On Tuesday, the forces of Syrian president Bashar al-Assad unleashed a chemical attack on the civilian residents of Khan Sheikun, a town situated near the rough borderlines between resistance and regime. It was a brutal strike, taking 80 lives and wounding hundreds more. It underscored, also, a painful truth about the nature of sarin gas and other internationally banned nerve agents: If a country wants to produce them, and use them, there’s precious little anyone can do to stop it.

Syria’s use of sarin gas this week echoes another in 2013, when Assad murdered more than 1,400 residents of a Damascus suburb with the same nerve agent. In response to that attack, the international community faced a choice: Direct military intervention, or wringing a promise out of Assad that he would destroy any remaining chemical stockpiles. After at times heated political maneuvering, the United States opted for the later.

“Obviously we were dealing with the shock and horror of the attacks,” says Laura Holgate, who served as senior director for weapons of mass destruction in the Obama administration. “We really did feel like there was an important need to respond.”

They accomplished plenty. Syria ultimately signed onto the Chemical Weapons Convention, and voluntarily opened up its chemical weapons program to inspectors, who seized and destroyed 1,300 metric tons of banned material. It unquestionably limited the scope of Assad’s potential destructiveness. The world’s safer for it.

But it couldn't stop Tuesday’s massacre, just as the Trump administration’s retaliatory airstrike Thursday won’t necessarily stop the next. The reason is as simple as it alarming: Sarin’s too easy to make, too hard to find, and the politics of policing it far too complex.

Few things unify the world’s nations more than their opposition to chemical weapons. As of late 2015, 197 countries had signed onto the Chemical Weapons Convention, with only North Korea, Egypt, Palestine, and South Sudan as holdouts. (Israel has signed, but not ratified.) Almost the entire civilized world has agreed not only not to use them, but to forgo any development, production, or stockpiling as well.

What's so terrible about these weapons is not merely how quickly and widely they can kill, but the manner in which they do so. There are a variety of categories, but all cause brutal, painful deaths.

Sarin, specifically, falls under the “nerve agent” category, meaning it disrupts the communication between a body’s nervous system and its muscles. “It creates paralysis by muscle spasm, and an inability to control muscles---particularly the muscles of breathing. That’s typically why people succumb to it,” says Tom Inglesby, director of the Johns Hopkins University Center for Health Security. A victim’s diaphragm contracts, and never recovers.

Two other attributes compound sarin’s insidiousness. First, it’s not especially hard to produce, in terms of both resources and expertise. “A competent chemist could make it, and possibly very quickly, in a matter of days,” says John Gilbert, a senior science fellow at the Center for Arms Control and Non-Proliferation, who spent much of his Air Force career assessing countries’ WMD capabilities. Producing sarin doesn’t require any kind of massive facility; a roughly 200 square foot room would do.

Attackers also don’t require much of it to do serious damage. Gilbert estimates that the Khan Sheikun devastation came from roughly 20 liters of sarin. (Remember: At one point Syria had stockpiled nearly 1,300 tons of banned chemical substances.)

“It would be possible to obtain, retain, or make relatively small amounts of sarin that would be hard to detect, if somebody really didn't want them detected by an international organization,” says Gilbert.

And while sarin garners most of the attention, the Assad regime has also made plentiful use of chlorine gas, a choking agent that can cause just as much devastation. Unlike sarin, though, it’s perfectly legal for companies to stockpile as much of it as they like. “The difficulty with chlorine is that because it has primarily legitimate uses, and it’s ubiquitous, and it’s easier to make, it’s not possible to outlaw it. Nor should it be,” says Inglesby.

Those factors make it nearly impossible to totally extinguish a country’s chemical weapon ambitions. After all, they can always just make more.

The regime would only need to make more, of course, if the 2013 effort really did clear out all of its earlier reserves of chemical weapons. It didn’t. And everyone knew it.

“It was always clear, and always clear in US statements, that we did not believe the declaration of the Syrians was complete,” says Holgate. Which to an outside observer sounds strange; what’s the point of a disarmament treaty if it doesn’t go fully enforced?

In truth, full enforcement was never an option. There are both scientific and political realities that makes assessing a country’s entire chemical weapons stockpile virtually impossible. “There is no magic scanner in the sky that can tell you whether there’s a chemical weapon in a particular crate, or a particular car, or on a person,” says Inglesby. “It requires a very close-in understanding of what’s going on in a particular building.”

These are not like large munitions, which leave large industrial footprints that a satellite can spy on. A small-scale chemical weapons facility, by contrast, could exist in any warehouse.

The specific language of the Chemical Weapons Convention feeds into this reality as well. The process lets the country declare its chemical weapons stash to outside inspectors, but also puts it on something like an honor system. It could hypothetically---as Syria almost certainly did---house additional material at secret locationss. “[Inspectors] don’t do a door-to-door search,” says Mark Bishop, a chemical weapons expert at the Middlebury Institute of International Studies at Monterey.

The Organisation for the Prohibition of Chemical Weapons, which monitors compliance of Syria's chemical weapon ban, has since 2013 reserved the right to call a “challenge inspection,” which allows them to inspect any facility at all with between two and 24 hours notice. But the reality of life on the ground complicates that proposition.

“What’s particularly novel about this situation is that it’s happening in the middle of a war zone,” says Holgate. “There hasn’t been a smoking gun. You’ve got to be really, really, really sure,” and not just to make it worth the risk of physical harm to the inspectors. If the OPCW goes into Syria and finds nothing, Holgate says, that just gives Assad-allied Russia ammunition to discredit the organization’s efforts.

That’s not just an assumption; Russia and China blocked UN sanctioning Syria for using chemical weapons just this past February. And as long as those two giants are in its corner, Assad's regime can act with something close to impunity.

So no, clearing Syria of chemical weapons---sarin, chlorine, mustard, you name it---won’t happen any time soon. Inspections haven’t done it. One airstrike won’t do it.

“It’s not possible to completely stop the creation of a chemical weapons program, or the use of chemical weapons, from afar,” says Johns Hopkins’ Inglesby.

Syria can either give up its gasses voluntarily, or not at all. And right now, with Russia and China blocking more aggressive UN actions, and the country’s civil war having no foreseeable end, the safe bet is that it won’t.