US and UK support for Saudi-led military operations in Yemen have received significant attention recently due to reports of strikes that damaged or destroyed hospitals, schools, and other infrastructure vital to the civilian population. When international lawyers consider the rules that apply to such assistance, the so-called law of State responsibility consumes most, if not all, of the attention (see Miles Jackson and my accompanying post that does a deep dive into that area of law). Given the allegations that some Saudi-led coalition operations may amount to war crimes, it is relevant to consider another specific area of international law: the law of aiding and abetting war crimes. Importantly, as I explain below, that area of law is likely broader in its attribution of vicarious liability for assisting States than the law of State responsibility. It should be a centerpiece of any discussion of this topic.

One clarification is necessary at the outset. In this analysis, I make no assessment of the alleged actions by the Saudi-led coalition or any reported instance of US or UK support. This post discusses the generally applicable legal framework that might apply. My discussion therefore also has relevance for all sorts of other situations as well, including reported Russian support related to the Syrian government’s alleged war crimes, reported Iranian support related to the Houthis’ alleged war crimes, and State support for terrorist organizations elsewhere.

I. War crimes
It is important to recall the mental state required for the commission of a war crime. I have separately posted a brief explainer of the basics of this area of law. For now, it may bear reminding that “willful killing” is a war crime, which includes purposeful attacks on civilians as well as civilian casualties that result from certain forms of recklessness in targeting.

Below the line of war crimes: There are plenty of violations of the law of armed conflict that do not amount to a criminal offence (e.g., a failure to take all feasible precautions to minimize harm to civilians). What is more, attacks that result in civilian casualties, even if those casualties were expected and significant in number, are not necessarily violations of the law of armed conflict.

All of these considerations go to the question of whether a Principal has committed a war crime. Our focus here is on the potential accomplice, or aider and abettor, in situations in which war crimes have been committed.

II. Aiding and abetting war crimes
Let’s start with a truism: US and UK departments, agencies and personnel would presumably be precluded from providing assistance to a foreign State if the assistance constituted aiding and abetting war crimes or other grave international crimes.

And let me add a caveat: My discussion considers only government policies and practices. I do not assess what immunities or individual liability, if any, might apply. It is important enough to focus on when government activities would involve a substantial legal risk of aiding and abetting war crimes, and, accordingly, at what point a government may need to desist in providing assistance.

Under customary international law, aiding and abetting war crimes includes three elements:

(1) A Principal person or entity committed a war crime;
(2) Another actor committed an act that had a substantial effect upon the commission of the underlying offence; and
(3) Required mental state: The other actor knew that that such an act would assist, or had the substantial likelihood of assisting, the commission of the underlying offense.

Notice what is missing. There is no requirement that the accomplice had the purpose or desire to facilitate the underlying offence.

In that respect, the law of aiding and abetting war crimes avoids one of the more vexing issues in the rules on State responsibility for assisting another State in internationally wrongful conduct. As Miles Jackson and I note in our accompanying post on State responsibility, an ongoing debate is whether an intent to facilitate the internationally wrongful act is an element of that customary international rule, and what the precise scope and meaning of “intent” might be in different circumstances.

So then, what sources or authorities suggest that “knowledge alone” is the correct test for aiding and abetting war crimes as a matter of customary international law?

I want to discuss three sources coincidentally listed by the DoD Law of War Manual as support for the theory of aiding and abetting war crimes: (1) contemporary prosecutions by U.S. military commissions; (2) statutes of international criminal tribunals; and (3) a 1994 Opinion by the Justice Department’s Office of Legal Counsel.

Turning to US military commissions, the Department of Defense’s Regulations provide that knowledge alone is sufficient: “The accused intended to or knew that the act would aid or abet such other person or entity in the commission of the substantive offense.” But how clear is it that the Government believes that standard reflects customary international law? In the prosecution of Khalid Shaikh Mohammad et al., the Chief Prosecutor at the Office of U.S. Military Commissions provided a detailed explanation of aiding and abetting liability under customary international law (see Beth Van Schaack’s very useful post on this topic). The Government’s 2013 Supplemental Finding states: “The knowledge required is simply a knowing participation that the acts would assist the commission of a crime. A conscious desire or willingness to achieve the criminal result is not required.”

Turning to international criminal tribunals, it is useful to note that the Government’s Supplemental Filing drew extensively from the 2013 appellate decision of the international criminal tribunal in the prosecution of Charles Taylor. The Special Court for Sierra Leone held that the mens rea for aiding and abetting war crimes under customary law requires only knowledge and not purpose: “the accused knew that his acts would assist the commission of the crime by the perpetrator or that he was aware of the substantial likelihood that his acts would assist the commission of a crime by the perpetrator.” In reaching that result, the tribunal referred to a consistent line of cases from the Nuremberg-era trials, the International Criminal Tribunal for the Former Yugoslavia, the International Criminal Tribunal for Rwanda, and the Extraordinary Chambers in the Courts of Cambodia.

