Chemical Weapons Under International Law: The CWC, OPCW and Accountability

Introduction

Chemical weapons are prohibited under international law through a combination of treaty obligations and customary rules governing armed conflict. The principal legal instrument is the Chemical Weapons Convention (CWC), which prohibits States Parties from developing, producing, acquiring, stockpiling, transferring or using chemical weapons. The Organisation for the Prohibition of Chemical Weapons (OPCW) oversees the Convention's implementation, verifies declarations and destruction activities, and investigates alleged chemical weapons use within its mandate.

The prohibition extends beyond conventional battlefields. Toxic agents such as VX and sarin, as well as industrial chemicals such as chlorine when used for prohibited purposes, fall within the Convention's legal framework. Its obligations apply in peacetime and during armed conflict, while national implementing legislation addresses prohibited activities by individuals and organizations.

The CWC has achieved substantial results. In July 2023, the OPCW confirmed the irreversible destruction of all chemical weapons stockpiles declared by States Parties up to that point, representing approximately 72,304 metric tons of chemical agents. Yet subsequent discoveries of undeclared weapons in Syria demonstrate that the destruction of known stockpiles does not establish the absence of prohibited weapons elsewhere.

Three questions determine the legal treatment of chemical weapons: what conduct international law prohibits, how compliance is established, and which institutions can hold violators accountable. Although the CWC establishes extensive obligations and an international verification system, responsibility for chemical attacks must still be determined through the applicable rules of state responsibility, domestic criminal law, and international criminal jurisdiction.

1. Chemical Weapons: Definition and Legal Scope

Article II of the CWC defines chemical weapons by reference to toxic chemicals, their precursors, specially designed munitions, and equipment associated with their use. The definition extends beyond bombs, artillery shells, and poisonous gases deployed during military operations.

Under Article II(2), a toxic chemical is any chemical capable, through its action on life processes, of causing death, temporary incapacitation, or permanent harm to humans or animals. The definition applies regardless of the chemical's origin, method of production, or physical form.

A substance does not become a chemical weapon solely because it is toxic. Its intended purpose, the quantities involved, and the equipment associated with its use may determine whether it falls within the Convention's prohibitions.

1.1 The General-Purpose Criterion

The CWC adopts a general-purpose criterion under Article II(1). Toxic chemicals and their precursors fall within the definition of chemical weapons unless intended for purposes not prohibited under the Convention, provided their types and quantities remain consistent with those purposes.

Article II(9) identifies permitted categories, including industrial, agricultural, research, medical, and pharmaceutical activities. It also recognizes protective purposes, certain military purposes unrelated to chemical warfare, and law enforcement. These categories do not provide unrestricted exceptions. The nature and scale of the activity must remain consistent with its stated purpose.

This approach addresses the problem of dual-use chemicals. Chlorine, for example, has legitimate applications in water treatment and industrial production. Its ordinary manufacture and use do not constitute chemical weapons activities. Deliberately releasing chlorine to exploit its toxic effects against people, however, may bring the substance within the Convention's definition of a chemical weapon.

The general-purpose criterion prevents circumvention through technical classification. A substance does not escape the prohibition merely because it was never designed as a military agent or does not appear in the Convention's schedules. Its intended use remains legally decisive.

1.2 VX, Sarin and Chlorine: Agents and Schedules

Chemical warfare agents differ in their physiological effects, persistence, and potential to cause harm. Nerve agents interfere with the normal functioning of the nervous system, while blister agents damage exposed tissue. Choking agents primarily injure the respiratory system.

VX is an exceptionally toxic organophosphorus nerve agent. It inhibits acetylcholinesterase, an enzyme required for normal nerve signaling. Severe exposure can cause respiratory failure and death. Sarin operates through a similar toxic mechanism, although its physical characteristics and persistence differ from those of VX.

Sulfur mustard is a blister agent associated with severe skin, eye, and respiratory injuries. Chlorine, by contrast, is a common industrial chemical that can cause serious lung damage when inhaled at harmful concentrations. Its civilian applications do not prevent it from being used as a chemical weapon.

The CWC's Annex on Chemicals establishes three schedules for applying verification and control measures. Schedule 1 includes substances such as VX, sarin, and sulfur mustard, which present particularly serious risks under the treaty's scheduling criteria and have limited legitimate applications.

Schedule 2 covers other toxic chemicals and precursors presenting significant risks, while Schedule 3 includes substances with relevant weapons-related risks that may also be produced in substantial quantities for peaceful purposes.

The schedules serve regulatory purposes rather than defining the outer limits of the prohibition. Chlorine is not listed in them, but its use as a chemical weapon remains prohibited. The legal classification depends on the Convention's general-purpose criterion, not simply the presence of a substance on a schedule.

