Three Lingering Questions about the Legality of Withdrawal from Syria: Part I – Complicity by Omission

Three Lingering Questions about the Legality of Withdrawal from Syria: Part I – Complicity by Omission

[Beatrice Walton is a 2018 graduate of Yale Law School and served as Judicial Fellow at the International Court of Justice in The Hague in 2018-2019. Paul Strauch graduated from Yale Law School in 2018, where he was a Herbert J. Hansell student fellow at the Center for Global Legal Challenges and Editor-in-Chief of the Yale Journal of International Law.] 

This post — the first in a two-part series — considers the law relevant to the U.S. withdrawal from Syria in early October. Experts condemned Turkey’s invasion as a violation of Article 2(4) of the UN Charter, an unlawful act of aggression, and as potentially entailing crimes against humanity. Turkish forces and their proxies have reportedly committed a host of violations of international humanitarian law (IHL) in addition, including through indiscriminate attacks, summary killings, torture, and rape against Kurdish populations.

But as attention has turned to the consequences of these violations for Turkey and third States, and the U.S.’ response, the legal issues implicated by the withdrawal decision itself have notably escaped analysis. Diplomatic reports have shone light on the U.S.’ decision to withdraw its forces, which, when combined with its decision not to criticize Turkey’s invasion as it was beginning, is said to have amounted to an effective “green light” to President Erdogan. Turkey had made clear its plans to intervene since at least January, and in a dramatic move, the U.S. began withdrawing the majority of its 1,000 troops in northern Syria after a phone call in early October. Until then, U.S. forces had been working to fight ISIS with the Kurdish-led Syrian Democratic Forces (SDF), who had in turn been maintaining some 11,000 ISIS fighters and tens of thousands of ISIS family members.

We take up three legal issues relevant to understanding whether the U.S. was able to abruptly remove its troops and support for the SDF just ahead of Turkey’s invasion without incurring international responsibility.

Responsibility for Failing to Stop an Invasion? Questions about Complicity for “Omissions”

The U.S.’ position has been that the Turkish invasion was inevitable, and that the abrupt decision to withdraw was the only way to prevent U.S. forces from being caught in Turkish and SDF cross-fire. An internal report by the top U.S. diplomat in northern Syria, released in early November, has questioned this view, as has a second senior defense official, several politicians, a former CIA agent, and other sources.

Though the facts surrounding the U.S.’ decision to withdraw remain unclear, an important legal question remains: Can a State be deemed complicit in the wrongdoings of another State for failing to stop violations committed by the other?

Article 16 – The Requirements for “Complicity” Responsibility

As is familiar to international lawyers, Article 16 of the International Law Commission (ILC) Articles on the Responsibility of States for Internationally Wrongful Acts (ASR) provides that a State incurs responsibility where it “aids or assists another State in the commission of an internationally wrongful act.” The State must (a) “do[] so with knowledge of the circumstances of the internationally wrongful act,” and (b) the act must be one “which would be internationally wrongful if committed by [the assisting] State.” While in many situations, as here, the (b) requirement is easily satisfied (neither Turkey nor the U.S. is permitted to use force outside of the parameters of Article 2(4), or commit violations of IHL), the (a) requirement poses more challenges. In effect, it entails two sub-elements: a “material” element (or what some call a “nexus” or “causal” element), and a “knowledge” or “intent” element.

The U.S.’ withdrawal sounds generally in “omission”: an (alleged) failure to take precautionary measures aimed at avoiding unlawful acts by the incurring state, Turkey. Under-explored, though, is whether an omission, as opposed to an act, can satisfy Article 16’s materiality requirement. The ILC Articles and Commentary offer little guidance.

