The U.S. Turkish Drone Shootdown Over Syria and the Jus Ad Bellum
In early October, the United States shot down an armed Turkish unmanned aerial vehicle (drone) operating over Syrian airspace with an F-16 fighter jet. The aerial engagement was unusual, if not unprecedented, in that it targeted an aircraft belonging to “one of Washington’s strongest and most-valued” NATO allies. Not surprisingly, it resulted in an immediate exchange of security concerns between U.S. and Turkish officials, including the U.S. Secretary of Defense and his Turkish counterpart. Thankfully, however, it did not result in any human casualties or further hostilities.
The incident followed a series of airstrikes earlier that morning by multiple Turkish drones targeting Kurdish forces located within a U.S.-declared Restricted Operating Zone (ROZ) in northern Syria, where U.S. forces are stationed in support of the global campaign to defeat ISIS. The proximity of the blasts caused the American troops, who often partner with the Kurdish forces, some of whom Türkiye considers to be part of an insurgent terrorist group (see, e.g., here and here), to seek shelter in bunkers. Shortly after the drones departed the area, a single drone reentered the ROZ on a course headed directly for U.S. forces.
Having observed the airstrikes on their Kurdish counterparts, American forces “made a dozen calls to Turkish military officials saying that American forces were on the ground in the area” and provided their location. Out of an abundance of caution, they further warned that they “would engage in self-defense if the drone did not leave the area.” After an exchange that lasted approximately ten minutes, the F-16 destroyed the drone once it came within 550 yards of U.S. forces. In a press briefing following the engagement, a Pentagon spokesman emphasized that “Turkey is one of our strongest and most valued NATO allies,” labeling the event “a regrettable incident.”
Likely in the minds of U.S. commanders throughout the event were the frequent attacks that Iranian-backed militants have launched against U.S. forces operating in Syria and Iraq in recent months and weeks (see, e.g., here and here). In July, for instance, in response to intelligence reports that armed groups were preparing for drone and rocket attacks against U.S. forces, a senior defense official remarked that, “It’s not a question of ‘if.’ It’s a question of when those would happen again.” Indeed, a flurry of attacks in Syria and Iraq in recent weeks demonstrates that security risks to U.S. forces in the area from drones and other aerial threats are steadily increasing (see, e.g., here, here, and here).
The downing of the armed military drone using an F-16 raises the question of whether the engagement was a use of force as that phrase is understood within the jus ad bellum. If it was, absent a recognized justification (or “circumstance precluding wrongfulness”), it constituted a violation of international law (or “internationally wrongful act”). Among other response options, Türkiye could seek reparations for the loss of the drone or, if its destruction rose to the level of an “armed attack,” Turkish forces could respond with a proportionate and necessary use of force in self-defense (see Draft Articles on State Responsibility, arts. 2, 21, 22, and 31).
This post undertakes those challenging use-of-force inquiries. It first assesses whether the targeting of the drone was a use of force. Assuming that it was, analysis then turns to whether its wrongfulness was precluded as an act of self-defense. It does not examine other issues under that framework, such as the lawfulness of American and Turkish military operations in Syria. Before turning to the rules for the use of force, however, it is helpful to summarize the context and relevance of the United States’ declaration of a ROZ with a brief, general overview of zones.
Military Zones
Although controversial in the past, it is widely accepted today that parties to an armed conflict may establish zones to aid in the execution of military operations. Such zones should not be confused with protected zones established pursuant to specific rules of international humanitarian law. With respect to the former, for instance, maritime zones may be declared to enforce a blockade or interdict contraband (see 2023 U.S. DoD Law of War Manual, § 13.9; NWP 1-14M, § 7.9; Newport Manual, § 7.2.1). A notable example is the exclusion zone declared by the United Kingdom during the Falklands/Malvinas War. Similarly, as it pertains to the law of air warfare generally,
States may establish airspace zones and associated procedures intended to prohibit aircraft from entering or flying in designated areas, including areas in international airspace. Such zones may be established for a variety of purposes, including to decrease the risk of inadvertent attack of civil or neutral aircraft, to control the scope of the conflict, or to enhance the predictability and effectiveness of ongoing operations (DoD Law of War Manual, § 14.7; see also Air & Missile Warfare (AMW) Manual, Section P).
