This piece is the second of two on U.S. operations in Yemen and the War Crimes Act, and the latest article in our forum on the Yemen crisis and the law.

In September 2017, Ryan Goodman suggested that the United States might be complicit in war crimes committed by the Saudi-led coalition. Here we reexamine whether a member of the United States government or armed forces could be susceptible to liability for aiding and abetting war crimes committed by the Saudi-led coalition in Yemen under U.S. domestic law in federal courts.

On 13 March 2018, U.S. Army Gen. Joseph Votel, the head of U.S. Central Command (CENTCOM), acknowledged before the Senate Armed Services Committee that the command does not track the strikes carried out during the Saudi-led coalition missions it refuels. This admission is striking, given allegations that the Saudi-led coalition has been alleged to have engaged in repeated violations of international humanitarian law in Yemen. Numerous reports suggest that the Saudi-led coalition has targeted protected persons and objects, including civilians,  hospitals, and food supplies. Additional reports allege that U.S. military interrogators cooperated with United Arab Emirates officials accused of torturing AQAP detainees in secret prisons across southern Yemen.

The federal statute for aiding and abetting (18 U.S.C. § 2) provides:

(a) Whoever commits an offense against the United States or aids, abets, counsels, commands, induces or procures its commission, is punishable as a principal.

(b) Whoever willfully causes an act to be done which if directly performed by him or another would be an offense against the United States, is punishable as a principal.

Offense” is defined as “any criminal offense, other than an offense triable by court-martial, military commission, provost court, or other military tribunal, which is in violation of an Act of Congress and is triable in any court established by Act of Congress”—and thus includes violations of the War Crimes Act.

Under Section 2(a), U.S. persons assisting the Saudi-led coalition can only be found liable for aiding and abetting a violation of the War Crimes Act where there has been an underlying violation of the War Crimes Act. Because the War Crimes Act only applies to U.S. nationals and members of the U.S. armed forces—not to members of foreign armed forces—individuals in the U.S. armed forces cannot be held liable for aiding and abetting violations under § 2(a) for their assistance to the Saudi-led coalition. As noted in the prior article, it appears unlikely that a court would find a primary violation by a U.S. person under the War Crimes Act, therefore liability under § 2(a) is unlikely as well. (If a primary violation of the Act were found, however, then those who assisted that crime could potentially be liable under § 2(a).)

U.S. persons participating in assisting the Saudi-led coalition are more likely to be held liable under Section 2(b), the “perpetration by means” clause of the statute, which does not require an underlying violation of the War Crimes Act. This clause gives rise to the “innocent-instrumentality doctrine” in criminal law, which applies in cases where an agent who causes harm has not actually committed any crime—but the assisting individual would have been guilty had she committed the act herself.  The U.S. Attorneys’ Manual explains that § 2(b) “removes all doubt that one who puts in motion or assists in the illegal enterprise or causes the commission of an indispensable element of the offense by an innocent agent or instrumentality is guilty as a principal even though he intentionally refrained from the direct act constituting the completed offense.” Under 2(b), then, even though those participating directly in the Saudi-led coalition could not be liable under the War Crimes Act (because of the nationality requirement in the War Crimes Act), members of the U.S. armed forces might be prosecutable under § 2(b) for causing violations that would amount to war crimes under the Act.

In order to establish liability under § 2(b), the prosecution must prove four things:

1) the Saudi-led forces committed an act, which if directly performed by United States officials would constitute a violation of the War Crimes Act

2) assistance from the United States officials caused the Saudi-led coalition to perform said act

3) the United States officials did so “willfully”

4) United States officials possessed the requisite mens rea elements required by the War Crimes Act.

Below, we briefly address each element in turn, concluding that it is possible, though perhaps not likely, that members of the U.S. Armed Forces could be found liable for aiding and abetting violations of the War Crimes Act.