1.3 Riot Control and Law Enforcement

The CWC distinguishes the use of chemicals for legitimate law enforcement from their employment as methods of warfare. Article II(7) defines riot control agents by their capacity to produce rapid sensory irritation or disabling physical effects that disappear within a short period after exposure ends.

Article I(5) expressly prohibits States Parties from using riot control agents as a method of warfare. The prohibition applies even where the same agent may be lawfully employed for certain domestic policing activities.

The distinction becomes more difficult when substances produce incapacitation through the central nervous system rather than temporary sensory irritation. On December 1, 2021, the OPCW Conference of the States Parties adopted Decision C-26/DEC.10, addressing the aerosolized use of central nervous system-acting chemicals for law-enforcement purposes.

The Conference determined that such aerosolized use is inconsistent with law enforcement as a permitted purpose under the Convention. The decision drew on scientific findings concerning the substantial risks associated with these substances, including the possibility of permanent injury or death.

It did not prohibit their legitimate medical or other permitted applications. The decision specifically addressed aerosolized use in the context of law enforcement.

A government's designation of a substance as an incapacitating agent does not independently establish that its deployment is permissible. The applicable legal assessment must consider the chemical's effects, its intended purpose, and the Convention's requirements.

2. From the Geneva Protocol to a Comprehensive Prohibition

International restrictions on chemical warfare developed before the establishment of the CWC. Early efforts addressed particular methods of combat, including the use of poison and poisonous gases. Industrialized chemical warfare during the First World War demonstrated the destructive consequences of large-scale toxic-agent deployment.

The use of chlorine, phosgene, and mustard agents during that conflict caused extensive casualties and long-term injuries. The experience contributed to international efforts to prohibit chemical warfare, although early legal instruments did not eliminate the production or possession of the substances involved.

2.1 The Limited Reach of the 1925 Geneva Protocol

The 1925 Geneva Protocol prohibited the use in war of asphyxiating, poisonous, or other gases and analogous liquids, materials, and devices. It reinforced earlier restrictions on poisonous weapons and became a central instrument regulating chemical warfare.

Its principal limitation concerned the activities that it regulated. The Protocol prohibited use but did not establish a comprehensive ban on developing, manufacturing, or stockpiling chemical weapons. States could maintain substantial arsenals without necessarily breaching its terms.

Reservations concerning retaliation further complicated the historical regime. Some States accepted the prohibition while reserving the possibility of responding in kind if an opposing party used chemical weapons first. Such reservations reflected an approach in which deterrence and the possibility of retaliation remained prominent considerations.

Chemical warfare nevertheless continued after 1925. Iraqi forces employed chemical weapons during the Iran–Iraq War, including against Iranian forces and Kurdish civilians. The March 1988 attack on Halabja became one of the most prominent examples of chemical weapons use against a civilian population.

These incidents exposed the limitations of restrictions on battlefield use without corresponding international mechanisms to eliminate chemical arsenals and verify compliance. Subsequent negotiations sought a comprehensive system that would regulate the entire life cycle of chemical weapons.

The Chemical Weapons Convention opened for signature in Paris on January 13, 1993, and entered into force on April 29, 1997. Unlike the Geneva Protocol, it prohibited an entire category of weapons and established international verification of destruction and relevant chemical activities.

2.2 Customary Prohibition in Armed Conflict

The prohibition on using chemical weapons also exists independently of the CWC as a rule of customary international humanitarian law. Rule 74 of the International Committee of the Red Cross's study of customary international humanitarian law identifies the prohibition as applicable in both international and non-international armed conflicts.

Customary international law develops through sufficiently general state practice accepted as law. In the chemical weapons context, treaty practice, national legislation, military manuals, and repeated international condemnation support the existence of a prohibition extending beyond CWC membership.

The International Criminal Tribunal for the former Yugoslavia examined this development in Prosecutor v. Tadić, Decision on the Defence Motion for Interlocutory Appeal on Jurisdiction, Case No. IT-94-1-AR72, October 2, 1995.

In paragraph 124, the Appeals Chamber concluded that international consensus had emerged on the prohibition of chemical weapons in internal armed conflicts. Its reasoning drew partly on international reactions to Iraq's use of chemical weapons against Kurdish civilians.

The decision supported the development of customary rules regulating methods of warfare in non-international conflicts. It did not establish that every treaty rule applicable to international armed conflicts had automatically become applicable to internal conflicts.

This distinction is central to international humanitarian law. Customary rules prohibit chemical weapons use by parties to armed conflicts, including organized non-state armed groups. The CWC imposes additional treaty obligations on States Parties concerning production, possession, transfer, verification, and destruction.

The two legal regimes are complementary, but their sources and obligations are not identical.

