This is not the first time that Islamic State fighters have been accused of using the region’s loose chemical weapons on pro-Western forces, but this might be the most disturbing reported use of chemical agents by ISIS. The allegation that ISIS militants used chlorine gas on Kurdish soldiers in Iraq likely represents the ultimate failure of the Obama administration’s policy toward to the Syrian civil war.

According to reports, Kurdish authorities have provided evidence to international investigators that indicates ISIS used low-grade chemical weapons, likely chlorine gas canisters, against Peshmerga fighters in an attack near the Syrian border in Iraq.

The allegation by the Kurdistan Region Security Council, stemming from a Jan. 23 suicide truck bomb attack in northern Iraq, did not immediately draw a reaction from the ISIS, which holds a third of Iraq and neighboring Syria in its self-declared caliphate. However, Iraqi officials and Kurds fighting in Syria have made similar allegations about the militants using the low-grade chemical weapons against them.

In a statement, the council said the alleged chemical attack took place on a road between Iraq’s second-largest city, Mosul, and the Syrian border, as peshmerga forces fought to seize a vital supply line used by the Sunni militants. It said its fighters later found “around 20 gas canisters” that had been loaded onto the truck involved in the attack.

Video provided by the council showed a truck racing down a road, white smoke pouring out of it as it came under heavy fire from peshmerga fighters. It later showed a white, billowing cloud after the truck exploded and the remnants of it scattered across a road.

An official with the Kurdish council told The Associated Press that dozens of peshmerga fighters were treated for “dizziness, nausea, vomiting and general weakness” after the attack. The Kurds say samples of clothing and soil from the site were analyzed by an unnamed lab in an unnamed coalition partner nation, which found chlorine traces.

The Obama administration’s approach toward the problem of containing the Syrian civil war, which by 2012 had come to be characterized by precedent-setting atrocities, was designed to prevent just this event.

In 2013, President Barack Obama agreed to abide by the terms of a deal negotiated between Damascus and Moscow in which Syrian President Bashar al-Assad pledged to surrender all of his chemical weapons stockpiles to international monitors. It was a way for the president to avoid having to follow through on his promise of military action against the Syrian government as a result of Assad’s repeated and flagrant violations of Obama’s “red line.” By June of last year, the Syrian government had supposedly surrendered the last of its declared chemical weapons, though international investigators did not include chlorine on its list of prohibited chemical munitions.

“We struck a deal where we got 100 percent of the chemical weapons out [of Syria],” Secretary of State John Kerry said on Meet the Press as part of a victory tour on all five of the Sunday news shows.

The unimpeachable fact-checking institution Politifact rated Kerry’s comment “mostly true.”

But it was not long before even administration officials began to concede that his victory celebration was premature. “We must ensure that the Syrian government destroys its remaining facilities for producing chemical weapons within the mandated time frames and without the repeated delays by the Assad regime that plagued earlier removal efforts,” United Nations Amb. Samantha Power said in September. “Certainly, if there are chemical weapons left in Syria, there will be a risk that those weapons fall into [ISIS’s] hands.”

Her warnings proved prescient. In October of last year, ISIS was accused of using chlorine gas in an attack on Iraqi police officers in a roadside bomb.

“If the UN won’t act when WMD is used, especially by non-state actors, then whatever use still remains of the UNSC other than a debating society will have evaporated for good — and what’s more, Russia and China have to know that,” Ed Morrissey wrote at the time. Not only have chemical weapons been deployed by non-state actors in the region, they continue to be used by Assad’s forces in Syria with the sanction of the regime in Damascus.

By December, international investigators began to cast some doubt on Kerry’s claim that the Syrian chemical weapon threat had been neutralized as the evidence indicating Assad’s forces had continued to use weapons of mass destruction mounted. “At the Hague Wednesday, the U.S. accused the Syrian regime of continuing to use chemical weapons on civilians as recently as September, despite claims that it has fully destroyed its stockpile as required under a 2013 U.N. treaty,” a CBS News report in December read.

The Obama administration frequently cites that U.S.-Russia brokered deal as a major foreign policy success, in part because the diplomatic agreement helped avert U.S. military strikes triggered by a sarin gas attack that killed hundreds of civilians outside Damascus.

Yet the Syrian regime has not been completely disarmed of that lethal cache, Undersecretary of State Rose Gottemoeller told members of the Organization for the Prohibition of Chemical Weapons (OPCW.)

“Now there is compelling evidence that Syria continues to use chemical weapons systematically and repeatedly,” Gottemoeller said.

The abhorrent precedent set by the flagrant and repeated use of weapons of mass destruction both by state and non-state actors in the region is one of this administration’s most loathsome legacies. The American president had a responsibility to marshal the forces of civilization to condemn these atrocities, punish those militaries using chemical weapons them with military force, and establish the clear standard that the deployment of chemical munitions backfires disastrously on their users. Obama failed to meet the demands of posterity by looking the other way as the Middle East became a non-conventional battlefield. Historians will not look back fondly on that irresponsibility.