Admittedly, the International Court of Justice (ICJ) seemed to foreclose the possibility of complicity by omission in the Bosnian Genocide case, where the Court considered complicity with non-State actors under Article III(e) of the Genocide Convention and articulated that complicity “requires that some positive action has been taken to furnish aid or assistance to the perpetrators.” However, this holding has attracted significant criticism. For one, it is difficult to square with situations, particularly in the post-9/11 context, where responsibility has been invoked on the basis of what arguably can be understood as an omission, including at regional human rights courts. Scholars have also assailed it. Jackson, for example, suggests that the ICJ erred in constraining Article 16 to positive acts, as in his view there is no good reason to exclude what he considers certain “particularly culpable omissions,” such as where a State fails to act when given a strong warning of impending harm to entities or persons under its control. Lanovoy has similarly argued that the ICJ’s reasoning shouldn’t extend generally to responsibility under Article 16, pointing to cases where States have facilitated internationally wrongful acts by “knowingly fail[ing] to object to the use of its territory, intelligence, or personnel.” Aust also finds a basis for omission liability, reasoning that even if the ILC failed to consider the issue, it hardly deviated from its general rule that wrongful conduct can consist of an action or omission.

Though similar, these views are difficult to reconcile precisely, leaving unsettled the circumstances in which complicity by omission would be appropriate, if indeed it is not categorically excluded. Domestic criminal law helps shine light on the possibilities. In the common law, omissions can generally substitute for acts only where the defendant had an independent legal duty to act in the circumstances. The Commentary to Article 2 ASR, which sets forth the elements of an internationally wrongful act, provides an analogous rule. It observes that “[c]onduct attributable to the State can consist of actions or omissions,” and describes such omissions as occurring in “circumstances where such steps were evidently called for,” citing Corfu Channel. Drawing on this language, Aust has suggested that to the extent complicity liability exists for omissions in international law, it is limited to circumstances in which a State otherwise had a duty to act — a view shared by the common law.

Complicity by an Omission — Predicated on a Duty to Act

Where might such a duty to act come from? The ILC Commentary’s citation to Corfu Channel calls into view the importance of territory, and the paradigm that many positive duties flow from control over it. With respect to aggression — the offense many commentators have focused on regarding Turkey — such a territorial requirement is explicit, limiting Article 16 in that context. Indeed, Article 3(f) of the Definition of Aggression, adopted by the General Assembly in Resolution 3314 (XXIX) of 1974, requires the “plac[ing]” of territory “at the disposal” of another. The level of control the U.S. retained at the border likely falls short of this requirement, at least looking to State practice concerning overflights in wartime. Nonetheless, it is interesting to note that a retired Army vice chief of staff condemned the U.S.’ “[g]iving up control of airspace” as a factor “enabl[ing] [the] Turk[ish] invasion”

Outside of the aggression context, such a strong territorial requirement is less obviously required for Article 16 responsibility based on omission. Aust points to the hypothetical example of a request by Israel to the U.S. asking the U.S. whether it can use Iraqi airspace to attack Iran. According to him, a status of forces agreement between Iraq and the U.S. requiring the U.S. to take “appropriate measures” to halt a threat of aggression against Iraq could, in such a case, serve as the preexisting positive duty making it “justifiable to consider [the U.S.’] omission as a form of aid or assistance.” Aust thus seems to suggest that even where a duty is required for Article 16 omission responsibility, it need not be a duty to prevent the ultimate wrongful act. This view is shared by the common law, where the relevant duty for complicity by omission need not be a duty to prevent the entire crime.

In the situation in Syria, no such formal arrangement appears to have existed between the U.S. and the SDF. This seems to suggest that the U.S. thus did not incur a duty to act for purposes of Article 16 based on its relationship with the SDF. Reports nonetheless indicate that the U.S. may not have been forthcoming with the SDF about how quickly the relationship could come to an end, though one State Department official apparently called it “temporary, transactional, and tactical” in 2017. Further, a senior U.S. diplomat in Syria wrote in a memo that he did specifically tell the Kurds that the presence of U.S. troops and air power in northeast Syria would keep the Turkish military from committing an attack similar to the one Turkish forces committed on the city of Afrin in 2018.

Another relevant duty might be a duty to prevent, as called for under treaties and custom. Schabas characterizes Bosnian Genocide as “precedent for the extension of the obligation of prevention [of genocide] beyond the borders of a state.” The ILC Draft Articles on Crimes Against Humanity provide a similar obligation to take preventative measures. Should it be established that U.S. airspace or areas formerly under U.S. military control have been used by Turkey in the invasion, such duties could in theory be implicated if the underlying crimes were proven.

