SMU Law Professor Anthony Colangelo:
What international laws justify U.S. military action?

September 5, 2013

Anthony Colangelo
Anthony Colangelo

Does international law allow U.S. military intervention in Syria? The Obama Administration has advanced a number of possible justifications including self-defense, halting civilian deaths, and debilitating the Assad regime’s chemical weapons capabilities. None of these justify intervention on the current state of international law.

Yet that doesn’t mean international law would view a U.S. intervention as illegal in the long run. International law is a tricky sort of law, and the United States could make a strong legal argument that intervention would help change the law to allow interventions to halt mass human rights abuses. Going forward, this argument could retrospectively ratify U.S. intervention in Syria and give the United States a central role in formulating international legal criteria for future interventions.  

Legal arguments against intervention are straightforward and rely principally on treaty law. Most prominently, the U.N. Charter prohibits the “use of force against the territorial integrity or political independence of any state.” There are only two ways intervention is permissible under the Charter: the Security Council authorizes it, which has not and will not happen, or the United States acts in self-defense. Even the very best international lawyers can’t stretch the doctrine of self-defense to cover a U.S. strike in these circumstances. Even if they could, that’s both an awful and an awfully expansive precedent.

The Administration has also seized upon the Assad regime’s evident use of chemical weapons as a violation of international law that justifies intervention. Yet here too, treaty law cuts the other way. Syria is not a party to recent treaties banning the use of such weapons. Because treaties bind only states that have agreed to be bound by them, Syria’s use of chemical weapons cannot violate the treaties. The only treaty banning the use of chemical weapons Syria is a party to addresses international, not internal, conflicts. And in any event, it doesn’t authorize states to attack other states that violate it. In sum, treaty law does not allow intervention in Syria.

There is, however, another type of international law that might allow intervention, called customary international law. Unlike treaty law, customary international law doesn’t derive from formal agreements among states. Instead, it arises from the practice of states accompanied by what international lawyers call opinio juris, or states’ intent that their practice carries legal significance.

States can usually treaty around custom much the same way private individuals can contract around the norms that govern our everyday behavior. But there are some rules of customary international law that cannot be contracted around and that override treaties inconsistent with their rules. These are called jus cogens, or peremptory norms of international law. Jus cogens contain prohibitions on serious international law violations like genocide, torture, slavery, and crimes against humanity. To illustrate, Hitler and Mussolini can agree by treaty to afford each other’s nations certain preferential trade treatment. But they cannot enter into a treaty legalizing genocide. Jus cogens would swoop in to invalidate that treaty as contrary to a peremptory norm of international law.               

Where does this leave the international legal justification for intervention in Syria? Many favoring intervention have cited jus cogens prohibitions on mass human rights abuses as justification. But that argument is flawed because the jus cogens norm does not directly conflict with the U.N. Charter’s prohibition on the use of force. That is, the Charter doesn’t authorize human rights abuses—in fact, just the opposite: it seeks to “promot[e] and encourage[e] respect for human rights.” Thus even if one can safely classify the Assad regime’s abuses as violations of jus cogens, that only gets the argument halfway to intervention. To justify intervention, the jus cogens norm would need to encompass not only a prohibition on human rights abuses but also the capacity of other states to enforce that prohibition. This latter enforcement component is presently lacking under the law. Finally, the Charter’s prohibition on the use of force isn’t some run-of-the-mill international rule. It is a cornerstone of the postwar international legal system that outlaws aggressive war. For this reason, the prohibition on the use of force is itself considered a jus cogens norm.        

Nonetheless, it may be time for a change. Because customary international law arises from state practice, as practices change so too does the law, including the law of jus cogens. One way customary international law changes is states break it to form new norms; breaches effectively plant the seeds from which new norms grow. Although a breach may violate international law when it occurs, the law may develop to ratify that breach as the early stage of a new norm’s development.  

If the United States intervenes in Syria, it has an initial international legal choice to make: it can ignore international law or seek to justify intervention within it. The second option’s benefit is that if state practice develops to allow intervention the illegality of U.S. action will wilt as the new norm blossoms. Yet some may view this option as undesirable precisely because it may prompt other states to accept the legality of intervention. Reciprocity is also a cardinal rule of international law: if it’s legal for us, it’s legal for you.

The question then becomes whether it’s better to operate within the law or outside it. For other states also will face that same initial choice above, to which this first-order reciprocity norm also extends; that is, the initial choice to ignore law or to justify their actions within it. In this respect, the United States may actually derive two distinct benefits from choosing to justify its actions under international law: a retrospective ratification of U.S. intervention and the ability to formulate criteria for a budding international law of humanitarian intervention.   


Anthony J. Colangelo is associate professor at SMU Dedman School of Law. His scholarly and teaching interests are in the fields of conflict of laws, civil procedure, U.S. foreign relations law, and private and public international law.