Acts Committed by Saudi-led coalition

The Saudi-led coalition has allegedly engaged in “intentional targeting of civilians” in Yemen. The UN Panel of Experts on Yemen, established pursuant to Security Council resolution 2140 implicated the Saudi-led coalition in a numerous violations of international humanitarian law. Non-governmental organizations, including Amnesty International and Human Rights Watch, have also gathered evidence of Saudi-led coalition violations. These violations might amount to war crimes under the statutory definitions provided § (c)(3) of the War Crimes Act, which addresses “grave breaches” of Common Article 3.

As noted above, the prohibition on “grave breaches” of Common Article 3, as defined by the War Crimes Act, likely applies to the conduct of hostilities. As noted in earlier Articles in this series, there have been numerous credible reports that the Saudi-led coalition has engaged in the intentional targeting of civilians. Most recently, on March 9, 2018, in a hearing before the Senate Armed Services, Senator Elizabeth Warren alleged that a strike undertaken by the Saudi-led coalition in February in Saada, a northern Yemeni town, where the Saudi-led coalition engaged in a “double-tap” airstrike, targeting medical personnel. This is not the first “double tap” allegation against the Saudi-led coalition. A video provided online by The Telegraph, claims to show a “double tap” strike against a funeral hall in Sanaa, Yemen’s most populous city, from October 9, 2016, which allegedly killed more than 140 people and injured at least 525 others. The allegations of Saudi Arabia’s “intentional targeting of civilians,” if substantiated in federal court, would almost certainly amount to war crimes under the standard prescribed by the War Crimes Act as a “grave breach” of Common Article 3.

In addition to U.S. support for airstrikes by the Saudi-led coalition, there have been reports that the U.S. may have been complicit in torture conducted by the United Arab Emirates (UAE) as part of the Yemen conflict.  As previously noted, the UAE, a member of the Saudi-led coalition, has been accused of operating terrorism detention centers in Yemen. The Associated Press reports that the United States has assisted UAE interrogations of individuals suspected of being members of AQAP in these detention facilities. According to the AP, “Several U.S. defense officials speaking on condition of anonymity to discuss the topic, told AP that American forces do participate in interrogations of detainees at locations in Yemen, provide questions for others to ask, and receive transcripts of interrogations from Emirati allies.” Acts of torture, as well as cruel and inhumane treatment, would amount to “grave breaches” of Common Article 3, as defined by subsection (a) and (b) of the War Crimes Act. (See also: Steve Vladeck’s excellent Just Security post from June 2017 assessing the potential legal implications for the U.S.’ role in UAE interrogations.) If substantiated, these acts would meet the first aiding and abetting element, because they would constitute a violation of the War Crimes Act if United States officials had directly performed the acts. Indeed, the case here is even stronger than for targeting violations, as the alleged victims were held in detention by UAE authorities, which eliminates any doubt about the applicability of Common Article 3.


Any prosecution under the “perpetration by means” statute (18 U.S.C. § 2(b)) would have to prove a clear causal link between U.S. support for the Saudi-led coalition and Saudi violations. Although there is some ambiguity in the case law about the precise causal standard, it appears that in order to establish causation, the prosecution would have to demonstrate that without the support of the United States, the Saudi-led coalition would not have carried out the attacks that allegedly violated international humanitarian law.

Such a causal connection would be difficult to establish without a deep factual inquiry. However, based on publicly accessible information, a colorable argument exists that U.S. support plays a causal role in violations committed by the Saudi-led coalition for two reasons.

First, the Saudi-led coalition is heavily reliant on U.S. weapons transfers to continue its operations in Yemen. The UN Panel of Experts on Yemen and human rights organizations documenting alleged violations in Yemen report that U.S. weapons manufacturers produced many weapons recovered during post-strike documentation. The UN Panel of Experts on Yemen found that U.S.-manufactured weapons were used in at least half the documented unlawful attacks committed by the Saudi-led coalition in 2017. These include attacks committed with bombs from the Mark 80 series, which are “general purpose” bombs produced by the U.S, and U.S.-produced “Paveway” guidance units. Between March 2015 and December 2016, Human Rights Watch found U.S.-supplied weapons at the site of 23 unlawful coalition airstrikes. Human Rights Watch also reported that U.S. weapons were used in two of the deadliest airstrikes in the conflict—including in strikes that killed at least 97 and 100 people respectively.

