The April 6 cruise missile strikes against Syria in response to its chemical weapon attacks on Khan Sheikhoun have generated considerable commentary on Just Security and elsewhere.  In this article, we offer our thoughts on the jus ad bellum framework (the international legal rules that regulate the resort to force) in assessing the U.S. strikes, with an emphasis on humanitarian intervention.

First, though, let us put to rest the central issue under jus in bello (the rules regulating the conduct of belligerents once an armed conflict has begun).  Syria and the United States are now involved in an international armed conflict.  The ICRC’s commentary to Common Article 2 of the 1949 Geneva Conventions (2016) makes clear that an international armed conflict exists whenever there are hostilities between States.  This is so irrespective of “how long the conflict lasts, or how much slaughter takes places,” an approach adopted by the Department of Defense (see DoD Law of War Manual, ¶3.4.2).  The armed conflict with Syria will continue until there is a prolonged cessation of hostilities.  This temporal conclusion precludes what an international criminal tribunal referred to as a “revolving door between applicability and nonapplicability” (the ICTY trial chamber decision in Gotovina,1694). Accepting that an IAC exists between the United States and Syria to which international humanitarian law applies, we turn to the lawfulness of the missile strike under the jus ad bellum.

Article 2(4) of the UN Charter prohibits the “threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the purposes of the United Nations.”  There are two Charter-based exceptions – UN Security Council action pursuant to Article 42 (a “Chapter VII operation”) and self-defense based on Article 51 of the Charter and its customary international law analogue. Extra-charter exceptions include consent and, perhaps, humanitarian intervention or “forcible countermeasures” (a term we describe below).

The Department of Defense statement justifying the operation refers to a “proportional response,” but goes on to note “the strike was intended to deter the regime from using chemical weapons again.”  The President on the other hand, pointed to the “vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons.”  Neither is useful in providing granularity as to the legal basis for the operation. This being so, we will serially address each of the potential legal bases for the strikes, with the exception of consent, which obviously was not granted by Syria.

Security Council

The Security Council has long taken measures designed to limit the proliferation of weapons of mass destruction, including chemical weapons.  Notable in this context is Security Council Resolution 2118, which endorsed the Framework for the Elimination of Syrian Chemical Weapons and the associated procedures for destroying Syrian chemical weapons that were adopted by the Organization for the Prohibition of Chemical Weapons.  The resolution declared “that the use of chemical weapons anywhere constitutes a threat to international peace and security” and that non-compliance would result in the imposition of “measures under Chapter VII of the United Nations Charter.” Yet, in the face of Syrian use of such weapons, Russia and China vetoed the 28 February Security Council Draft Resolution (S/2017/172) that would have imposed sanctions for that use.  This was the seventh and sixth veto respectively for Russia and China on matters related to Syria (S/2016/1026, S/2016/846, S/2014/348, S/2012/538, S/2012/77, and S/2011/612).  Thus, to date, the Security Council has not authorized the use of force against Syria.


Article 51 of the UN Charter provides that States have an “inherent right of individual or collective self-defense if an armed attack occurs.” Self-defense as a basis for the missile strike can be quickly dispensed with as the chemical weapons attack was not directed at the United States or any State with regard to which the United States might have been acting in collective defense.  Suggestions of anticipatory self-defense would be equally untenable, as there is no basis for concluding that Syria will (or even might) conduct a chemical attack against any other State.  This would be the case even with respect to US troops in the country unless there was reason to believe that Syria was imminently going to target them or otherwise conduct chemical operations in which it would be reasonably forseeable that they would be harmed. There is no basis for drawing such a conclusion, at least not yet.

Forcible Countermeasures

The International Court of Justice confirmed the existence of obligations erga omnes, which are those obligations owed to the entire international community, in the Barcelona Traction case. As the commentary to the Draft Articles on State Responsibility notes, “every State, by virtue of its membership in the international community, has a legal interest in the protection of certain basic rights and the fulfillment of certain essential obligations.”  This raises the question of whether the use of chemical weapons is a violation of an obligation erga omnes and, if so, who has the right to enforce it and how?