3. The Convention's Prohibitions and State Duties

The CWC establishes a comprehensive legal system for chemical disarmament and non-proliferation. Its general prohibitions are accompanied by obligations concerning declarations, destruction, verification, and national implementation.

The Convention also preserves legitimate chemical activities. Article VI recognizes that States Parties may develop, produce, acquire, retain, transfer, and use toxic chemicals and precursors for purposes not prohibited under the treaty, subject to its regulatory conditions.

The Convention's operation reflects the principle of pacta sunt servanda, expressed in Article 26 of the Vienna Convention on the Law of Treaties. Treaties in force bind their parties and must be performed in good faith. The specific requirements of chemical disarmament arise from the CWC itself.

3.1 Article I: Prohibited Conduct Without Exception

Article I establishes the CWC's principal prohibitions. Each State Party undertakes never under any circumstances to develop, produce, otherwise acquire, stockpile, or retain chemical weapons, or transfer them directly or indirectly to anyone.

The provision separately prohibits chemical weapons use, military preparations for use, and assistance, encouragement, or inducement of prohibited activities.

The Convention does not provide an exception permitting chemical warfare in self-defense, military necessity, or retaliation against another party's chemical attack. Its general prohibitions apply regardless of the circumstances in which a State might seek to employ these weapons.

Article XXII also prohibits reservations to the Convention's articles. Reservations to its annexes are permitted only if compatible with the Convention's object and purpose. States Parties cannot selectively exclude the central prohibition on chemical weapons use through a treaty reservation.

The obligations extend beyond the conduct of government armed forces. Prohibited transfers or assistance may constitute treaty breaches, and States Parties must adopt domestic measures addressing activities undertaken by natural and legal persons within the Convention's jurisdictional requirements.

3.2 Declarations, Destruction and National Legislation

Article III requires States Parties to submit declarations concerning chemical weapons, relevant production facilities, historical transfers, and other specified activities. These declarations provide the basis for international verification.

Articles IV and V establish obligations concerning the destruction of chemical weapons and chemical weapons production facilities. Destruction must follow prescribed procedures and verification requirements. The Convention also contains separate arrangements for old and abandoned chemical weapons, reflecting their different historical and technical circumstances.

Article VII requires States Parties to adopt the necessary national implementation measures. Under Article VII(1)(a), these measures must prohibit natural and legal persons from undertaking activities forbidden by the Convention within the specified territorial and jurisdictional scope, including through penal legislation.

Article VII(1)(c) extends the relevant penal legislation to prohibited activities undertaken abroad by natural persons possessing the State Party's nationality, in conformity with international law. Article VII(2) requires cooperation and appropriate legal assistance between States Parties to facilitate implementation.

These obligations give the Convention a domestic enforcement dimension. National authorities must possess the legal means to address prohibited activities, while international verification examines compliance with the treaty's requirements.

The Convention also addresses assistance and protection against chemical attacks through Article X. Article XI promotes international cooperation in peaceful chemical activities, reflecting the objective of eliminating chemical weapons without unnecessarily obstructing legitimate scientific and industrial development.

4. The OPCW: Verification, Inspections and Compliance

The OPCW was established under Article VIII of the CWC to implement its provisions and oversee the international verification system. Its headquarters are located in The Hague, Netherlands.

The organization monitors declarations, conducts inspections, verifies destruction, and assists States Parties with implementation. It also addresses compliance concerns through the institutional procedures established by the Convention.

The OPCW's authority is defined by its constituent treaty and subsequent decisions adopted within that legal framework. It can establish technical findings and support compliance decisions, but it does not possess general criminal jurisdiction or the authority to convict individuals.

4.1 The Conference, Executive Council and Secretariat

The Conference of the States Parties is the OPCW's principal policy-making body. It considers matters relating to treaty implementation, reviews compliance, and exercises the powers assigned to it under the Convention.

The Executive Council supervises aspects of the organization's activities and addresses implementation and compliance concerns within its mandate. It may recommend measures to the Conference and exercise powers delegated to it in accordance with the Convention.

The Technical Secretariat conducts much of the organization's scientific and operational work. Its responsibilities include inspecting relevant facilities, examining declarations, analyzing technical information, and preparing reports for the policy-making organs.

Designated laboratories provide specialized analytical capabilities. National authorities coordinate the domestic implementation of treaty obligations and serve as points of contact with the OPCW.

These institutional arrangements allow technical findings to inform compliance decisions while preserving the different functions assigned to the organization's constituent bodies.

4.2 Routine Verification and Challenge Inspections

The CWC's Verification Annex establishes inspection procedures according to the activities and facilities concerned. Routine verification covers declared chemical weapons, destruction operations, scheduled chemicals, and other relevant industrial production.

For legitimate chemical facilities, inspections assess whether declarations correspond with observable activities and whether operations are consistent with treaty obligations. The applicable requirements depend on the substances, quantities, and verification categories involved.

