The Global Hawk Incident: Self-Defense against Aerial Incursions – Reflections on the Applicable Law

Mohamed S. Helal is an Assistant Professor of Law at The Ohio State University Mortiz College of Law and an Affiliated Faculty with the Mershon Center for International Security Studies.

I don’t want to say it, but I will anyway: I told you so! On June 4th, I posted a two part post on Opinio Juris (here and here) on the current situation in the Persian Gulf and evaluated the possible justifications that might be proffered in favor of war. And, three weeks later, here we are. On June 20th, Iran shot down a U.S. MQ-4 Triton/Global Hawk surveillance drone. Although it has no offensive capabilities, the Global Hawk is an impressive piece of intelligence-gathering hardware. It is loaded with cutting edge surveillance equipment, it has a range of 12,000 nautical miles, it can fly at 60,000 feet, and costs $220 million apiece. According to Iran, the drone was shot down over its territorial waters. The U.S., however, insists that the Global Hawk was flying over international waters when it was destroyed. In response, the U.S. planned retaliatory attacks against Iranian radar and missile batteries. However, before these attacks were executed, President Trump ordered U.S. forces to stand down because, according to a Presidential Tweet, the U.S. response would have caused 150 fatalities, which the President felt was disproportionate to the downing of a drone. The next day, the New York Times reported that the U.S. carried out cyberattacks against Iranian intelligence agencies and computer systems that control Iranian missile launchers.     

This situation, which I’m calling the Global Hawk incident, raises difficult questions about the applicable law in cases of incursions by military aircraft into foreign airspace. The issue is whether this situation is governed by jus ad bellum and the general prohibition on the use of force enshrined in Article 2(4) of the U.N. Charter, or is this incident regulated by the general right of a state to police its airspace and protect its sovereignty against aerial intrusions by foreign aircraft? As I explain below, the answer is that frustrating lawyerly response: it depends. (See generally this excellent article by Tom Ruys)

But first, let’s see what the legal blogosphere has said about this incident. In an excellent post on Lawfare, Ashley Deeks and Scott Anderson argued that if, as Tehran alleged, the U.S. drone was in Iranian airspace, then it would be “permissible under international law for Iran to shoot it down, at least if Iran had first attempted to warn the drone or its operator.” However, the legal bases of this right to shoot down the drone appears unclear. Although they note that, pursuant to the Chicago Convention on International Civil Aviation, Iran has the right “to control access to its airspace,” Deeks and Anderson argue that “there is no black letter law on the question” of whether states may shoot down drones or other aircraft that enter their airspace without consent. Instead, they base their conclusion on examples of state practice that confirm that it is “lawful for a territorial state to shoot down a drone that enters its airspace.”

In an equally insightful piece on Just Security, Michael Schmitt discussed the legality of both Iran’s actions and the possible American responses. Schmitt argued that if, as the U.S. asserts, the drone was destroyed over international waters, then the incident could be characterized as a frontier incident (a concept that I will discuss further below), in which case the U.S. would not have the right to use force in self-defense. On the other hand, if the drone was over Iranian airspace, Schmitt agrees with Deeks and Anderson that Iran would be allowed under international law to shoot down the drone.   

I do not disagree with Deeks and Anderson, or Schmitt. But I do think the matter requires further reflection. I am not certain that the Chicago Convention is the most relevant body of law. The Convention applies “only to civil aircraft” and merely prohibits military aircraft (and, as Deeks and Anderson note, drones) from flying over the territory of another state without authorization. The Convention has nothing to say, however, about the appropriate response to aerial incursions by military aircraft. It only says that it does not “modify in any way the right and obligations of States set forth in the Charter of the United Nations.” In other words, the Convention has no bearing on questions of the use of force and self-defense against incursions by military aircraft.

I am also unsure about Schmitt’s reasoning. He appears to apply two separate bodies of law to the incident depending on the location of the Global Hawk. If the drone was in Iranian airspace, he agrees with Deeks and Anderson that Iran’s response would be lawful under the general right of states to protect their airspace. However, if the drone was outside Iranian airspace, Schmitt applies jus ad bellum and examines the legality of possible U.S. responses under jus ad bellum.

In my view, a single body of law ought to apply to the Global Hawk incident, regardless of whether the drone was over international or Iranian waters. In either case, I believe the Global Hawk incident is governed by jus ad bellum. The characterization of the shooting down of the drone and its legality will undoubtedly depend on its location and flight path. However, in all cases, this incident should be governed by jus ad bellum. 

Let me first explain why I am not convinced that Iran’s shooting down of the Global Hawk drone is a mere policing or law enforcement operation. To be sure, various rules of conventional and customary international law entitle states to monitor and secure their airspace, territorial sea, and other maritime zones, and to take measures, including forceful measures, to enforce their laws in those areas. Indeed, state practice, especially during the Cold War (see Oliver Lissitzyn’s classical article here) and in recent years (see this article on Turkey’s downing of Russian jets in 2015), indicates that in numerous incidents the general right to preserve sovereignty over national airspace was considered to be the applicable law to aerial incursions, including in cases that caused loss of life or damage to intruding aircraft.    

In other instances, however, jus ad bellum displaces these general rules of international law. In these cases, it is jus ad bellum that determines the nature of aerial incursions, assesses their legality, and outlines the permissible responses to these acts. If certain conditions are met, an aerial incursion becomes an act of inter-state violence that is covered by the general prohibition on the use of force by states. While scholarly opinion on this matter is far from unanimous, the conditions under which jus ad bellum becomes applicable can be encapsulated in a single concept: hostile intent.

 A whole range of factors should be examined to determine hostile intent. These include the means of the aerial incursion – i.e. was it undertaken using military aircraft; the gravity of the incursion – i.e. the number and type of aircraft involved; the duration of the incursion; the location of the incursion; and most importantly, the political circumstances surrounding the incursion.