In Barcelona Traction, the court characterized obligations erga omnes as deriving from the “outlawing of acts of aggression, and of genocide, [and] also from the principles and rules concerning the basic rights of the human person.” It is reasonable to contend that, given the universally accepted and long-standing ban on the use the weapons in any circumstance, an obligation to refrain from using chemical weapons against civilians qualifies as such an obligation.

Assuming, for the sake of analysis, that the prohibition on the use of a chemical weapons is an erga omnes norm, all States would effectively be “injured States” under the law of State responsibility. On this basis the United States would at least arguably be entitled to take countermeasures (acts that would be unlawful but for the fact that they are in response to another State’s breach of a legal obligation owed the former) to compel Syria to comply with international law. However, countermeasures are generally understood not to include the use of force, a view advanced by the International Law Commission in Article 50(1) of the Articles on State Responsibility and one with which the authors agree.  Although the permissibility of forcible countermeasures is the topic of some debate—primarily because of the logic set forth by Judge Simma in his Oil Platforms separate opinion—there seems to be agreement that even if forcible countermeasures are permissible, they may not reach the level of intensity of an armed attack.  Clearly, an operation involving the use of 59 cruise missile strikes launched from warships does so.  Therefore, there is no basis for styling the US operation as a countermeasure.

Humanitarian Intervention

A number of commentators have addressed the argument that the strikes might be lawful under the concept of humanitarian intervention (see here for a roundup of some).  Humanitarian intervention is a highly controversial basis for the use of force.  Indeed, the International Court of Justice in Nicaragua held that “the use of force could not be the appropriate method to monitor or ensure such respect” for human rights.  In the intervening three decades since the Nicaragua judgment, however, much has changed.

There is growing evidence in support of a right of humanitarian intervention.  First, the UN Security Council has approved humanitarian interventions, including in Somalia (1992 and 1993), Sudan (2005 and 2006), and Libya (2011). Admittedly those developments are confined to actions authorized by the Security Council, but they show how far a dynamic interpretation of the UN Charter has traveled. Second, there is sometimes authoritative ex post facto praise from the international community in response to humanitarian interventions mounted without Security Council authorization.  This includes statements of support from the Security Council in response to the Economic Community Of West African States (ECOWAS) interventions in Liberia in 1990 (e.g., here) and its 1997 intervention in Sierra Leone (here).  Third, ad hoc coalitions of States conducted no-fly zones over Iraq (Operations Provide Comfort, Northern Watch, and Southern Watch) for a dozen years without Security Council authorization in part based on humanitarian concerns.  Fourth, humanitarian interventions have been conducted by States that are clearly committed to the rule of law, most notably by the then 19 NATO States that conducted the 1999 military intervention in the Federal Republic of Yugoslavia.  Finally, when massive humanitarian crises occur, States are often criticized for their failure to intervene, as was the case in Rwanda in 1994 and in Darfur after the crisis began in 2003 until authorization for limited intervention two years later.

There are counterarguments to the crystallization of such a norm.  Most significant is the paucity of State opinio juris, a condition precedent to emergence of a customary law right or obligation.  Opinio juris is an expression by a State to the effect that particular practice is engaged in, or refrained from, out of a sense of legal obligation. A notable exception to the relative lack of opinio juris is that of the United Kingdom, which has embraced humanitarian intervention as a permissible ground for the use of force.  On August 29, 2013, the Prime Minister’s Office issued a statement arguing that the use of chemical weapons in Damascus on August 21, 2013, would justify the use of force “in order to alleviate the scale of the overwhelming humanitarian catastrophe in Syria by deterring and disrupting the further use of chemical weapons by the Syrian regime.”  Despite this pronouncement, in a letter to the Security Council on November 24, 2014, the UK cited only self-defense as a basis for its actions in Syria. Nevertheless, the Attorney General later confirmed the UK’s acceptance of a right of humanitarian intervention in a 2017 speech at the International Institute for Strategic Studies.