The potential outlier is the Rome Statute for the International Criminal Court. Article 25(3) of the ICC Statute limits aiding and abetting liability to cases in which the individual acts “for the purpose of facilitating the commission of such a crime.” That qualification on aiding and abetting is widely thought to be peculiar to the jurisdiction of the ICC, and not reflective of customary international law. The Court in the Taylor judgment, for example, noted that “Article 25(3) does not represent or purport to represent a complete statement of personal culpability under customary international law.” Furthermore, leading international criminal law experts (e.g., Kevin Jon Heller, James Stewart) consider that the proper standard under the Rome Statute is still one of knowledge, which may be due, in part, to Article 30 stipulating that the default mens rea standard for crimes under the Rome Statute is knowledge. Finally, for deep insight into the negotiating history of the ICC, which supports this same conclusion, a good resource is an amicus brief by David Scheffer who was Ambassador-at-Large for War Crimes Issues and led the US delegation that negotiated the Rome Statute.

Finally, let’s turn to the Opinion by the Office of Legal Counsel in 1994, which supports the same conclusion, albeit in an assessment of the US federal aider and abettor statute. The OLC Opinion addressed a set of facts remarkably similar to our subject at hand (and the federal aider and abettor statute notably applies to the War Crimes Act, 18 USC 2441). Specifically, the question involved the US Government’s providing intelligence information and other assistance to foreign governments engaged in military strikes to shoot down civil aircraft. For those who believe the OLC seldom opposes administration action, this opinion stands as a strong counterexample. The OLC concluded that there was a “very substantial danger” that the US Government’s actions would be aiding and abetting the unlawful conduct. Walter Dellinger’s OLC concluded: “USG agencies and personnel may not provide information (whether “real-time” or other) or other USG assistance (including training and equipment) to Colombia or Peru in circumstances in which there is a reasonably foreseeable possibility that such information or assistance will be used in shooting down civil aircraft.”

Most important for our discussion, the OLC explained that, in general, aiding and abetting liability does include the element of “desire to help the activity succeed” (a purpose element). However, the Opinion then goes into detail explaining that this feature of aiding and abetting liability is relaxed when dealing with particularly grave crimes. The Opinion states:

“[T]here is support for relaxing this requirement [of specific intent to bring about the criminal act] when the crime is particularly grave: . . . ‘the seller of gasoline who knew the buyer was using his product to make Molotov cocktails for terroristic use’” would be guilty of aiding and abetting the buyer’s subsequent use of the “cocktails” in an act of terrorism. Fountain, 768 F.2d at 798 (quoting with approval People v. Lauria, 251 Cal. App. 2d 471, 481 (1967) (dictum)). Where a person provides assistance that he or she knows will contribute directly and in an essential manner to a serious criminal act, a court readily may infer a desire to facilitate that act.

Notably, in the Fountain case, Judge Richard Posner explained that the legal standard for aiding and abetting, specifically in the situation of a most serious crime, excludes the element of purpose in order to deter such grave offences.

Notably, the DoD’s Law of War Manual cites and quotes the Dellinger Opinion as key support for aiding and abetting liability for war crimes. It stands to reason that what the OLC Opinion specifically said about the purpose element should be given great weight when applying its framework to the war crimes context.

III. Mitigation measures
What mitigation measures might an assisting State adopt to stop short of aiding and abetting liability? The OLC Opinion is instructive. It described the measures that the US Government could pursue to be assured that its assistance would not aid and abet the criminal actions. Those measures, however, were relatively onerous and resulted in structurally separating US assistance from the foreign governments’ targeting practices. In particular, the OLC stated that an agreement with the foreign governments would have to assure that USG information and other assistance would not be used to target civil aircraft for destruction; the agreement should “establish mechanisms by which USG personnel would obtain detailed and specific knowledge as to how the USG-provided information and assistance were in fact being used, and thus be able to identify at an operational level any instances of non-compliance with the agreement;” the agreement should “stipulate that if any incident should occur in which the foreign government’s agents fired on a civil aircraft, USG personnel would be able to verify whether USG provided information and assistance had been used in that instance;” and the agreement should “provide for the termination of USG-supplied information and assistance in the event of material noncompliance.”

It is important to consider whether mitigation measures in other contexts—such as arms sales and intelligence information sharing—should include provision of training and other assistance to improve the targeting practices of the foreign recipient. This may be especially important in cases in which a foreign State’s practices are not an intentional policy—such as Colombia and Peru’s shootdown policy—but rather an incidental and unintended effect of imperfect targeting practices. I disagree with those who suggest that any targeting assistance that the US Defense Department may provide the Saudi-led coalition “as a matter of law means [the United States] is liable for unlawful strikes in which it takes part.” Such a rule would discourage States from providing any assistance in the form of helping ensure that a recipient’s target selection and military strikes comply with the law of armed conflict. In a future post, I will suggest that there should, indeed, be a safe harbor from liability for assistance that is designed to ensure a recipient’s practices comply with international law (see also Part IV of this article by Oona Hathaway et al.).

In a subsequent post, I will also outline the elements of a framework that governments—including the next US Administration—might adopt as best practices for dealing with the promise and problem of assisting foreign partners in the conduct of hostilities.

For now, the important point is this one for States contemplating assistance to another State or non-State armed group: there is substantial legal risk that aiding and abetting liability for war crimes would be found under international law even absent any intent or purpose to promote the crimes.