control agents to be used for "law enforcement." Whether that use extends beyond a nation's borders is a matter of debate. The issue will be discussed in April when the treaty comes up for international review in The Hague.
Chemical warfare was used to great extent in World War I; the first large-scale attack took place in Ypres, Belgium, on April 22, 1915. By the end of the war, about 124,200 tons of chlorine, mustard gas and other agents were released and more than 90,000 soldiers died painful deaths after being exposed to them. Millions of others were left blind or disfigured.
News of chemical attacks in Iraq in 1988 helped spur the treaty on.
But when it comes to urban warfare, military experts say firing tear gas instead of bullets saves lives.
"It would incapacitate them, allowing them to be controlled by our military policemen and by our soldiers," said Lt. Col. Robert Maginnis, a Fox News military analyst. "You want to be able to segregate those hostiles out, so you can take appropriate action against them, at the same time, protect the civilians."
Backers of the international treaty say that using any chemical agents in the coming conflict would violate the same agreements the United States accuses Iraqi President Saddam Hussein of flouting. Others worry a presidential waiver to allow their use could set a dangerous precedent.
"If there were exceptions made, it would allow in fact going from a mace-type gas into something much more deadly ... and so that was the reason to adhere to it strictly," said Thomas Henriksen, a foreign policy expert at the Hoover Institution.
Military planners are drafting new guidelines to allow the use of riot-control agents such as tear gas and pepper spray to control prisoners and separate enemy soldiers from civilians -- in spite of the treaty, Rumsfeld said last month. The Pentagon argues that their most important consideration is saving lives -- on both sides.
"We are doing our best to live within the straitjacket that has been imposed on us on this subject," Rumsfeld said. "We are trying to find ways that non-lethal agents could be used within the law."
Army Major General David Grange ordered his troops to use tear gas on hostile crowds of Serbs in Bosnia six years ago but complained that red tape prevented him from using it more often.
"We didn't kill anyone," Grange, who is retired, told The Associated Press. "It saved lives."
But weapons-control activists cite myriad reasons for banning non-lethal chemical weapons in war. They say these agents can kill people when used in war environments. They could also lead to militaries to using deadlier chemicals.
Non-lethal weapons are used to "delay, disrupt, or degrade threat forces, combat functions," and to pursue military objectives "with the minimum force necessary," according to the Defense Department.
The U.S. military has also explored mind-altering drugs such as opiates, along with genetically engineered microorganisms that can destroy objects such as runways, vehicles and buildings.
The research is spearheaded by the U.S. Marine Corps' Joint Non-Lethal Weapons Directorate, which was created in 1997 to equip soldiers on overseas peacekeeping and other non-combat duties.
A Pennsylvania State University institute prepared a 50-page report with Pentagon funding in October 2000 that explored a range of drugs -- including Prozac, Valium and Zoloft -- for use as "calmatives" for crowds.
The researchers found "use of non-lethal calmative techniques is achievable and desirable."
Despite the endorsement, Marine Capt. Shawn Turner of the non-lethal weapons directorate said the military stopped "calmative" research because such drug-weapons could violate international law.
Fox News' Claudia Cowan and Liza Porteus and The Associated Press contributed to this report.