The Syrian Civil War has been a defining humanitarian crisis of the 21st century, highlighting the grave threat posed by chemical weapons. During the course of the war, Assad used chemical weapons more than 300 times against his own citizens, causing thousands of casualties. What happened to those chemicals is an urgent question in the wake of last month’s takeover by Syrian rebels.

To learn more about the status of chemical weapons in Syria right now, along with international efforts to prevent chemical weapon proliferation, we turned to American University Professor of Chemistry Stefano Costanzi. Costanzi holds degrees in both the chemical sciences and international affairs. His work at the intersection of science and policy seeks to combat the proliferation of chemical and biological weapons. He is a member of the International Center for Chemical Safety and Security Global team, the Scientists Working Group on Biological and Chemical Security, and the Chemical Weapons Convention Coalition.

PH: Can you bring us back to the beginning? When did the world first learn that Assad had chemical weapons?  

SC: The Assad regime first publicly acknowledged Syria's chemical weapons arsenal in 2012. A year later, the regime carried out the Ghouta sarin attack, killing at least 1,200 people in one of the deadliest uses of chemical weapons in history. This atrocity shocked the world and spurred an international response that ultimately led to Syria’s accession to the Chemical Weapons Convention (CWC) in October 2013, under intense diplomatic pressure. The regime declared its stockpiles – more than 1,300 metric tons of chemical agents and precursors – leading to their verified destruction by 2016. 

PH: And the world breathed a sigh of relief. But what happened next?  

SC: Syria’s accession to the CWC was a watershed moment, but it did not end chemical weapons attacks. Post-2013, the nature of these attacks changed. While large-scale sarin strikes like Ghouta were curtailed, chemical chlorine became the weapon of choice. Chlorine’s accessibility due to its civilian applications (in water sanitation, for example) made it harder to regulate.

Between 2013 and 2019, the Global Public Policy Institute confirmed 311 chemical attacks, primarily using chlorine, with a handful involving sarin. The reduction in casualties – 517 deaths compared to 1,444 prior to Syria’s CWC membership – demonstrates the life-saving impact of destroying Syria’s declared chemical arsenal. However, it also suggests that undeclared stockpiles or ongoing production enabled the sporadic use of sarin, raising questions about the completeness of Syria’s declarations under the CWC, while also highlighting the challenges of regulating dual-use chemicals.

PH: Were there any consequences?  

SC: Investigations by the Organization for the Prohibition of Chemical Weapons attributed the majority of the chemical weapons attacks to the Assad regime, while some were attributed to the Islamic State. 

There were two distinct military responses to these continued attacks. In 2017, a unilateral US strike followed the Khan Shaykhun sarin attack that killed at least 90 people. A year later, a multilateral response involving the US, UK, and France targeted chemical facilities after the Douma chlorine attack that killed over 40 people.

PH: Do we believe that Syria really destroyed all its chemical weapons capabilities?  

SC: The use of sarin in seven post-2013 attacks revealed alarming gaps in compliance. These findings point to either incomplete declarations, hidden stockpiles, or the ability to produce new chemical agents – a stark reminder of the challenges in enforcing the CWC’s provisions. In 2021, the CWC suspended some of Syria's rights and privileges under the convention due to apparent non-compliance. 

PH: And what is the status now? What has happened to Syria’s existing chemical weapons after the fall of Assad?

SC: In December 2024, a rebel coalition led by Hayat Tahrir al-Sham (HTS) toppled the Assad regime, ending 53 years of rule by the Assad family. Rebel factions immediately took steps to address the issue of chemical weapons.

HTS issued a statement reaffirming its commitment to preventing the use or storage of chemical weapons in territories under its control and expressed readiness to collaborate with the Organization for the Prohibition of Chemical Weapons to dismantle remaining stockpiles. Meanwhile, rebels in southern Syria reached out to international organizations, emphasizing the humanitarian toll of chemical weapons and calling for global assistance in ensuring their safe disposal.

PH: Does the international community believe these statements are truthful?

SC: The OPCW, for its part, remains vigilant. In a statement issued on December 9, 2024, the organization emphasized the importance of monitoring chemical weapons-related sites and ensuring compliance with the CWC. This ongoing engagement reflects the international community's commitment to preventing a resurgence of chemical warfare in Syria.

The ongoing efforts to secure and destroy chemical weapons in Syria have been underscored by both diplomatic and military actions. The US Department of State has emphasized the necessity of safeguarding and ultimately dismantling chemical weapons sites to prevent their misuse. Simultaneously, Israel has reportedly conducted strikes on suspected chemical weapons facilities, such as the Barzah research site, which was previously targeted by the United States in 2018 following the Douma chlorine attack. 

While these strikes aim to prevent strategic weapons systems from falling into the hands of extremist groups, they also risk destroying critical evidence needed to verify Syria’s compliance with the Chemical Weapons Convention. Such developments highlight the intricate balance between immediate security measures and the long-term objectives of accountability and verification, both of which remain central to ensuring the success of the global nonproliferation regime.

PH: What are the takeaways and lessons learned?  

SC: As Syria embarks on a new chapter, the legacy of chemical weapons in its civil war serves as both a cautionary tale and a testament to the power of international cooperation. The destruction of declared stockpiles under the CWC saved countless lives, demonstrating the convention’s resilience even amid conflict. Yet, the continued use of chlorine and sporadic sarin attacks underscore the need for vigilance, innovation, and enforcement to address the dual-use nature of many chemicals and the challenges of verifying compliance. Moving forward, the lessons learned in Syria must guide global efforts to prevent the proliferation and use of these devastating weapons, ensuring they remain tools of the past, not the future.

About Professor Stefano Costanzi

Dr. Stefano Costanzi is a Professor of Chemistry, holding degrees in both the chemical sciences and international affairs. On February 1, he will become the Isbell Chair of Chemistry at American University. Costanzi’s research interests rest at the nexus of chemistry, policy, security, and the application of computational methods to address practical problems. Focusing on countering chemical weapons, he studies arms control, nonproliferation, and counterproliferation strategies, policies, and practices with the goal of conceptualizing and developing solutions and tools to strengthen them, through a blend of chemistry, information technology, and science-based policy.

Costanzi is a member of several international organizations operating at the intersection of science and policy to combat the proliferation of chemical and biological weapons. His affiliations include the International Center for Chemical Safety and Security Global team, the Scientists Working Group on Biological and Chemical Security, and the Chemical Weapons Convention Coalition.