Yesterday, the United States launched a missile strike against an airbase of the Syrian armed forces, in response to the recent chemical attack that the US claims was launched from this airbase. This is the first time that the US has directly used force against the Syrian regime. It is also the first time that its use of force in Syria is clearly illegal. Clearly, in the sense that I can’t imagine even a remotely plausible argument (let alone a persuasive one) as to why this act is not a breach of Article 2(4) of the Charter. (And arguably of US constitutional rules on the use of force – for which see Marty Lederman’s post on Just Security).
While the US use of force against ISIS on Syrian territory also implicates Article 2(4) of the Charter, the US at least has a reasonably plausible claim to collective and/or individual self-defense in that respect, even if this issue is hugely controversial. In this case, however, no self-defense claim can be made, since the Assad regime targeted its own population (assuming that the facts as alleged by the US are correct). Nor is the US publicly making such a claim. The official statement of the Pentagon quoted in Marty’s post states that ‘[t]he strike was intended to deter the regime from using chemical weapons again.’ Its purpose was therefore clearly retaliatory or deterrent, rather than defensive.
International law does not permit forcible reprisals that would breach Article 2(4), even if the purpose of the reprisal is to induce the other party to comply with its legal obligations. The US also has no Security Council authorization to do this act. Nor is the US claiming, or has ever espoused, a doctrine of humanitarian intervention (like the UK government does, for instance). And even if there was a customary humanitarian intervention exception from the prohibition on the use of force (and there isn’t), its requirements would clearly not be met in this instance. Hundreds of thousands of people have died in Syria even without the use of chemical weapons, and thousands of people will continue to die even if the Assad regime never uses such weapons again. There is, in other words, nothing legally or morally unique about the use of chemical weapons as opposed to other war crimes and crimes against humanity in Syria which did not (and will not) provoke an interventionist response.
In short, this is a situation in which the US government doesn’t have even a colourable argument that its conduct is lawful. It may, of course, decide to break the law (as it did), by thinking that the breach of the law is justified by higher moral considerations (‘illegal but legitimate,’ etc), and by thinking that under the circumstances it is unlikely to pay a high political cost for its breach. At a moral or political plane, this argument rests on an (at this time untestable) assumption that the strike will do more good than harm. But the Charter has nonetheless been broken, and at that with a rare clarity.