Article IX provides a mechanism for resolving concerns about possible non-compliance. A State Party may request a challenge inspection of another State Party's facility or location within the Convention's jurisdictional framework.

The inspected State Party is obliged to accept the inspection in accordance with treaty procedures. The Convention also provides safeguards for confidential information and mechanisms intended to prevent abusive requests.

Challenge inspections differ from routine verification and investigations of alleged attacks. They are designed to clarify compliance concerns arising under the treaty rather than automatically determine criminal responsibility.

4.3 Destruction of Declared Stockpiles in 2023

On July 7, 2023, the OPCW confirmed that the last chemical weapon from the stockpiles declared by States Parties had been irreversibly destroyed. The final destruction occurred at the Blue Grass Chemical Agent-Destruction Pilot Plant in Kentucky, United States.

The verified destruction covered approximately 72,304 metric tons of declared chemical agents accumulated under the Convention's declaration and verification system. It marked the completion of a central objective of the CWC.

The achievement concerned declared stockpiles. It did not establish that all national declarations were complete, that undisclosed weapons could not exist, or that every remaining treaty obligation had been fulfilled.

Old and abandoned chemical weapons continue to require identification and destruction under the Convention's applicable procedures. Industrial verification must also continue to prevent renewed prohibited production.

Subsequent discoveries in Syria demonstrated the practical significance of these remaining obligations. Earlier verified destruction activities had not revealed the full extent of the former Syrian chemical weapons programme.

5. Investigation and Attribution of Alleged Attacks

Investigations of alleged chemical weapons attacks involve several factual questions. Investigators must determine whether a toxic chemical was present, whether it was employed as a weapon, how the incident occurred, and whether the available evidence identifies those responsible.

Scientific confirmation of chemical exposure does not establish the identity of the perpetrator. Evidence linking an attack to a military unit or organization may support an attribution finding, but individual criminal liability requires additional legal determinations.

International investigative mechanisms operate under defined mandates and evidentiary standards. Their findings may assist subsequent legal proceedings without replacing the functions of national or international courts.

5.1 OPCW Missions and Their Different Mandates

The OPCW established its Fact-Finding Mission (FFM) in Syria in 2014 to determine whether toxic chemicals had been used as weapons. Its mandate did not include identifying the perpetrators of alleged attacks.

The OPCW–United Nations Joint Investigative Mechanism (JIM), established through Security Council Resolution 2235 (2015), subsequently investigated responsibility for specified incidents. Its mandate expired in November 2017.

On June 27, 2018, the OPCW Conference of the States Parties adopted Decision C-SS-4/DEC.3, authorizing arrangements to identify perpetrators of chemical weapons use in Syria. The Investigation and Identification Team (IIT) was established pursuant to that decision.

The IIT examines qualifying incidents for which the FFM determined that chemical weapons use or likely use occurred and for which the former JIM had not issued a final attribution finding.

The Declaration Assessment Team (DAT) performed a different function. Established in 2014, it examined gaps, inconsistencies, and discrepancies in Syria's declared chemical weapons programme.

Following the change of government in Syria, these activities were coordinated through the OPCW Office of Special Missions, established on June 1, 2025.

The IIT's legal authority remains limited. Its fifth report, issued as OPCW Technical Secretariat document S/2478/2026 on January 21, 2026, expressly states that the team is not a judicial body authorized to assign individual criminal responsibility. Nor does it possess authority to make final findings of non-compliance with the CWC.

The team's mandate is to establish facts and identify perpetrators within the framework of the relevant OPCW decisions. Formal treaty compliance measures and criminal adjudication belong to different institutional processes.

Outside these Syria-specific arrangements, the United Nations Secretary-General's Mechanism for Investigation of Alleged Use of Chemical and Biological Weapons provides a separate investigative framework. Its mandate and procedures should not be equated with those of the IIT.

5.2 Evidence, Attribution and Evidentiary Thresholds

Chemical weapons investigations may involve environmental samples, biomedical specimens, laboratory findings, witness interviews, medical records, satellite imagery, and authenticated photographs or videos.

Investigators may examine munition remnants, delivery systems, and the circumstances of military operations. Technical findings are assessed alongside witness testimony and other available information to determine whether the evidence supports a coherent account of an incident.

Chain-of-custody procedures help establish the origin, handling, and integrity of physical evidence. They are particularly important when investigators gain access to a site long after an alleged attack or when materials have passed through several custodians.

The IIT applies a reasonable-grounds evidentiary standard. Its conclusions depend on the collective assessment and corroboration of available information, including the consideration of alternative explanations.

The fifth IIT report concerning Kafr Zeita describes the examination of witness statements, samples, munition remnants, computer modeling, satellite imagery, and other documentary evidence. The investigators assessed the consistency and probative value of that material before reaching their attribution findings.

