The Assad regime has been accused of using chemical agents on rebels and civilians several times over more than six years of civil war in Syria. The first prominent attack came in 2013 in Ghouta, a Damascus suburb—about a year after Barack Obama declared that using such weapons would cross a “red line” and “would change our calculus” about intervening. That attack killed 1,000 people. The latest, on Tuesday in Idlib province, is said to have killed dozens, and came days after Nikki Haley, the U.S. ambassador to the UN, said that America’s “priority is no longer to sit and focus on getting Assad out.”

In the intervening period, the U.S. and Russia formulated an agreement under which Syria agreed to allow international monitors to destroy its chemical-weapons stockpile by 2014, but that deal has apparently done little to deter the continued use of chemical weapons in the conflict.

Syria acknowledged in July 2012 that it possessed chemical weapons, which a foreign-ministry spokesman said would be used only against “external aggression;” a month later, President Obama drew his famous “red line.” There were several reports in the ensuing months of that the Assad regime had used chemical weapons. Then in August 2013, following the Ghouta attack, the U.S. government assessment said it was “highly likely” Assad’s regime was responsible. John Kerry, who was U.S. secretary of state at the time, said the regime used chemical weapons “multiple” times that year. Obama then sought congressional approval for the use of limited military force in Syria. A few days later, Kerry said a military strike could be averted if Syria turned over its chemical weapons to the international community. Russian Foreign Minister Sergey Lavrov proposed such a plan almost immediately—and Syria signed onto it.

On September 14, 2013, Kerry and Lavrov reached an agreement to eliminate Syria’s chemical weapons. The plan called for Syria to disclose its stockpile within a week, and provide international inspectors access to all its chemical-weapons sites. It also called for the destruction of the Syrian stockpile by the first half of 2014. Kerry hailed the diplomacy that made the deal possible. Obama told The Atlantic’s Jeffrey Goldberg he was “very proud” of his decision not to strike Assad.

Despite the agreement, however, there were subsequent reports that Assad used chlorine gas against civilians. The deal struck with the international community did not include chlorine because the Assad regime hadn’t added it to a list it submitted to international monitors of the chemical weapons it possessed. News reports at the time said Syria had 1,000 tons of chemical weapons, including mustard gas, sarin, and VX, the nerve agent. There were other problems with the agreement, as well: Among several other deadlines in the agreement, Syria missed the June 30, 2014, deadline for the weapons’ destruction. As Paul Wolfowitz wrote in May 2014 in The Wall Street Journal:

Earlier this month on C-Span, National Security Adviser Susan Rice said that, “with 92.5% of the declared chemical weapons out of the country” we have accomplished more than any “number of airstrikes that might have been contemplated would have done.” Yet much more important than what’s been removed is what’s still left, and it seems likely that more remains than just what was declared by Syria last year. For one thing, it appears that the regime is now using chlorine gas as a kind of poor man's chemical weapon even though, as Time reported on May 23, it has made no declaration about its chlorine stocks. Chlorine as such is not a prohibited substance, since it has many civilian uses. But its use as a weapon is prohibited by the Chemical Weapons Convention that Syria joined in 2013.

The possibility that Syria is using chlorine gas is not the most disturbing factor. The experience with Libya demonstrates that there is no reason to be confident that Syria has even declared its entire stockpile of its more lethal weapons, such as sarin.

It’s unclear so far what type of chemical agent was used in Tuesday’s attack in Idlib province. Chlorine typically kills just a few people. The death toll in this attack—58 so far by one estimate—is the largest since the Ghouta attack in 2013, when sarin was used.

The New York Times reports the symptoms in Tuesday’s attack were also different from those associated with chlorine: “They included the pinpoint pupils of victims that characterize nerve agents and other banned toxins. One doctor posted a video of a patient’s eye, showing the pupil reduced to a dot. Several people were sickened simply by coming into contact with the victims.”

Assad’s regime has consistently denied using such weapons—despite evidence to the contrary. Sean Spicer, the White House spokesman, called the attack a “reprehensible” act, but added the “political reality” in Syria made it unlikely the U.S. would pursue a policy of regime change there. Indeed, Assad is more in control of Syria today than at any point since the civil war began more than six years ago.