Distinct from (albeit often overlapping with) the right to establish zones is the customary right of a belligerent to control the immediate area of military operations, which is considered to include areas where hostilities are taking place or where hostile forces are located. The right of control ensures both the security of a State’s military forces and its right to conduct hostilities without interference (see DoD Law of War Manual, § 13.8.1; AMW Manual, Rule 106 and accompanying commentary). As the expert AMW Manual helpfully explains, the right of a belligerent to take appropriate force protection measures
is a generally recognized belligerent right. Such measures may include the establishment and enforcement of “warning zones” around naval units (“defence bubbles”) or around military units stationed on the ground, and other measures the responsible commander considers necessary in view of a given threat.
Such “warning zones” merely serve to keep aviation or navigation at a distance from the force subject to protection, and to indicate that — should they enter the zone — they are at increased risk of defensive action. The establishment of a “warning zone” may never result in attacks without prior warning. However, aircraft approaching a “warning zone” may become liable to attack if, after prior warning, they continue on their course and military necessity warrants attack (Commentary to Rule 106(b)).
As this restatement implies, regardless of a zone’s purpose, the same rules apply inside and outside. Thus, there is broad agreement that establishing such a zone does not create a “free fire” area automatically precluding the wrongfulness of a use of force inside it. But breaching a zone may indicate that an intruder—such as an armed drone that is aware of the zone and has been repeatedly warned to leave it—possesses a hostile intent or is undertaking a hostile act.
Use of Force and Self-Defense
Article 2(4) of the UN Charter, which is widely recognized to reflect customary international law, provides that “All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the Purposes of the United Nations.” Although interpretation of the rule continues to evolve, there is broad consensus that it prohibits a State’s use of armed force against another State. There is little question that destroying another State’s military aircraft, outside of very narrow contexts not applicable here, qualifies as a use of force (for an examination of the distinction between domestic law enforcement measures as exercises of a State’s sovereignty over its own territory and uses of force, see Corten, Chapter 2).
It is obvious that the United States did not down the drone pursuant to an authorization from the Security Council under Chapter VII of the Charter. The critical inquiry, therefore, is whether any presumptive wrongfulness of the strike was precluded by a right of individual self-defense under Article 51 of the Charter and its customary international law analog. The former provides that, “Nothing 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.” As is evident from the provision, the availability of self-defense turns on the existence of an “armed attack.”
Armed Attack
Before proceeding, however, it is important to emphasize that the location of the American troops outside of U.S. sovereign territory at the time of the incident is immaterial to an armed attack analysis, for “it is undisputed that military units and military installations abroad can be regarded as ‘external manifestations of the State’ for the purpose of triggering the right of self-defence” (Ruys, § 3.4.1; see also Dinstein, para. 567). Instead, the assessment depends on whether the threshold of an armed attack was met.
There is no consensus, however, as to the precise quantum of force required to qualify as an armed attack permitting a State to act in self-defense. By the prevailing view, one articulated by the International Court of Justice (ICJ), it is “necessary to distinguish the most grave forms of the use of force (those constituting an armed attack) from other less grave forms” (Paramilitary Activities, para. 191). In the Court’s view, only those uses of force of sufficient “scale and effects” qualify as armed attacks, whereas “mere frontier incident[s]” do not (para. 195).
Applying this standard, reasonable minds may disagree as to whether a single drone strike—though certainly qualifying as a use of force—rises to the level of an armed attack. In 2003, for instance, the ICJ notably demurred from answering a comparable question in the context of Iranian attacks on U.S. vessels in the Persian Gulf during the 1980s Iran-Iraq Tanker War. Instead, after asserting that a series of hostile attacks, “even taken cumulatively,” would not necessarily “constitute an armed attack on the United States, of the kind that the Court, in [Paramilitary Activities], qualified as a ‘most grave’ form of the use of force,” the Court further opined that it 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’” (Oil Platforms, paras. 64, 72). As these seemingly contradictory observations indicate, the line between armed attacks and less grave uses of force under the Court’s view is anything but bright.
