It has been reported that President Obama has authorized the US armed forces to come to the defense of the new non-State group the United States is training to fight ISIS (Islamic State) in Syria should that group be attacked by the Assad regime. Last week, US air power was employed to support the group, Division 30, when it came under attack by the al-Qaeda affiliated al-Nusra Front. The United States is ready to do the same should Division 30 find itself engaged in combat with the ISIS. However, any US use of force directly against Assad’s forces will take the conflict to a new level. It is therefore a propitious moment to review the normative bidding.

As Ryan Goodman and I opined in an earlier post (and as I explained in greater depth here), absent a legal justification such as self- or collective defense, arming and training rebels to fight against a government amounts to a violation of the UN Charter Article 2(4) and the customary law prohibition on the use of force. The International Court of Justice took this position in its Nicaragua judgment, one that is well accepted despite significant State practice to the contrary. In my estimation, arming and training rebel forces for the express and only purpose of overthrowing the Assad regime is both an unlawful intervention into the internal affairs of Syria and a use of force against that State.

A State may, however, arm and train non-State forces if they are solely dedicated to supporting the State’s lawful exercise of self- or collective defense pursuant to Article 51 of the Charter and customary law against an armed attack by another State (or, in my view, a non-State group). Moreover, a State that may lawfully use force to defend itself or come to the defense of another State is entitled to turn to non-State groups to assist with, or even carry out, the defensive operations. With respect to collective self-defense, the State’s activities are, as noted in Nicaragua, conditioned on a request for assistance from the State facing the armed attack; the latter may limit the scope and conduct of the former’s activities in any manner it deems appropriate.

This raises the question of the legal basis for Division 30 operations. Louise Arimatsu and I previously analyzed assertions that US airstrikes against ISIS and Khorasan Group forces are based on implicit consent by Syria, self-defense against on-going or imminent armed attack against the United States, or an exercise of collective self-defense in support of Iraq. Although a colorable argument may be fashioned for the first two rationales, the last is, as we explained, on much firmer legal footing. But even this justification rests on two contentious preconditions: 1) interpretation of the right of self-defense as extending to attacks by non-State actors; and 2) the legal viability of the “unwilling or unable” justification for conducting operations against non-State groups in other countries. Louise and I came to differing conclusions on these issues.

Assuming, for the sake of analysis, that Iraq is entitled to act in self-defense, it may turn to the United States, other States, and non-State groups like Division 30 to mount its defense against ISIS. There would be no prohibition, therefore, on the United States arming, training, or supporting Division 30 because the wrongfulness of doing so under the use of force prohibition is precluded as a matter of law (Article 21 of the Articles on State Responsibility) by the fact that the United States is acting in the collective defense of Iraq. This is only logical, for if the United States may lawfully use force against ISIS to defend Iraq, it may surely support other entities or States from which Iraq has sought assistance. It must be cautioned, however, that such US support must be based on Iraq’s right to self-defense and Iraq has to therefore acquiesce to the activities concerned.

The quandary is that the US air strikes at issue are directed against groups other than ISIS and, perhaps in the future, will target Syrian armed forces. Thus, they are much more attenuated from the notion of the collective defense of Iraq than those operations directly targeting ISIS assets. A reasonable claim can be made that the former nevertheless qualify as acts of collective defense in that they are designed to preserve the ability of Division 30 to itself lawfully assist in the defense of Iraq; the US operations would have a clear causal relationship to the right of Iraq to defend itself in accordance with international law. Indeed, it might be argued that Syria’s forceful interference with that right would constitute an “armed attack” by Syria against Iraq, even though Iraqi forces are not being directly attacked.

But any argument that US operations against Syrian forces and organized armed groups other than ISIS are justifiable on the basis of collective defense is highly nuanced. Clearly, to justify the operations there must be a factual (and legal) nexus between the US operations and the defense of Iraq (or perhaps to US self-defense against ISIS by the aforementioned weaker justification for counter-ISIS ops). This being so, if Division 30 becomes directly involved in hostilities against Assad’s forces unrelated to the ISIS conflict with Iraq, the legal basis for US military action to defend Division 30 would be tenuous because neither Iraq nor the United States have been subjected to an armed attack by Syria.

Enter Turkey. It is now at least arguable that Turkey has been the victim of an armed attack by ISIS. If so, find “Iraq” in the discussion above and replace it with “Turkey.”

It’s complicated….

What is not complicated is that if the US military becomes engaged in hostilities, however minor, with Syrian forces, the applicable international humanitarian law rules would unquestionably be those pertaining to international armed conflict. This is so irrespective of the jus ad bellum grounds the United States may have to conduct the operations. And should US forces engage in hostilities with an armed group not under the “overall control” of Syria, it will be involved in a non-international armed conflict with that group if the “organization” and “intensity” requirements set forth by the International Criminal Tribunal for Yugoslavia in the Tadic Appeals Chamber judgment are met (this is unmistakably the case vis-à-vis ISIS). The fact that the US operations against Syria or the organized armed groups might be entirely “defensive” in nature has no bearing on the legal classification of conflict.

Finally, I would caution that the analysis set forth above is offered solely for the purpose of deconstructing the legal questions that the situation poses. In fact, US offers of protection from regime attacks may be little more than a recruiting tool for the Pentagon’s train and equip program, which has yet to field more than a relative handful of fighters. According to The Wall Street Journal, US officials “played down the chances of a direct confrontation [between Division 30 and Assad], at least in the near term.” Those officials later told the paper that promises of support “could help persuade prospective recruits the Pentagon is serious about protecting them, including against the regime.” And whether operations against Assad in support of Division 30 would even make sense from a military, policy, and humanitarian perspective is a different matter. But, whatever the case, decision-making regarding the use of force always benefits from an understanding of which international law norms are applicable and settled…and which are not.

The views expressed are those of the author in his personal capacity and do not necessarily represent those of the United States government.