LONDON » Wilfred Owen, the British soldier-poet, wrote in his best-known work, "Dulce et Decorum Est," an effort to depict the horrors of chemical warfare, "If you could hear, at every jolt, the blood/Come gargling from the froth-corrupted lungs."

Germany is recognized as the first to use chemical weapons on a mass scale, on April 22, 1915, at Ypres, Belgium, where 6,000 British and French troops succumbed. Chemical weapons, rarely used since that war, have once again emerged as an issue after the massacre in Syria last month, in which the United States says nearly 1,500 people, men, women and children, were killed, many as they slept.

As in World War I, that represents only a small fraction of the more than 100,000 lives that have been lost during the 2 1/2 years of Syria’s civil war. Yet, President Barack Obama is prepared to initiate a military attack in response.

Why, it is fair to ask, does the killing of 100,000 or more with conventional weapons elicit little more than a concerned shrug, while the killing of a relative few from poison gas is enough to trigger an intervention?

Whatever the reasons for the distinction, it has long been recognized.

Roughly 16 million people died and 20 million were wounded during World War I, that "war to end all wars," yet only about 2 percent of the casualties and less than 1 percent of the deaths are estimated to have resulted from chemical warfare.

Nevertheless, the universal revulsion that followed World War I led to the 1925 Geneva Protocol that banned the use, though not the possession, of chemical and biological weapons. In effect from 1928, the protocol is one of the few treaties that has been almost universally accepted and become an international norm. Syria, too, is a signatory.

No Western army used gas on the battlefield during the global slaughter of World War II. Hitler, himself gassed during World War I, refused to order its use against combatants, however willing he and the Nazis were to gas noncombatant Jews, Gypsies and others.

Since World War II and the atomic bomb, which redefined warfare, chemical weapons have been categorized as "weapons of mass destruction," even if they do not have the killing power of nuclear weapons.

The Geneva Protocol was not even the first effort to ban the use of poison in war, said Joanna Kidd of King’s College London. "Throughout history, there has been a general revulsion against the use of poisons against human beings in warfare, going back to the Greeks," she said. Some date a first effort to ban such weaponry to 1675, when France and the Holy Roman Empire agreed in Strasbourg not to use poisoned bullets.

With the industrial revolution and advances in chemistry, many nations agreed in the Hague Convention of 1899 not to use "projectiles the sole objective of which is the diffusion of asphyxiating or deleterious gases," which were not then widely understood. There was a follow-on agreement in 1907, but World War I proved just how hollow that effort was.

There have been only a few known instances of poison gas being used since 1925, and in each case the perpetrator never openly admitted it. In the first two cases, gas was used by authoritarian regimes against those they considered lesser races. In 1935-36, Mussolini used several hundred tons of mustard gas in Abyssinia, now Ethiopia, and in 1940-41, the Japanese used chemical and biological weapons widely in China, where unexploded poison gas shells are still being dug up at the expense of the Japanese government.

Frangois Heisbourg, a special adviser to the Foundation for Strategic Research in Paris, argued that one reason Japan stopped the use of chemical weapons, while then denying their use, is that President Franklin D. Roosevelt stepped in and, in quiet diplomacy, "told the Japanese that we knew of the use and that there would be consequences."

In 1965-67, President Gamal Abdel Nasser of Egypt ordered intermittent use of chemical weapons in the course of a long and disastrous war in Yemen, and the U.S. use of Agent Orange in Vietnam was widely criticized, but it was legally considered a defoliant, despite its impact on human health.

It was only in the Iran-Iraq war of 1980-88, started by Iraq after the Islamic Revolution in Iran, that chemical weapons were again used in large amounts, and by Iraqi leader Saddam Hussein against Iranian forces and his own Kurds. The Iraqis used both first- and second-generation nerve gases to blunt Iranian offensives in southern Iraq and forestall defeat. Given U.S. and Western unease with Iran’s revolution, there was little public outrage as Muslims used poison on other Muslims.

The world’s indifference altered sharply, however, in March 1988, when Saddam killed between 3,200 and 5,000 Kurds around the town of Halabja and injured thousands more, most of them civilians, some of whom died later from complications.

The Halabja killings, considered the largest chemical warfare attack ever directed at civilians, led to the Chemical Weapons Convention of 1993, in force since 1997, which bans not just the use but also the possession, manufacture and transfer of chemical weapons. It has since been signed and ratified by 189 states, according to the Organization for the Prohibition of Chemical Weapons, which carries out the treaty. Syria is among only five states — with North Korea, South Sudan, Angola and Egypt — that have neither signed nor ratified it.

Tellingly, said Camille Grand, who worked on chemical disarmament for the French Foreign Ministry, Iraq never used its chemical weapons again — not in the Persian Gulf War in 1991, although U.S. troops were prepared for their use, or in the second U.S. invasion, in 2003, which overthrew Saddam.

"Halabja changed nothing but changed everything," Heisbourg said. "In the Gulf War the dechemicalization of Iraq became a war aim, and we achieved it, even though we didn’t know or believe it."

That still leaves the question of why Syria’s use of chemical weapons to kill perhaps 1,500 people has elicited such a strong response.

Former Sen. Richard G. Lugar said the difference lay in the danger of proliferation.

"We are talking about weapons of mass destruction, we are talking about chemical weapons in particular, which may be the greatest threat to our country of any security risk that we have, much more than another government, for example, or another nation because they can be used by terrorists, by very small groups," he told the BBC. "The use of these weapons of mass destruction has got to concern us, and concern us to the point that we take action whenever any country crosses that line and uses these weapons as have the Syrians."

Others say that by using gas against its own civilians Syria is violating taboos built up over more than a century that need to be defended.

"We signed up for over 100 years to not use these weapons," Kidd of King’s College said, "and if we just stand by and not do anything, what is the value of the treaty and the norm?"

Grand agreed, saying that "it really breaks a taboo and puts Syria in breach of its own commitments in Geneva and a long list of international norms."

While militaries find chemical weapons hard to control, given the vagaries of wind and weather, they can be effective against the unprepared, and especially deadly to unsuspecting civilians.

"You just have to watch the videos from Syria from Aug. 21," Heisbourg said. "This is killing people like cockroaches and using the same chemicals to do it."

Thousands of people were killed by machetes in Rwanda, he noted. "That’s gruesome," Heisbourg said, "but the production and sale of machetes is not considered a threat to international security."

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