Iranians know better than most the horrors of chemical warfare.

During the Iran-Iraq war of the 1980s, they were on the receiving end of mustard and nerve gas shells launched by the regime of Saddam Hussein. Some 50,000 Iranians were killed or injured, mostly young soldiers on the battlefield. A dwindling number of survivors live with the awful aftereffects of exposure, including blindness and severe respiratory problems that bring constant pain.

This gruesome experience compounds Iran’s moral imperative to prevent the regime of Bashar al-Assad from continuing horrific attacks such as the ones perpetrated on Tuesday against the northern Syrian city of Khan Sheikhoun.

Videos and other evidence of the attacks suggest that Assad’s forces used sarin gas against civilians, including dozens of children. The death toll is at least 69 and expected to rise.

Khan Sheikhoun is in Idlib province, one of the few areas in Syria under the control of Assad opponents but not the group that calls itself the Islamic State. With a US-led coalition fighting ISIS and pro-Assad forces successful in retaking Syria’s most populous city, Aleppo, from the opposition, the Damascus government appears intent on reclaiming and consolidating control over all of western Syria.

While the goal is understandable, the tactics are beyond the pale.

Perhaps Assad, who in a just world would already be standing trial for war crimes, does not care about adding to a long list of atrocities committed by his regime including torture and summary execution of innocent people. However, in 2013, his government agreed to give up its chemical weapons stockpile under a Russia-brokered deal that averted a possible US attack on Syria’s chemical weapons sites.

The Organization for the Prohibition of Chemical Weapons (OPCW) was put in charge of finding and removing those stockpiles and it was thought that the bulk of them had been destroyed. The OPCW had already been looking into Syrian government attacks using chlorine, a less potent substance that has legitimate peaceful uses. Judging from the horror in Idlib, the Syrian regime held on to some quantities of more lethal and illegal compounds.

In the aftermath of Tuesday’s attacks, the OPCW said its fact finders were “in the process of gathering and analyzing information from all available sources” and would “report its findings to the OPCW’s Executive Council and States Parties to the Chemical Weapons Convention… The OPCW strongly condemns the use of chemical weapons by anyone, anywhere and under any circumstances.”

Iran, Syria and Assad’s other great supporter, Russia, are all signatories of the Chemical Weapons Convention, as are countries representing 98 percent of humanity. The convention prohibits “the use, development, production, stockpiling and transfer of chemical weapons.”

In the aftermath of the attacks, Russia resorted to its habitual fake news and asserted that Syrian airstrikes had hit a rebel stockpile of chemical weapons.

The Iranian government had the intelligence to at least condemn “all use of chemical weapons” in Syria but did not ascribe responsibility to the Assad regime.

It is possible that authorities in Tehran and Moscow did not have foreknowledge of the attack. But as Secretary of State Rex Tillerson noted in a statement, Russia and Iran have the ability to influence the Syrian regime and “guarantee that this sort of horrific attack never happens again.”

Without Russia’s airpower and Iran’s contribution of advisers and Shiite militia, Assad would not still be the president of Syria. Given his willingness to resort to such vile methods to stay in power, it is time for Moscow and Tehran to begin to seriously plan for a post-Assad Syria. That is a project that the Trump administration, despite its apparent allergy to regime change, should enthusiastically embrace.

Barbara Slavin is acting director of the Future of Iran Project at the Atlantic Council.