If U.S. troops entering Baghdad encounter Iraqi soldiers hiding among civilians, President Bush has authorized them to use tear gas to try to reduce casualties.

But if they do, the U.S. military may be accused by other countries of violating the international ban on chemical weapons - an ironic prospect in a war whose goal is to rid Iraq of weapons of mass destruction.

The United States has long asserted that the Chemical Weapons Convention permits the use of tear gas to save lives when soldiers and civilians are tangled together in battle. But nearly every other country - even such staunch allies as Britain - disagrees, as do some prominent U.S. legal experts.

"I admit it's a terrible ethical conundrum," says David Fidler, an Indiana University law professor and one of the country's leading authorities on laws governing nonlethal weapons. "But I would not support the use of tear gas in that situation. I think it would create a lot of problems for our troops down the road."

Elisa D. Harris, a former National Security Council official now at the University of Maryland, also believes it would be a legal and tactical mistake to use tear gas against Iraqi fighters hiding behind civilians.

"I am genuinely concerned that this could give Iraq a legal justification for using chemical weapons," Harris says, because Iraq could claim the right under the 1925 Geneva Protocol to retaliate with lethal chemicals.

Apart from legal technicalities, Harris says, history suggests that using tear gas could start a destructive cycle.

"Every time lethal chemical weapons have been used in war, they've been preceded by the use of nonlethal riot-control agents," she says. "You want to have a bright line that does not allow any gas on the battlefield."

The dilemma U.S. forces face on tear gas - which Defense Secretary Donald H. Rumsfeld complained about in February - reflects a paradox at the heart of the Chemical Weapons Convention, which bans the use of such riot-control agents as a "method of warfare."

That means that by the rules of war, U.S. troops meeting Iraqi soldiers in combat are permitted to kill them using a terrifying arsenal of guns, bombs and missiles. And they can spray tear gas on rioting Iraqi civilians away from the battlefield, as police around the world do routinely during civil disorders.

But they cannot spray enemy soldiers with tear gas.

"Yes, strange as it seems, you can shoot 'em with your M-16, but you can't hit 'em with tear gas," Fidler says. "There is a sort of disconnect from common sense. But this is driven by military concerns" - chiefly the fear that tear gas would open the door to lethal chemical weapons.

The United States accepts that tear gas is banned from ordinary battle. But with Bush's explicit approval, the Defense Department has authorized field commanders to use tear gas to save lives when Iraqi soldiers are mixed with civilians, military sources say.

Testifying before Congress on Feb. 5, Rumsfeld complained about the ambiguity of restrictions on nonlethal tear gas: "We are doing our best to live within the straitjacket that has been imposed on us on this subject."

Calling the issue "enormously complex," Rumsfeld suggested that the Pentagon was trying to write rules for troops "so that a soldier, a single human being, a private, a sergeant, knows what to do" with tear gas. "Is he going to break the law or not?"

What exact guidelines were developed is not known; Pentagon officials say the rules are classified. But a government official who declined to be identified said Friday that tear gas "would be used only in a manner consistent with our legal obligations" to save civilian lives.

The reason chemical weapons, and biological agents, are subject to a taboo that does not extend even to nuclear bombs is traceable largely to World War I, says Michael Moodie, president of the Chemical and Biological Arms Control Institute in Washington.

By the late 1890s, there was already "concern about the impact of industrialization on weapons of war," he says. "It was reinforced by the horrible experience of the troops in World War I with mustard gas.

"Even though the level of casualties from chemical weapons wasn't greater than with conventional weapons, the psychological impact was profound."

So the Geneva Protocol of 1925 banned chemical arms - but with the reservation demanded by most countries that they could retaliate with gas if it were used against them.

As a result, countries kept stockpiling deadly gas. The protocol really became a "no-first-use" agreement, Moodie says.

U.S. forces used huge quantities of tear gas in Vietnam, sometimes asserting that it was to save civilians, says Milton Leitenberg, a weapons expert at the University of Maryland. But a far more common purpose, he says, was to flush enemy fighters from tunnels and other cover so they could be killed with conventional weapons.

The legal situation changed with the Chemical Weapons Convention, signed in 1993 and ratified by the U.S. Senate in 1997 after a bitter debate that focused in part on riot-control agents.

The United States insisted on retaining the right to use tear gas in circumstances first laid out in a 1975 executive order issued by President Gerald R. Ford. The order allows gas "in situations in which civilians are used to mask or screen attacks and civilian casualties can be reduced or avoided."

Other countries objected, but the signatories "sort of agreed to disagree" so the treaty could be signed, Moodie says.

His institute has sought to work out a compromise, but "we can't figure out a way to bring the sides together without creating a shouting match. It's almost a theological debate."

The issue will only grow more complicated, he says, because the Defense Department wants to develop new kinds of nonlethal weapons based on advances in chemistry and biology.

That desire was increased by the experience of Russian authorities in October, when an opiate-based gas was pumped into a Moscow theater where Chechen militants had seized 700 hostages. At least 118 hostages died from the gas.

"Department of Defense officials have felt quite strongly for some time they would prefer not to have their hands tied as they plan for 21st-century warfare," Moodie says. "But as we saw in the Russian theater, incapacitating agents can be quite lethal."