The reasonable-grounds standard differs from the criminal standard of proof beyond reasonable doubt. An IIT finding may identify a perpetrator for institutional purposes without establishing every element required for conviction.

Criminal courts must independently assess jurisdiction, the elements of the alleged offense, individual responsibility, and the procedural rights of the accused.

6. Syria: Attribution, Disarmament and Treaty Compliance

Syria illustrates the difficulty of enforcing a comprehensive chemical weapons prohibition during armed conflict and political instability. Its experience includes verified chemical weapons use, incomplete declarations, disarmament operations, and successive investigative mechanisms.

The Syrian case demonstrates that compliance cannot be assessed solely by measuring the quantity of weapons destroyed. Complete declarations, access to relevant facilities, cooperation with investigators, and the elimination of undeclared capabilities remain independent legal requirements.

The obligations of the Syrian state did not disappear following the fall of Bashar al-Assad's government in December 2024. A change of government does not ordinarily terminate the international legal personality of the State or extinguish its existing treaty commitments.

6.1 Accession and Security Council Resolution 2118

The large-scale chemical attack in the Ghouta area near Damascus on August 21, 2013, intensified international efforts to address Syria's chemical weapons programme.

The United Nations Mission to Investigate Allegations of the Use of Chemical Weapons in the Syrian Arab Republic established that chemical weapons had been used in Ghouta, including the nerve agent sarin. Its September 2013 findings were supported by biomedical and environmental evidence, laboratory analysis, and the examination of affected sites and munitions.

The mission's mandate concerned determining whether chemical weapons had been used. It did not include formally attributing the attack to a particular perpetrator.

Syria deposited its instrument of accession to the CWC on September 14, 2013, and the Convention entered into force for Syria on October 14, 2013. These developments accompanied international arrangements for eliminating its declared chemical weapons.

On September 27, 2013, the United Nations Security Council adopted Resolution 2118, endorsing arrangements for the destruction of Syria's chemical weapons programme and reinforcing the relevant international obligations.

The OPCW subsequently supervised the removal and destruction of declared chemical agents and other relevant materials. These operations were significant, but concerns about the accuracy and completeness of Syria's declarations remained.

Subsequent OPCW investigations established additional instances of chemical weapons use. The former Syrian authorities' incomplete declarations and failure to disclose the full scope of their programme prevented earlier destruction activities from resolving Syria's compliance obligations.

6.2 Attribution Findings for State and Non-State Actors

The OPCW's investigations identified perpetrators of several chemical attacks in Syria. The findings differ according to the incident, the available evidence, and the actors involved.

In its third IIT report, S/2125/2023, issued on January 27, 2023, the OPCW concluded that there were reasonable grounds to believe that Syrian Arab Air Forces were responsible for the April 7, 2018 chemical attack in Douma.

The report found that at least one helicopter operating under the control of the Syrian Tiger Forces dropped two cylinders containing toxic chlorine gas onto residential buildings. The attack killed 43 named individuals and affected dozens more.

The fourth IIT report, S/2255/2024, issued on February 22, 2024, concerned the September 1, 2015 attack in Marea. It concluded that there were reasonable grounds to believe units of the Islamic State in Iraq and the Levant (ISIL) had deployed sulfur mustard during military operations against the town.

The Marea investigation identified modified artillery projectiles used to disperse the chemical agent. It also examined evidence concerning the organizational structures and command arrangements involved in the operation.

The findings demonstrate that chemical weapons use in Syria involved both state forces and organized armed groups. The applicable obligations of non-state actors in international law depend on the relevant legal regime. Parties to an armed conflict are bound by applicable international humanitarian law, while the CWC imposes treaty obligations on its States Parties.

In January 2026, the IIT issued its fifth report, S/2478/2026, addressing an incident in Kafr Zeita on October 1, 2016.

The investigation concluded that there were reasonable grounds to believe a Syrian Arab Air Force helicopter dropped at least one pressurized chlorine cylinder near a cave system in the Wadi al-Aanz valley. The cylinder struck an area near the ventilation openings of a hospital operating within the cave complex.

The cylinder ruptured and released chlorine gas, injuring 35 named individuals and affecting dozens more. The IIT considered physical evidence, witness statements, imagery, computer modeling, and other information before identifying the Syrian Arab Air Forces as the perpetrators within its mandate.

These reports provide documented attribution findings, not criminal convictions. They do not establish that every individual associated with the military units or organizations involved bears criminal responsibility.

6.3 Undeclared Weapons and the 2026 Disarmament Effort

The fall of the Assad government in December 2024 created new opportunities for OPCW access to facilities, documentation, and other locations associated with Syria's chemical weapons programme.

