WASHINGTON – On April 22, 1915, a gray-green gas cloud crept over men in the heart of Europe. The toxic fog was unnatural; unleashed by German armed forces, it broke Allied lines. Hundreds of French soldiers were killed by the chemical attack.

The weapon of choice that day was chlorine – the same toxin used to clean swimming pools, to make the plastic bottles we purchase daily, to produce widely used home cleaning products.

“To kill a lot of people, you need an awful lot of chlorine,” says Matt Meselson, a professor at the Belfer Center for Science and International Affairs at Harvard University.

“It’s just less lethal. It’s not much of a chemical weapon.”

And yet the Germans were determined to make it one – at least until other, more potent chemicals were weaponized. To maximize chlorine’s effect, German forces deployed more than 5,700 canisters of the toxin. More than 168 tons were used.

As Europe prepares to mark 100 years since the outbreak of World War I this summer, it appears as if chlorine has been weaponized once again. Few died from the latest attack, broadcast on YouTube and acknowledged by the United States.

But if history is any indication, low potency will not stop embattled President Bashar Assad of Syria from continuing to use the chemical against his own people.

“We have indications of the use of a toxic industrial chemical” in Kafr Zeita on April 11, State Department spokeswoman Jen Psaki said at a news briefing last week. “We are examining allegations that the government was responsible. Obviously there needs to be an investigation of what’s happened here.”

If confirmed, the attack marks another sobering anniversary. In April 2013, the Obama administration published evidence that Assad had used chemical weapons on a small scale “on multiple occasions” against rebels fighting for his ouster – and that, in response, the US would begin providing moderate opposition fighters with light arms.

What took place in the months to come was historic. Assad brushed off warnings to adhere to international norms against the use of chemical weapons in warfare, and in late August of last year, killed more than 1,400 civilians in their sleep in a Damascus suburb. Some 500 of the victims were children.

The White House threatened to retaliate with force. And under pressure from Russia, Assad agreed to sign the United Nations’ Chemical Weapons Convention, to rid himself of his massive chemical stockpile.

According to the Chemical Weapons Convention, the use of any chemical to inflict harm or cause death is illegal. But the convention is intentionally vague: if its authors had included an exhaustive list of forbidden chemicals, they would have risked missing some or might have banned others with legitimate scientific or industrial value.

Instead, the convention’s authors chose to classify chemicals into three tiers, and to define criminality of their use based on intent. In doing so, the Chemical Weapons Convention differentiates between chemical weapons and chemicals that have the potential to be weaponized.

Schedule 1 chemicals, such as sarin and VX, have virtually no civilian purpose; they are the most potent, and can kill the most people with the least material. Schedule 3 items are widely produced industrial toxins with a multitude of peaceful uses, such as rat poison, hydrogen cyanide and chlorine.

By certain measures, the deal Russia and the US brokered in September has been a success. As of this writing, 92 percent of Schedule 1 chemicals have been removed from Syria, and months ago, the production and mixing plants used to weaponize the materials were rendered inoperable.

Even if Assad had produced a hidden stockpile of sarin before the deal was implemented, those chemical arms have a shelf life of mere months once mixed. Thus, experts agree that the majority of Assad’s deadliest chemical arms – chemicals that are, by definition, weapons of mass destruction – are no longer in his possession.

But because of the very nature of Schedule 3 chemicals, the US has not sought their collection or destruction. And while the use of such chemicals is in clear violation of the Chemical Weapons Convention, they are almost impossible to banish from a country, and the use of them equally difficult to punish.

Chlorine “combines with water to form hydrochloric acid and hypochlorous acid, which damage the lungs and draw moisture to the lungs, leading to what has been called dry land drowning,” says Mark Bishop, an adjunct professor at the Monterey Institute of International Studies, noting that, if used with the intent to kill or maim, it is in clear violation of the convention.

The State Department echoed that conclusion a week ago, calling the use of chlorine a violation of the Chemical Weapons Convention, while declining to state whether that also meant a violation of the September agreement.

When that agreement was forged, Secretary of State John Kerry said that “anything but full compliance” with the deal would justify the use of force by the United States. And the US-Russia deal, indeed, explicitly requires Syria join and uphold all tenets of the CWC.

Thus, the use of chlorine to kill even two men – the death toll of the alleged April 11 attack – presents an inconvenient truth to US President Barack Obama: A CWC violation, of any kind, crosses redlines drawn on chemical warfare by this president and his secretary of state last August.

The question challenging the White House is whether America will tier violations similar to how the CWC tiers chemicals; whether the US will allow perfect to be the enemy of the good, or else tolerate Assad’s use of low-grade chemical arms for the sake of a greater deal on WMD.

“A country is not going to go to war because a couple people got sick from chlorine,” Meselson says. “It is for governments to decide their reaction. But in terms of the law, there is no delineation.”

One senior administration official tells this paper the White House will never publicly acknowledge the prioritization of one chemical over another. Nor will the US lead an investigation into the matter on the ground: that is the responsibility of the Organization for the Prohibition of Chemical Weapons.

A policy decision appears to have been made, for the time being, to tolerate the proliferation of industrial toxins so long as they are not used to murder on an industrial scale.

Such mass use has stark historical precedent. Hydrogen cyanide, a gas, was the Nazis’ weapon of choice used to kill millions herded into chambers. That gas is categorized as Schedule 3.

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