The United States, by contrast, rejects such a high armed attack threshold. In 2004, for instance, in response to the Oil Platforms judgment, U.S. State Department Legal Adviser William H. Taft IV explained that “For its part, if the United States is attacked with deadly force by the military personnel of another State, it reserves its inherent right preserved by the U.N. Charter to defend itself and its citizens” (p. 302). Further clarifying the U.S. position, he elaborated,
The gravity of an attack may affect the proper scope of the defensive use of force (that is, its proportionality . . . ), but it is not relevant to determining whether there is a right of self-defense in the first instance.
A requirement that an attack reach a certain level of gravity before triggering a right of self-defense would make the use of force more rather than less likely, because it would encourage States to engage in a series of small-scale military attacks, in the hope that they could do so without being subject to defensive responses. Moreover, if States were required to wait until attacks reached a high level of gravity before responding with force, their eventual response would likely be much greater, making it more difficult to prevent disputes from escalating into full-scale military conflicts (p. 300–01; see also Dinstein, para. 552).
Harold Koh, also a Legal Adviser to the State Department, echoed the position in a 2012 speech at U.S. Cyber Command. He observed,
[T]the United States has for a long time taken the position that the inherent right of self-defense potentially applies against any illegal use of force. In our view, there is no threshold for a use of deadly force to qualify as an ‘armed attack’ that may warrant a forcible response.
Thus, by the U.S. view, all attacks are “armed attacks.” But considering that a drone strike likely would have had fatal consequences for a military unit of U.S. soldiers, it is not unreasonable to conclude that it qualified as an armed attack under either view.
Intent
Apart from the applicable threshold of force, and perhaps more critically here considering the U.S-Turkish relationship, the degree of intent required for armed attacks is similarly unsettled. While some suggest that intent is better assessed as a component of necessity, addressed below, the resulting analysis is likely the same under either approach.
On one end of the spectrum, some argue that an offending State must intend to engage in an armed attack before the right to self-defense vests (see, e.g., here). In its Oil Platforms judgment, for instance, the ICJ observed that
There is no evidence that the minelaying alleged to have been carried out by the Iran Ajr, at a time when Iran was at war with Iraq, was aimed specifically at the United States; and similarly it has not been established that the mine struck by the Bridgeton was laid with the specific intention of harming that ship, or other United States vessels. (para. 64).
Professor Yoram Dinstein adopts a similar position indicating, “For an ‘armed attack’ to occur, the use of force must be deliberate: it cannot be caused by mistake or through an accident” (para. 548(i)). Professor Tom Ruys likewise claims that “hostile intent is a relevant factor for determining whether an ‘armed attack’ has occurred,” and is “particularly important for qualifying small-scale uses of force” (§ 3.2.2.a).
On the opposite end of the spectrum, however, some States and scholars do not foreclose the possibility that a State may resort to self-defense absent clear indications of an attacker’s hostile intent. For example, in his riposte to the ICJ’s suggestion that hostile intent is a necessary element of an armed attack, former U.S. State Department Legal Adviser William Howard Taft IV asserted,
States have a right of self-defense so that they can protect their national security and deter attacks against them, concerns that are implicated just as much when States are subjected to indiscriminate attacks as when they are subjected to targeted attacks . . . .
The United States accepts—as it believes all responsible States do—the need under international law to observe the principle of distinction in the use of force. If another State violates this principle and attacks the United States by unlawfully using indiscriminate force, the United States, like any State, is legally entitled to defend itself and its citizens (p. 302-03).
Depending on the circumstances, therefore, a lack of unequivocal hostile intent may not be dispositive, particularly when a State recklessly engages in potentially deadly behavior. As Professor Michael Schmitt summarized (rightly, in my view) in the context of Russia’s destructive interception of a U.S. drone over the Black Sea in March,
Although the issue of intent remains unsettled in international law, it is easier to make the use of force case in situations of recklessness, as seems to be the case in the Russian intercept, than those involving a mistake of fact, even a reasonable one. Assuming that the acts were indeed reckless but unintentional, an assertion that the Russian action amounted to a use of force is colorable but not unassailable. (For an assessment of intent and recklessness in the context of “indirect” force, see Schmitt-Biggerstaff, p. 206-10).