On February 8, 2025, the OPCW Director-General visited Damascus and met representatives of the new Syrian authorities. Subsequent cooperation allowed investigators to examine suspected locations, collect documentation, and continue verification activities.

In May 2026, an OPCW expert team discovered a significant quantity of previously undeclared chemical weapons-related materials. The findings included chemical munitions, separately stored substances, equipment, and thousands of pages of documentation concerning the former Syrian programme.

Among the discoveries were aerial bombs resembling types previously associated with chemical attacks in Syria, along with rockets of a type connected to the 2013 Ghouta attack.

The discoveries confirmed the OPCW's longstanding assessment that the former Syrian authorities had withheld information about the extent of their chemical weapons activities.

On July 9, 2026, the OPCW Executive Council adopted Decision EC-112/DEC.7, reinstating Syria's rights and privileges under the Convention. Those rights had been suspended in 2021 following findings concerning the former Syrian government's non-compliance.

The reinstatement followed progress in cooperation, including amendments to Syria's initial declaration, preparations for systematic verification, and initial steps toward the destruction of identified remnants.

The Executive Council also approved a detailed plan for verifying the destruction of Category 3 chemical weapons at Al Qutayfah and facility agreements relating to the Al Qutayfah and Homs chemical weapons storage facilities.

The decision did not certify complete compliance. Syria remained responsible for fulfilling its outstanding obligations, and the OPCW retained its verification responsibilities.

Further progress was confirmed later in 2026. In an address delivered on October 26, OPCW Deputy Director-General Mekondjo Kaapanda-Girnus reported that 42 previously undeclared chemical aerial bombs associated with the former Syrian programme had been discovered, declared, and irreversibly destroyed during the year under OPCW verification.

The destruction marked the first OPCW-verified elimination of chemical munitions in Syria since 2014. The available official confirmation does not establish that all 42 bombs had been destroyed before the July reinstatement decision.

Syria's experience demonstrates the difference between verified destruction of identified weapons and complete fulfillment of treaty obligations. International verification must continue until remaining declarations, materials, and facilities have been addressed in accordance with the Convention.

7. From Treaty Violations to Legal Accountability

International law distinguishes the responsibility of States from the criminal responsibility of individuals. A chemical weapons attack may engage both forms of responsibility, but the applicable rules, evidentiary requirements, and institutions differ.

A State may incur international responsibility when conduct attributable to it breaches an obligation binding upon that State. Criminal proceedings require a separate legal basis and proof that an individual committed an offense within the jurisdiction of the court concerned.

The OPCW contributes to accountability by investigating incidents, preserving relevant information, and reporting findings. It does not prosecute or convict alleged perpetrators.

7.1 State Responsibility and Article XII Measures

The international responsibility of a State arises when conduct attributable to it breaches an international obligation. The general framework is reflected in the International Law Commission's 2001 Articles on Responsibility of States for Internationally Wrongful Acts.

The Articles are not a treaty, although significant provisions reflect customary international law. Article 2 identifies attribution and breach as the elements of an internationally wrongful act.

Where a State's armed forces violate the CWC, attribution may follow from their status as state organs under the general law of responsibility. Different questions arise where private persons or organized armed groups are involved, particularly when instructions, direction, or control by the State are alleged.

The consequences of state responsibility in international law may include cessation of the wrongful conduct, assurances of non-repetition where appropriate, and reparation for legally established injury.

Article XII of the CWC establishes additional procedures for addressing treaty compliance concerns.

Under Article XII(2), the Conference of the States Parties may restrict or suspend a State Party's rights and privileges when the applicable conditions are satisfied, including failure to comply with a request by the Executive Council to remedy a compliance problem.

Article XII(3) authorizes the Conference to recommend collective measures to States Parties, in conformity with international law, where activities prohibited by the Convention may cause serious damage to its object and purpose.

Article XII(4) imposes a different obligation. In cases of particular gravity, the Conference shall bring the issue, including relevant information and conclusions, to the attention of the United Nations General Assembly and Security Council.

This mandatory referral provision does not itself authorize military force or impose Security Council sanctions. Any subsequent UN action must rest on the legal powers and procedures applicable to the relevant UN organ.

The suspension of certain Syrian rights and privileges in 2021, followed by their reinstatement in July 2026, illustrates the operation of the Convention's compliance procedures. Reinstatement reflected progress under the treaty without extinguishing questions concerning earlier violations or outstanding obligations.

7.2 War Crimes and the Limits of ICC Jurisdiction

Chemical weapons use during armed conflict may constitute a war crime under international criminal law. Article 8 of the Rome Statute identifies prohibited methods of warfare, including the employment of poison, poisoned weapons, and asphyxiating or poisonous gases.

Articles 8(2)(b)(xvii) and 8(2)(b)(xviii) address these offenses in international armed conflicts.