Second, the Saudi-led coalition depends on mid-air refueling by U.S. jets to continue its operations in Yemen. The United States has engaged in mid-air refueling for coalition planes since 2015. A spokeswoman for the U.S. Central Air Force Command reported that the United States transferred over 88 million pounds of fuel to more than 2,800 aircraft refueling operations in the Horn of Africa between January and mid-March 2018. The U.S. Air Force does not publicly report the numbers of specific refueled flights to Yemen or where the refueling takes place—but many of these refueling operations are suspected to be for the Saudi-led coalition.

U.S. Air Force service tankers, like the U.S. Air Force KC-135 Stratotanker and the KC-10 Extender, are engaged in mid-air refueling operations for the coalition. The U.S. Armed Forces have lauded these aircraft as “critical” in previous operations. For example, the U.S. Air Force states that “[d]uring operations Enduring Freedom and Iraqi Freedom, KC-10s flew more than 1,390 missions delivering critical air refueling support to numerous joint and coalition receiver aircraft.” In an acquisition report, the Office of the Under Secretary of Defense for Acquisition, Technology, and Logistics wrote that Operations Enduring and Iraqi Freedom “could not have happened without these aerial refueling capabilities.” The Pentagon said that aerial refueling is a “vital” capability and allows “rapid deployment of forces to contingencies.”

Given the Saudi-led coalition’s apparent reliance on U.S. provided weapons and mid-air refueling operations, it is likely that without U.S. support, the Saudi-led coalition would not be able to carry out many of its airstrikes. Thus, there is a strong claim that the U.S. support has “caused” the Saudi-led coalition’s alleged violations of international humanitarian law in Yemen.

Similarly, if the facts alleged were substantiated, there is a strong argument to be made that the U.S. has “caused” UAE torture of Yemeni detainees, which would amount to a “grave breach” of Common Article 3.  For example, if the United States has provided questions to UAE interrogators and detainees were subjected to torture to obtain answers to those questions, the U.S. officials could reasonably be said to have “caused” that torture.

(3) Willfully

Federal courts have interpreted the term “willfully” in three different ways. In the first interpretation, the term “willfully” means an intentional violation of a known duty. In the second, the term requires that the defendant knowingly violate a known duty. In the third, “willfully” means that a defendant knowingly engages in an act, without regard for whether or not he knows that act violates a duty (effectively merging “willfully” with the Model Penal Code definition of “knowingly” with regard to conduct.) (Indeed, the matter is the subject of a recently filed cert petition to the U.S. Supreme Court.)

No legislative history exists suggesting why the U.S. Senate revised 18 U.S.C. § 2(b), to read “willfully causes” instead of its original form, “causes.” Interpreting 18 U.S.C. § 2(b), various federal courts have suggested that § 2(b) requires the defendant to intentionally cause an intermediary to commit an act prohibited by the underlying statute—in other words, “an individual ‘willfully’ causes an offense when he intends the commission of conduct that constitutes a crime and then intentionally uses someone else to commit it.” It seems clear that the United States does not desire the Saudi-led coalition to engage in international humanitarian law violations. In fact, documents acquired from the U.S. Department of State in a 2016 FOIA request indicate that the United States endeavors to ensure international humanitarian law compliance in Yemen (See those documents here, here, here, here, and here).

Yet it is not clear that such intent is necessary to establish that the U.S. officials have acted “willfully.” If a court were to interpret “willfully” to mean the defendant “knowingly” enabled the Saudi-led coalition to commit international humanitarian law violations, U.S. officials might be liable. In the context of § 2(a) (the classic aiding and abetting clause) federal courts have inferred intent where an actor “knowingly provides essential assistance.” For example, in United States v. Zaftro (7th Cir. 1991), the Seventh Circuit held that where a defendant “knowingly provides essential assistance, we can infer that [that person] does want [the primary actor] to succeed, for that is the natural consequence of his deliberate act.”