Comparing the Duty to Prevent and Complicity by Omission – Causation

Even if a duty to prevent cannot be found to apply to the Syria situation, distinguishing between the duty to prevent and complicity responsibility by omission — both of which Bosnian Genocide dealt with — is essential to understanding Article 16. Notably, international law commentators do not seem to have considered whether and how a pre-existing duty, let alone duties to prevent, could (if at all) serve as the grounds on which complicity by omission liability is based.

One reason this issue matters is that the same elemental standards — particularly for materiality — may not necessarily apply to both an underlying duty and complicity by omission. As an analogy, while a domestic prosecutor is typically obligated to prove but-for causation where an omission replaces an act requirement, she is not typically required in a complicity case to prove that the defendant would have succeeded in stopping the principal’s crimes had she acted differently (regardless of whether complicity is founded on an act or omission). (p. 731). Whether the same is true in international law — a question pertaining to how Article 16’s materiality requirement might interact with the causation requirements of an underlying duty — has not yet been examined. This interaction is critical to understanding what to make of the U.S.’ view that the Turkish invasion was inevitable. As Defense Secretary Mark Esper remarked, “[f]ifty service members [we]re not going to stop a Turkish advance,” which was “15,000 strong.” Still, Schabas critically reminds that Bosnian Genocide did not contemplate military activity but instead “lawful measures involving diplomatic activity and other forms of peaceful pressure.”

Looking at the relevant causation standards closely, it appears, perhaps surprisingly, that even a relatively monumental event — Turkey’s invasion — would not necessarily impede a finding of complicity by omission, at least under current law. Article 16, like domestic criminal law, sets a low “minimum threshold” for materiality or causation, allowing complicity liability even where assistance is an “incidental factor in the commission of the primary act.” Even Goodman and Jackson’s heightened standard that the aid must “significantly contribute” may not be that onerous. And like the common law, there is no suggestion that Article 16 imposes a different materiality standard in cases of complicity based on an omission, versus an act. Further, due diligence — the standard applying to most potential underlying duties, such as the duty to prevent — does not itself require but-for causation. As the ICJ has explained, “it is irrelevant whether the State . . . claims, or even proves, that even if it had employed all means reasonably at its disposal, they would not have sufficed to prevent the commission of genocide.”

Taken together, a rather low causal threshold for complicity by omission seems to emerge, one which some might find troubling. It means that States may be required to act — diplomatically, not militarily — even if they cannot do all that much to prevent another actor’s wrongful conduct. However, what could help assuage fears that complicity by omission would function like a runaway train is that there may rarely be an incentive to argue it, in contrast to the underlying duty. This is because unlike domestic law, many duties to act involve a due diligence standard, a standard which is already similar to complicity’s low materiality threshold. That said, complicity by omission could be more favorable to argue, on the causation front, if the relevant underlying duty is more akin to Aust’s example of a duty to do something dramatically less than prevent the ultimate offense, such as in the status of forces agreement example.

In sum, international law seems to permit complicity by omission responsibility where complicity is predicated on an underlying duty. The standard of causation is likely a low one, and roughly the same as that required of the underlying duty.

Knowledge / Intent

Adding in the final requirement of Article 16 — the mental element — makes plain though that even if a duty can be found, and causation shown, complicity is difficult to prove. The text of the Article requires that “the aid or assistance must be given with a view to facilitating the commission of the act, and must actually do so.”

Article 16’s mental requirement has been subject to debate, particularly in situations like the Yemen conflict where it was arguably not always obvious that wrongful acts might follow from U.S. support. The Syria situation presents the more basic issue, though one which is difficult to overcome. While Turkey’s acts — at least its unlawful use of force — were clearly foreseen as unlawful, it is much less clear that any aid or assistance was provided by the U.S. “with a view to facilitate” them.


Complicity by omission may well be possible under international law, though clarification of the duties which can potentially supply a basis for such responsibility is needed. In the Syria case, some argument might be made that a pre-existing duty to act operated on the U.S., although the lack of formal partnership likely points in the other direction. In any event, the main hurdle to complicity by omission remains — not causation — but the intent requirement. And in the Syria situation, it’s a long route to satisfying the “view to facilitate” standard, at least given current information.

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