At the Kampala Review Conference on June 10, 2010, States Parties adopted Resolution RC/Res.5, introducing corresponding provisions for non-international armed conflicts through Articles 8(2)(e)(xiii) and 8(2)(e)(xiv).

The amendments did not become applicable to every State Party automatically. Under Article 121(5) of the Rome Statute, amendments to Article 8 enter into force for accepting States Parties one year after the deposit of their instruments of ratification or acceptance.

Article 121(5) also provides that, regarding a crime covered by an amendment, the Court shall not exercise jurisdiction when the crime is committed by the nationals of a State Party that has not accepted the amendment or on that State Party's territory.

These amendment-specific conditions must be considered alongside the ordinary jurisdictional provisions of Articles 11, 12, and 13 and any relevant question concerning Security Council referral.

The Kampala amendments to Article 8 first entered into force on September 26, 2012, for San Marino, following its ratification on September 26, 2011. Their subsequent application depends on the relevant State's treaty position and the applicable jurisdictional rules.

A violation of the CWC does not automatically constitute a war crime within the jurisdiction of the ICC. The conduct must satisfy the elements of an offense defined in the Rome Statute.

For the provisions concerning poison and prohibited gases, those elements include the employment of substances or weapons meeting the specified harmful characteristics, a connection with the relevant category of armed conflict, and awareness of the factual circumstances establishing that conflict.

The ICC's Elements of Crimes specify additional requirements concerning the substances and the circumstances of their use. Treaty prohibitions concerning possession, production, or transfer cannot simply be converted into the Rome Statute's offenses concerning employment of prohibited weapons.

The Rome Statute of the International Criminal Court also establishes jurisdictional limitations. Syria is not a State Party, and the Court does not possess general jurisdiction over crimes committed on Syrian territory merely because the alleged conduct violates international criminal law.

A Security Council referral under Article 13(b), an applicable territorial or nationality connection, or another legally valid basis would be necessary. The relevant temporal and substantive jurisdictional requirements would still need to be examined.

Chemical weapons use may also form part of crimes against humanity where the separate elements of Article 7 are established. These include a widespread or systematic attack directed against a civilian population, undertaken pursuant to or in furtherance of a State or organizational policy, and the accused's knowledge of the attack.

The use of a chemical agent alone does not establish those additional requirements.

7.3 Domestic Prosecutions and the VX Incident

Domestic criminal jurisdiction provides an important means of enforcing chemical weapons prohibitions. Article VII of the CWC requires States Parties to adopt implementing legislation, including penal measures addressing prohibited activities by natural and legal persons.

The Convention also requires nationality-based extensions of the relevant penal legislation and cooperation between States Parties. These provisions recognize that prohibited chemical weapons activities may involve cross-border transfers, foreign nationals, and conduct occurring outside the territory of the prosecuting State.

United Nations Security Council Resolution 1540 (2004), adopted under Chapter VII of the UN Charter, reinforces national obligations concerning weapons of mass destruction. It requires States to adopt and enforce appropriate laws preventing non-state actors from manufacturing, acquiring, possessing, developing, transporting, transferring, or using nuclear, chemical, or biological weapons and their means of delivery, as well as establishing relevant domestic controls.

The fatal VX incident at Kuala Lumpur International Airport 2 on February 13, 2017, illustrates chemical weapons use outside conventional warfare.

Malaysian authorities reported that VX had been identified in connection with the death of a North Korean national. The deceased was widely identified as Kim Jong-nam. The OPCW's official decision addressed the incident on the basis of information supplied by Malaysia rather than an independent OPCW criminal determination.

On March 9, 2017, the OPCW Executive Council adopted Decision EC-84/DEC.8, expressing grave concern over Malaysia's report that VX had been used in the fatal incident. The Council supported Malaysia's investigation and requested technical assistance from the Director-General upon Malaysia's request.

The criminal investigation remained a matter for Malaysian authorities. Any separate allegation of responsibility by another State would require evidence and application of the relevant international attribution rules.

The incident demonstrates that intentional poisoning involving a chemical weapon does not automatically constitute a war crime. Without the necessary connection to an armed conflict, liability must be assessed under other applicable legal provisions, including domestic criminal law.

National proceedings may also involve international crimes arising during armed conflicts. The July 25, 2025 decisions of the French Court of Cassation concerning Syria provide an important illustration of the procedural questions that can arise.

In case No. 24-84.393, the Court examined an arrest warrant issued against Bashar al-Assad in November 2023 in connection with allegations of complicity in crimes against humanity and war crimes arising from chemical attacks in Syria in August 2013.

The Court held that incumbent foreign heads of state benefit from personal immunity before French criminal courts under the applicable international law. It determined that the legality of an arrest warrant must be assessed according to the circumstances existing when the warrant was issued.