Whether or not the United States support for the Saudi-led coalition would satisfy this element depends almost entirely on how the federal courts decide to interpret “willfully” in the statute. However, U.S. engagement with the UAE interrogating forces might give rise to liability under either standard.  In a chilling report from the Associated Press, one witness from the Yemeni security forces reported that American forces were present and “at times only yards (meters) away” from the commission of the alleged abuses. If substantiated, such behavior would certainly meet the “knowledge standard” and may even indicate that U.S. forces “intentionally” caused the UAE intermediary to commit the violations. It is important to note that “mere presence” will not always satisfy the ‘intent’ standard under legal theories of aiding and abetting, but presence in addition to an express or implied understanding can satisfy ‘intent.’

Requisite Mens Rea of Underlying Statute

To find a party guilty under the “perpetration by means test,” a defendant must also possess the mens rea required by the underlying statute. An individual can be guilty of the war crime of ‘murder’ if he or she “intentionally kills…one or more persons taking no active part in the hostilities.” And an individual can be guilty of a war crime of  “torture” or “cruel or inhumane treatment” if the person “intend[s] to inflict such treatment.”

With regard to the targeting violations, one factor that might militate against finding the requisite intent is the fact that the United States appears to have taken steps to reduce the likelihood that U.S. support would be used to intentionally kill persons “taking no active part in the hostilities” in Yemen. For example, as a condition for restarting U.S. military assistance in June 2017, Saudi Arabia assured the United States that it would take greater care to avoid hitting civilian targets. The measures taken by the United States to obtain assurances from Saudi Arabia that its practices are consistent with international humanitarian law might suggest members of the U.S. Armed Forces do not possess the requisite mens rea to be held accountable for aiding and abetting violations of the War Crimes Act under § 2(b).

Nonetheless, these assurances may not be sufficient to disprove mens rea if the United States knows the assurances to be false or ineffective. For example, Andrew Exum, a former Deputy Assistant Secretary of Defense for Middle Eastern Affairs, has argued that there are deep, systemic problems in the Saudi military that render it incapable of carrying out independent air operations without violating the international humanitarian law principle of discrimination.

Establishing the mens rea of intent in the context of the alleged ‘torture’ or ‘cruel and inhuman treatment’ of AQAP detainees by the UAE would rely on an in-depth fact analysis. However, if U.S. military personnel were indeed present at the interrogations where torture took place, a court might readily infer intent. (See our analysis above suggesting that courts have been willing to infer intent where a person “knowingly provides essential assistance.”) Even if U.S. forces were not present, but provided essential interrogation questions, a court might infer intent, satisfying this prong of the § 2(b) ‘perpetration by means’ analysis.


U.S. government personnel that have participated in mid-air refueling, targeting assistance, and other support to the Saudi-led coalition face limited legal risk of prosecution for aiding and abetting violations of the War Crimes Act, because establishing the requisite mens rea is likely to be difficult, if not impossible. However, if the UAE engaged in torture during interrogation with the participation of U.S. personnel, those personnel face greater potential liability for aiding and abetting violations of the War Crimes Act.

Regardless of their legal risk, U.S. government personnel should continue to endeavor to reduce the risk of IHL violations by the coalition. They could begin by continuing to place pressure on the Saudi-led coalition forces to abide by international humanitarian law.  They should also track Saudi-led coalition flights that they refuel, to ensure that those flights do not perpetrate violations of international humanitarian law.  And they should ensure that U.S. forces are not complicit in the torture of detainees by the United Arab Emirates: If they have reason to suspect that detainees are subject to torture, they should insist on proper treatment and cease any support or assistance for the detention and interrogation.

Photo: USAF