On June 20, the Iranian Islamic Revolutionary Guard Corps used a surface-to-air missile to shoot down a MQ-4 Triton, a Navy variant of the Global Hawk, over or near the Strait of Hormuz. The Global Hawk is a high altitude, long-endurance RPA (remotely piloted aircraft, colloquially a “drone”) equipped with a highly advanced sensor suite that operates in any weather, day or night, to provide ISR (intelligence, surveillance and reconnaissance) capabilities. The Navy version that was downed performs a “Broad Area Maritime Surveillance (BAMS-D)” ISR mission.

The incident follows on the heels of the United States pulling out of a multilateral nuclear agreement and reimposing economic sanctions last year and alleged attacks by the Revolutionary Guard using limpet mines against Norwegian and Japanese flagged tankers in the Gulf of Oman on 13 June. With tensions already high, the shootdown prompted the United States to plan strikes on Iran employing cruise missiles and manned aircraft. However, according to tweets from President Trump the day after the drone was downed,

Ashley Deeks and Scott Anderson have authored excellent pieces on Lawfare discussing the legal questions surrounding the maritime attacks (Deeks) and the shoot down (Deeks and Anderson), while Brian Egan and Tess Bridgeman have masterfully deconstructed the domestic legal concerns raised by the canceled U.S. response. In this post, I build on their analysis by examining the international law issues implicated by the aborted U.S. strikes, zeroing in on the jus ad bellum (the law that governs the resort to force by States) and international humanitarian law (IHL – the law governing how operations are conducted during an armed conflict).

The Right to Use Force

First the basics. States are prohibited from using force against other States by Article 2(4) of the UN Charter and customary international law. There are but three universally accepted exceptions to this prohibition – consent of the other State, UN Security Council authorization or mandate to use force pursuant to Chapter VII of the Charter, and the right of individual and collective self-defense reflected in both the Charter’s Article 51 and customary law. Other possible bases for the use of force, such as humanitarian intervention and rescue of nationals abroad, are less well-settled as a matter of law and do not apply factually in this situation.

Unfortunately, much of the discussion surrounding the U.S. strikes has been framed in terms of retaliation. To be clear, retaliation, that is, a tit-for-tat use of force, is unlawful in international law. What is not clear is whether the President and others are using the term loosely to describe a US response or actually believe that retaliation is a lawful basis for the use of force against another State. If the former, they should be more cautious, as rhetoric can prove escalatory. If the latter, they are simply wrong.

Obviously, Iran did not consent to US strikes and the UN has not authorized military action in response to the oil tanker attacks or the downing of the US drone. Accordingly, the only possible basis for a U.S. use of force against Iran is self-defense.

The Right to Use Force in Self-Defense

Article 51, which replicates its customary law counterpart, provides, in relevant part, that “[n]othing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security.”

With respect to the attacks on the oil tankers, the International Court of Justice (ICJ) noted in its Nicaragua judgement that a condition precedent to using force in collective defense on behalf of, or in collaboration with, a State that is the victim of an armed attack is a request from that State for assistance. It is unlikely that other States would ever request the United States to use force unless the Iranian actions crossed a threshold of force sufficient to rise to the level of an “armed attack” – more on that threshold in a moment.  But since such a request has not been made either expressly or implicitly by Japan or Norway, the sole remaining possible basis for a U.S. use of force is individual self-defense on the basis of the attack on its unmanned drone.

If the drone was in Iranian airspace at the time it was shot down, as claimed by the Iranian Foreign Minister, it would seem clear that the United States would not have a right of self-defense. Deeks and Anderson have correctly noted that States are entitled to control access to their national airspace (which extends to airspace above the territorial sea of a coastal State). Accordingly, States may use force to repel penetration of their airspace without their consent by another State’s military aircraft, although a warning requirement would logically attach if feasible in the circumstances.  But, for the sake of analysis, let us assume the US drone was located in international airspace at the time it was engaged by Iranian forces, as the Commander of U.S. Air Forces Central Command asserts. If the drone were over Iranian airspace, the rest of this analysis would be unnecessary.