Because Assad was a serving head of state in November 2023, the Court annulled the warrant. It expressly indicated that this annulment did not determine the validity of a warrant issued after the individual had ceased to hold office.

The judgment did not decide Assad's criminal guilt or innocence. It addressed the procedural consequences of immunity at the time the warrant was issued.

In a separate decision delivered the same day, case No. 24-84.071, the Court addressed functional immunity for foreign state officials. It held that, subject to applicable treaty provisions, functional immunity could not be invoked to prevent proceedings concerning genocide, crimes against humanity, or war crimes.

These rulings illustrate the importance of distinguishing substantive criminal responsibility from jurisdiction and immunity. They do not establish a universal rule eliminating the personal immunity of serving heads of state before foreign national courts.

Evidence preservation is another essential part of accountability. The United Nations General Assembly established the International, Impartial and Independent Mechanism for Syria (IIIM) through Resolution 71/248 in December 2016.

The IIIM collects, consolidates, preserves, and analyzes evidence relating to serious international crimes committed in Syria since March 2011, with the objective of assisting competent criminal jurisdictions.

Its cooperation with the OPCW is particularly relevant to chemical weapons investigations. The institutions have established arrangements allowing the IIIM to access relevant OPCW material, including information collected through IIT investigations.

The IIIM is not a court and does not possess authority to conduct criminal trials. Its contribution lies in preserving evidence and supporting jurisdictions legally empowered to investigate and prosecute international crimes.

8. The Remaining Limits of Chemical Disarmament

The CWC has achieved exceptionally broad participation for an international disarmament treaty. As of November 2026, it has 193 States Parties, representing approximately 98 percent of the world's population.

Egypt, Israel, North Korea, and South Sudan remain outside full participation in the Convention. Israel has signed but not ratified the treaty, while the other three States have not joined.

The continued absence of these States limits the Convention's universality. Their treaty positions differ from those of States Parties, although the customary prohibition on chemical weapons use during armed conflict remains applicable independently of CWC membership.

Undeclared chemical weapons programmes present another difficulty. Verification operates through national declarations, inspections, technical information, and other activities authorized by the Convention. Where States conceal facilities or materials, investigators may face substantial obstacles to establishing the full scale of prohibited activity.

Old and abandoned chemical weapons require continuing attention. They may present serious risks long after the conflicts or programmes responsible for their existence have ended. The Convention's provisions recognize that historical weapons can create ongoing obligations concerning identification, destruction, and international cooperation.

Non-state actors create additional security concerns. Chemical substances and relevant technical knowledge may be available through legitimate industries or research institutions. Preventing diversion requires effective national legislation, export and transfer controls, law enforcement, and international cooperation.

Scientific and technological developments also affect verification. New chemical compounds, changes in production methods, and increasingly sophisticated industrial processes may complicate the identification of prohibited weapons development.

The CWC's general-purpose criterion permits its prohibitions to apply beyond an exhaustive list of named substances. Practical monitoring, however, still depends on scientific expertise, accurate national declarations, and adequate institutional resources.

The OPCW's continuing responsibilities consequently extend beyond stockpile destruction. They include industrial inspections, verification of outstanding declarations, assistance and protection, support for national implementation, and measures designed to prevent renewed chemical weapons production.

These responsibilities concern the practical capacity of States and international institutions to establish compliance and respond to evidence of prohibited activities. The underlying legal prohibition remains firmly established.

Conclusion

The prohibition of chemical weapons is firmly established in international law. The Chemical Weapons Convention provides a comprehensive treaty system governing their development, production, acquisition, transfer, possession, and use. Customary international humanitarian law independently prohibits chemical warfare in both international and non-international armed conflicts.

The OPCW gives practical effect to the Convention through declarations, inspections, technical verification, and investigations. Its confirmation in 2023 that all stockpiles declared up to that point had been destroyed represents a major achievement in international disarmament. Subsequent discoveries in Syria demonstrate why the elimination of declared weapons cannot be equated with complete assurance that no prohibited materials remain.

Accountability depends on further legal mechanisms. Treaty compliance procedures address States Parties' obligations, the law of state responsibility governs internationally wrongful conduct attributable to States, and criminal jurisdiction determines whether individuals may be prosecuted for particular offenses.

The Syrian investigations and subsequent judicial proceedings illustrate the limits of institutional attribution. Even where credible evidence identifies those responsible for chemical attacks, prosecution may depend on jurisdiction, immunity, the availability of admissible evidence, and the applicable criminal law.

The effectiveness of the international chemical weapons regime rests on maintaining the connection between a comprehensive legal prohibition, independent verification, and enforceable accountability. Each performs a distinct function. The existence of one does not guarantee the operation of the others, and the continuing elimination of chemical weapons depends on all three.

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