That’s why I answered the question I posed above about the applicable law in cases of aerial incursions by saying “it depends.” Some aerial incursions are just that – simple, perhaps inadvertent, incursions of no political consequence. Other incursions, however, may be a prelude to all-out war. If the overall circumstances of the incursion, especially its political context and the nature of relations between the relevant states, demonstrate that the state that committed the incursion did so with hostile intent, then I would argue that jus ad bellum is the applicable law. Given the current tensions between the U.S. and Iran, I would argue that the Global Hawk incident is governed by jus ad bellum.

Applying the rules of jus ad bellum to the Global Hawk incident, however, is not a simple matter. This is because this incident falls in a particularly complicated gray area of jus ad bellum, which relates to the right of states to respond to minor uses of force. For the sake of analysis, let’s assume that the Global Hawk did indeed enter Iranian airspace. That would, undoubtedly, constitute a violation of Iranian sovereignty. It could also amount to an unlawful use of force. However, as Michael Schmitt explained on Just Security, states are not permitted to use force in response to any breach of sovereignty, nor are states allowed to use force in response to every unlawful use of force. In the absence of U.N. Security Council authorization and except in cases of intervention by invitation, states may use force in self-defense only “if an armed attack occurs.”

Therefore, Iran’s shooting down of the Global Hawk would be lawful, under the rules of the jus ad bellum, only if the drone had entered Iranian airspace and if that act can be categorized as an armed attack. Indeed, it seems that Iran considered itself to be acting in self-defense. In a letter to the U.N. Security Council and the U.N. Secretary General, Iran declared that it “reserves its inherent right, under Article 51 of the United Nations Charter, to take all appropriate necessary measures against any hostile act violating its territory, and is determined to vigorously defend its land, sea and air.”

It is conceivable, however, that this situation constitutes a so-called “frontier incident” – a minor unlawful use of force that does not amount to an armed attack that unlocks the right to self-defense. In the Nicaragua Case (and in subsequent judgments), the ICJ differentiated between “the most grave forms of the use of force (those constituting an armed attack)” and “other less grave forms” of force, one example of which are “frontier incidents.” These are limited cross-border exchanges of fire or isolated incursions by the forces of one state onto the territory, airspace, or waters of another state. There are two distinguishing features of frontier incidents. The first is scale and effects. Because the scale and effects (i.e. gravity) of a frontier incident are limited, such acts are not considered serious enough to warrant the use of force in self-defense. Second, frontier incidents occur in circumstances that do not evince an intent to engage in an armed attack. These are insignificant skirmishes of limited geographical scope that occur inadvertently, by mistake, or without a clear intent to execute an operation of sufficient gravity to constitute an armed attack.  

 Applied to the present case, it could be argued that an incursion by a single U.S. surveillance drone that does not have offensive capabilities does not amount to an armed attack. In that case, Iran would not have the right to invoke the right of self-defense to shoot down the drone. Alternatively, Iran could plausibly argue that the circumstances of this incident, especially the current escalation in the region, justify categorizing an incursion by the Global Hawk as an armed attack, or as the initial steps of an armed attack, which necessitated a forceful response. Interestingly, the U.S. Commander’s Handbook on the Law of Naval Operations would support such an Iranian argument. It states: “military aircraft intruding into foreign airspace on a military mission may constitute a sufficient threat to justify the use of force in self-defense. This appears true both for tactical military aircraft capable of directly attacking the overflown state and for unarmed military aircraft capable of being used for intelligence-gathering purposes.”

The ICJ’s distinction between armed attacks and less grave uses of force has been debated and criticized extensively. One view that is critical of the ICJ and which is espoused by several states, including the U.S. (see here), is that any unlawful use of force entitles a state to use force in self-defense, as long as the defensive force respects the principles of necessity and proportionality. This approach essentially removes the distinction between armed attacks and less grave uses of force. Alternatively, in a thoughtful opinion in the Oil Platforms Case, Judge Bruno Simma retained the distinction between armed attacks and less grave forms of force, but argued that states may undertake proportionate forceful countermeasures in response to uses of force that do not amount to armed attacks. Another approach proposed by Yoram Dinstein (p. 261-263) allows states to exercise “on-the-spot” defense in situations that may be characterized as frontier incidents. In this case, Dinstein writes, “the employment of counter-force must be temporally interwoven with the armed attack triggering it.” In other words, the response must be limited to terminating and repelling the small-scale incursion.

These arguments, which allow states to use force in response to frontier incidents, could be used by Iran to justify its shooting down of the Global Hawk. My view, however, is that these approaches do not reflect the lex lata of jus ad bellum, which continues to recognize the distinction between armed attacks allowing for the use of force in self-defense and less grave situations, such as frontier incidents.

If, however, the Global Hawk was outside Iranian airspace, I would agree entirely with Michael Schmitt’s analysis, which I outlined above. Iran’s downing of the U.S. drone would be governed by jus ad bellum. It would be unlawful, but it would be categorized as a frontier incident, which would not entitle the U.S. to use force in self-defense.

A final note: it may be plausibly contended that the rule on frontier incidents doesn’t make a whole lot of sense. The outcome, it could be argued, is that the U.S. is left helpless when its drone is attacked over international waters and Iran is left defenseless when a drone intrudes into its airspace. That’s a fair criticism, but it overlooks the logic underlying the frontier incidents rule and the overarching purposes of jus ad bellum. The rule proscribing the use of force in self-defense in response to frontier incidents is intended to short-circuit the logic of escalation. It limits the possibilities that minor, insignificant skirmishes could spin out of control and lead to the outbreak of an unnecessary, needless war.