Prerequisite: An “armed attack” by Iran

Operations in self-defense must be, as indicated in Article 51, in response to an “armed attack.” In the Nicaragua judgement, the ICJ, described armed attacks as the “most grave forms of the use of force,” such that not every use of force against a State allows for a forceful defensive response. Whether the downing of a single unmanned military aircraft crosses this threshold is highly doubtful. For instance, in its Oil Platforms judgment, which also involved hostilities between Iran and the United States, the ICJ could not “exclude the possibility that the mining of a single military vessel might be sufficient to bring into play the ‘inherent right of self-defence.’” This characterization is often cited as reflecting the lower end of the armed attack continuum. An attack on an unmanned military drone that places no lives at risk falls well below this level of severity. What’s more, in Nicaragua, the Court distinguished between an armed attack and “a mere frontier incident,” which this event with the U.S. drone could easily be labeled. Although a strict application of the frontier-incident distinction has been criticized, and rightly so, by scholars like Yoram Dinstein, the fact that no lives were lost in the drone downing would support its application in this case.

Nonetheless, the United States has consistently rejected the existence of the so-called “gap” between a use of force and an armed attack, asserting instead that the thresholds are identical. As noted in the Department of Defense’s Law of War Manual, “[t]he United States has long taken the position that the inherent right of self-defense potentially applies against any illegal use of force.”  This interpretation is not widely accepted to say the least, although notably Japan may be moving in the U.S. direction on the “no gap” theory.  By the U.S. view of the armed attack threshold, the Iranian surface-to-air missile strike on the drone would amount to an armed attack, for it is undoubtedly a use of force.

Prerequisite: necessity of the U.S. response

Adopting the U.S. view for the sake of continued analysis, acts of self-defense, such as the planned strikes against Iran, must comply with two requirements — necessity and proportionality — that the ICJ set forth in its Nicaragua judgement and reiterated in Oil Platforms. These two conditions bear, respectively, on whether a U.S. use of force in response to the Iranian action is permissible at all and, if so, the nature of that operation.

The DoD Manual accurately characterizes the condition of necessity as requiring “that no reasonable alternative means of redress are available.” It illustrates this condition by noting that “diplomatic means must be exhausted or provide no reasonable prospect of stopping the armed attack or threat thereof.” Beyond diplomacy, sanctions and any other viable means of causing the armed attack to end must also be unavailable before force may be used in self-defense. In this case, a reported Presidential warning to Iran through Oman that attacks were imminent and requesting talks met a cold response from Iran. The irony here is that President Donald Trump and Secretary of State Mike Pompeo have separately called that media report false. As to sanctions, economic sanctions were already in place but failed to deter the Iranian action against the drone.

Yet, reports that U.S. Cyber Command conducted a cyber operation against Iranian missile and rocket command and control systems in response to the drone attack would appear consistent with the requirement of necessity (assuming the shoot down occurred in international airspace). If the cyber operation did not cross the use of force threshold, it would be a “countermeasure” under the law of State responsibility and the use of force in self-defense would be unnecessary. And, acknowledging that the legal threshold for a cyber use of force is uncertain, even if the cyber operation did cross that threshold, its availability in response to the drone operation would nevertheless have rendered the proposed kinetic strikes questionable on the basis of the jus ad bellum proportionality criterion, which is discussed below.

Of greatest normative significance is the fact that the proposed U.S. air and missile strikes (and the cyber operation if it crossed the use of force threshold) would likely have failed the necessity test on the basis of “imminency or immediacy.” These temporal criteria provide that a defensive response must be in response to an armed attack that is imminent, underway, or reasonably believed to be, in the words of the DoD Law of War Manual, “part of an ongoing pattern of attacks” in which “force is reasonably necessary to discourage future armed attacks.”