It has been argued by one of the authors here (and agreed by the other) that a use of force under a humanitarian intervention paradigm is necessarily limited by at least two factors.  First, the quantum of suffering must be very high.  Second, the use of force must be limited to actions necessary to alleviate the suffering.  Others, such as Harold Koh, also have articulated taxonomies for the limits of humanitarian intervention.  Although they differ to an extent, there seems to be a high level of consensus that if a right of humanitarian intervention exists, at least these two conditions must be satisfied.

Consider the second factor first.  The cruise missile strikes targeted an airfield from which the chemical attacks were mounted.  As the airfield and associated facilities have a clear nexus with the humanitarian harms, they would be appropriate targets, for it is reasonable to conclude that their destruction would hinder further chemical attacks. As to the first factor, the quantum of harm, discussions of humanitarian intervention have almost exclusively focused on the scale of the crisis that can trigger the right.  For instance, the UK position is that the situation must be one of “extreme humanitarian distress on a large scale, requiring immediate and urgent relief.”  While impossible to quantify, anecdotally there seems to be agreement among supporters of the right that the casualties should at least be measured in the tens of thousands.

The casualty figures for the Khan Sheikhoun chemical attack indicate that there were less than a hundred deaths, with more than 500 injured victims.  Viewing the attack through an archetypal lens of humanitarian intervention, the attack, while unquestionably tragic and horrific, alone is insufficiently large to constitute an overwhelming humanitarian catastrophe.  This is probably so even if the casualties from prior uses of chemical weapons by the regime are factored in; the threshold is very high. There are two responses to this observation.

First, it could be argued that the missile attack was a response to the humanitarian situation writ large, not just the missile strike. The conflict in Syria has resulted in more than 450,000 deaths and 5 million internally displaced persons and over 6 million refugees.  The United Nations’ Commission of Inquiry on Syria has done a commendable job investigating and memorializing the countless atrocities in the conflict, including multiple uses of chemical weapons, and countless indiscriminate attacks using artillery, cluster munitions, barrel bombs, and fuel-air bombs.  The official US statements, however, are focused on the fact of the use of chemical weapons in the Khan Sheikhoun attack, and do not address the legal basis for the action. It would seem the United States remains unwilling to openly accept the right of humanitarian intervention as such, at least not yet.

Second, it could be argued that that the nature of harm should be considered as a basis for humanitarian intervention in addition to the quantum of harm. Chemical weapons are particularly abhorrent.  There is a long history of international instruments designed to regulate or ban the use and proliferation of chemical weapons. Recently, the Security Council has taken a number of actions to limit the proliferation of chemical weapons, including Security Council resolutions 1540 (2004), 2118 (2013), 2209 (2015), 2235 (2015), 2314 (2016), and 2319 (2016).  In our view, in circumstances in which the humanitarian consequences are especially abhorrent and universally condemned, as in the case of chemical weapons against civilians, it is legitimate, albeit not clearly lawful, to intervene to put an end to the offending actions.

By its nature, customary international law evolves based on the conduct of States combined with their opinion juris.  This evolutionary dynamic of crystallization is often driven by States acting in a manner that is not contemplated by existing law and, indeed, sometimes contrary to that law.  What we may be witnessing is the slow and rather painful birth of a nascent right in customary international law allowing States to act forcefully to put an end to the use of particularly repugnant weaponry against a civilian population, or perhaps even one countenancing forceful State responses to other egregious forms of terrorizing and massacring civilian populations in other countries.  It is not without note that a number of States have publically expressed support for the missile strike, including the United Kingdom, Australia, Canada, Israel, Japan, Spain, Italy, Saudi Arabia, Jordan, and Turkey.  Furthermore, France and Germany issued a joint statement implicitly supporting the US action and senior officials at the EU and NATO have spoken approvingly of the operation.  If, however, this commendable norm of international law is ever to fully crystallize, States must have the moral courage to set forth their legal basis for use of force in the form of explicit expressions of opinio juris. Their failure to do so, among other things, leaves the world less safe.

Image: Wikimedia Commons