Even in the absence of hostile intent, therefore, recklessness on Türkiye’s part when operating the drone could nonetheless satisfy the threshold for an armed attack under the U.S. view.
Despite the nations’ otherwise friendly, albeit politically complicated, relationship, one should not quickly dismiss the possibility that the drone objectively displayed a hostile intent immediately preceding its destruction. Although a Pentagon spokesman expressed that U.S. officials had “no indication that . . . Turkey was intentionally targeting U.S. forces,” that conclusion was announced only after speaking with their Turkish counterparts following the incident. Indeed, Türkiye’s foreign minister had warned that same week that “his country would target ‘third parties’ that got in the way of Turkish attacks on the Syrian Democratic Forces.”
Thus, nothing demonstrates that U.S. forces on the ground did not genuinely and reasonably believe that the drone—which had knowingly penetrated the ROZ, was headed directly for U.S. forces, was not responding to repeated U.S. warnings, and came within 550 yards of their location—was an imminent threat in the moment. Accordingly, while there is little difficulty in concluding that the drone’s behavior was, at best, reckless, there is also a colorable argument that U.S. forces were reacting in the face of what they objectively perceived to be hostile intent.
Necessity and Proportionality
Assuming that a potential Turkish drone strike, even if errant, on U.S. forces would have risen to the level of an armed attack, the United States was obliged comply with the additional requirements that any use of force in self-defense be both necessary and proportionate, as those terms are understood under the jus ad bellum (in contrast to the law of armed conflict).
Necessity requires, in essence, that a State only use force as a last resort or, in other words, that “no practicable alternative means of redress” would suffice to address the threat (Dinstein, para. 656; Ruys, § 2.2.2.b.i). A temporal component of the criterion requires that an armed attack must be “imminent” before a State may engage in anticipatory self-defense. The classic definition of imminency was set forth by U.S. Secretary of State Daniel Webster in the Caroline affair, during which he famously declared that the necessity to act forcibly in self-defense must be “instant, overwhelming, and leaving no choice of means, and no moment for deliberation.”
But as Ian Brownlie has presciently observed, considering modern military capabilities, “the difference between attack and imminent attack may now be negligible” (p. 368). The window in which States may act in self-defense has, therefore, progressively expanded (see, e.g., here and here). Any distinctions between the varying positions are likely of little practical significance in this context, however. Given that the Turkish drone armed with air-to-surface missiles was within 550 yards on a course directly headed for U.S. forces and was non-responsive to repeated U.S. warnings, it is challenging to see how perceived harm from the drone was not imminent under even the stringent Caroline standard.
With respect to the proportionality requirement, this demands that a responding State use only as much force as is necessary to terminate the circumstances giving rise to the right of self-defense. Considering the United States limited its response to eliminating the drone it believed—reasonably, in my view—was an imminent and deadly threat to its forces, that criterion is easily met here.
Concluding Thoughts
While there is broad agreement concerning many of the rules pertaining to the use of force, some interpretations are unfortunately, as the preceding analysis indicates, still unsettled. The Turkish drone incident illustrates that long-debated questions pertaining to armed attacks, such as their applicable threshold of force and the extent to which they require hostile intent, are still timely and relevant.
After considering the totality of the circumstances surrounding the incident, I believe that the U.S. decision to destroy the drone to protect its forces—although undeniably a “regrettable incident” involving a longstanding NATO ally—was lawful. At the same time, I acknowledge that some might conclude the contrary. While I would disagree, given the lack of consensus in the field, I could not find such conclusions patently unreasonable.
In closing, I must emphasize that this post does not advocate for relaxing the standards for the use of force. Nor should it suggest that all reckless and dangerous behavior in close proximity to military forces merits a forcible response in self-defense. Rather, under the unique facts of this case, it was not unreasonable, nor clearly unlawful, for the United States to act in the manner it did, destroying an unmanned drone to protect its forces from foreseeably deadly harm.
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Lieutenant Colonel William C. Biggerstaff is a military professor at the Stockton Center for International Law at the U.S. Naval War College, where he co-teaches a course on the Law of Armed Conflict.
Photo credit: Master Sgt. Lochner