As the U.S. response was conducted after the drone had been downed, it would only have satisfied the temporal requirements of self-defense if the United States possessed reliable intelligence indicating Iran would conduct further attacks against U.S. assets, such as military aircraft, warships or commercial shipping.  In that event, the drone attack could be characterized as but one strike in an Iranian campaign against the United States.  Indeed, the Defense Department has claimed that Iran attempted to shoot down a U.S. drone while conducting surveillance over the damaged oil tankers.

However, broadly speaking, open source material does not seem to support a conclusion that further attacks were planned by Iran, especially in light of the Iranian insistence that it only acted in response to a violation of its national airspace. What’s more, it should be cautioned that the earlier alleged Iranian attacks against the oil tankers might bear on Iran’s willingness to resort to force generally, but they do not signal a campaign directed at the United States.  Ultimately, without access to the available intelligence, no definitive conclusion can be drawn as to whether the drone attack was the initiation of a pattern of attacks against the United States.

Prerequisite: proportionality of the U.S. response

The second requirement is proportionality. This jus ad bellum criterion is often confused with a criterion of the same name in the context of countermeasures, international human rights law and international humanitarian law (discussed below). Indeed, Trump mischaracterized it in his tweet by suggesting that 150 deaths was disproportionate to shooting down a drone, thereby suggesting that the severity of the response must not be out of proportion to the severity of the armed attack. That is not the law.

As Geoff Corn has described the criterion in correspondence with the author, “Ad bellum proportionality is not a humanitarian rule; its sole purpose is to protect state interests by preventing a valid claim of self-help necessity being used as a subterfuge for unlawful aggression. Accordingly, its function is solely to define permissible scope and duration of an action born of necessity.” Consistent with this accurate characterization of the object and purpose of the self-defense proportionality requirement, the DoD Law of War Manual explains, “[f]orce may be used in self-defense, but only to the extent that it is required to repel the armed attack and to restore the security of the party attacked” (emphasis added). In some cases, this may be less force than used against the victim State, whereas in others it may be more.

Assuming for the sake of analysis that the United States reasonably concluded that further attacks would take place, a proportionality analysis would focus on the scale and scope of the forceful response that would be required to deprive Iranian forces of the ability to launch the pending attacks and/or convince Iranian authorities to refrain from conducting them. Should U.S. strikes exceed that threshold, they would be unlawful. Without knowing the intended targets and the effect of the proposed strikes upon them, it is difficult to make this proportionality assessment. However, in making such an assessment, it is essential to understand that the issue would not be the possible casualties that might result, but rather the effect of the strikes upon continued Iranian attacks. For instance, striking Iranian radar, surface to air missile sites or military airfields would be targets clearly related to degrading enemy capabilities against U.S. aircraft. Striking military assets that play no role in the continuing Iranian attacks would be dubious.

International Humanitarian Law Targeting Rules

Irrespective of the legality of the planned U.S. strikes under the jus ad bellum, the Iranian downing of the drone initiated an international armed conflict between the United States and Iran to which IHL applied. That conflict would have continued with the U.S. strikes and therefore those strikes would have been subject to IHL prohibitions and requirements. Without knowing the nature of the planned attacks, it is difficult to determine whether they would have complied with the limitation of targets to “military objectives” and the requirement to take feasible “precautions in attack” to avoid or minimize collateral damage to civilians and civilian objects. Such precautionary requirements require exhausting feasible means of verifying that the target is a military objective; selecting from among available weapons, attack tactics, and targets that can achieve the desired military effect with the least harm to civilians and civilian objects; and warning the civilian population of the attacks if it will be affected and a warning is militarily and practically feasible in the circumstances.

The one IHL requirement implicated by the Trump tweets is proportionality. As noted, IHL proportionality differs from jus ad bellum proportionality. The generally accepted definition of IHL proportionality is captured in Article 51(5)(b) of the 1977 Additional Protocol I to the 1949 Geneva Conventions. By that article, an attack violates the rule of proportionality if it “may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated.” This ex ante determination is made at the time the attack is planned, approved, and executed. It is not a post factum assessment based on the results of an attack. Thus, it was proper to conduct the proportionality assessment, as Trump appears to have done, prior to the strikes being conducted.

But, if the tweets are accurate, questions arise. First, the harm that is factored into the calculation is only that caused to civilians or civilian objects. Iranian military deaths would not be considered in making the determination. It is uncertain whether the 150 casualties were civilian or military, although it is very difficult to believe the U.S. military leadership would have recommended strikes causing 150 civilian deaths in response to a drone shoot down, especially when Iran is a target-rich environment, one presenting many lawful targets that can be struck with minimal risk of collateral damage. 150 civilian deaths would self-evidentially appear excessive relative to the military advantage (see below) and would also likely violate the selection of targets required by the obligation to take all feasible precautions.

Moreover, without knowing the intended targets it is difficult to gauge the “concrete and direct military advantage” anticipated to result from the strikes. U.S. targeting doctrine is “effects-based” in the sense that the military advantage is not measured in terms of attrition of enemy personnel and equipment, but rather is based on the effect on enemy operations or the benefit accruing to one’s own operations. In this case, the presumed advantage would be degradation of Iranian capability to conduct further attacks against U.S. assets. Of course, if the U.S. operations were being conducted solely in retaliation (see discussion above) and no further operations were planned by either side, there would be no meaningful military advantage to conducting them, thereby rendering them unlawful should any collateral damage result.

Open source information is simply insufficient to judge the proportionality of the proposed strikes. One would have to know whether the possible causalities were civilian, whether civilian objects would be incidentally damaged, and the nature of the military advantage that U.S. commanders expected to achieve by conducting the attacks. But what is clear is that the proportionality analysis would not include all deaths and would not be measured against the loss of the drone, as suggested in the Trump tweets and by news reports of what was on the President’s mind.

Concluding Thoughts

It is difficult to definitely conclude that the proposed U.S. kinetic strikes would have been valid exercises of self-defense. Even assuming the downed drone was in international airspace, and though the attack thereon would clearly qualify as a use of force, it is unclear that the Iranian action amounted to an armed attack except by the United States’ particular approach to the threshold for armed attacks. Yet, even if the U.S. position is applied, the proposed strikes would have violated the necessity criterion absent reliable intelligence that the Iranian downing of the drone was but the starting part of a series of attacks directed at U.S. assets. On its face, therefore, the planned U.S. response looks like what it was publicly labeled – retaliation.

As to the IHL prohibitions and requirements, without knowing the nature of the planned U.S. strikes or what was meant by Trump’s tweets regarding 150 casualties, the consistency of the operations with the IHL rule of proportionality and the requirement to take precautions in attack cannot be determined. However, it is clear that much of the discussion surrounding the proposed strikes, including the President’s statements, failed to accurately reflect that body of law.

Finally, what of a threat by Iran to interfere with international commerce passing through the Strait of Hormuz, a prospect that has long caused concern? Until such attacks occur or are imminent (as in an announcement by Iran that it will conduct them or reliable intelligence indicating Iranian forces will do so), self-defense affords no right to the United States to conduct any use of force to preclude the interference. Additionally, to justify U.S. military action, the attacks would have to be directed at U.S. vessels unless the flag States concerned asked the United States to defend their vessels in the event of actual or imminent Iranian attack. It should also be cautioned that taking action solely to ensure shipping lanes remain open, sometimes labeled a “vital national interest,” is not a justification for the use of force in international law. That said, if Iranian forces attack, or are about to attack, vessels exercising their lawful freedom of navigation, the States concerned are free to defend themselves either individually or collectively, and with respect to the latter, they may pre-arrange a system providing for robust collective defense of their shipping.


[Editor’s Note: You may also want to read, Brian Egan and Tess Bridgeman’s Top Experts’ Backgrounder: Military Action Against Iran and US Domestic Law]