On Sept. 19, three Russian MiG-31 Foxhound fighters penetrated Estonian airspace over the Gulf of Finland for approximately 12 minutes. Finnish jets intercepted them, NATO scrambled Italian F-35s deployed to Estonia as part of the NATO Air Policing mission, and Swedish fighters monitored the Russian aircraft after they left Estonian airspace. Predictably, Russia claimed its aircraft were on a “scheduled flight … in strict compliance with international airspace regulations and did not violate the borders of other states, as confirmed by objective monitoring.”
This was only the latest in a series of Russian intrusions into NATO airspace. On Sept. 13, a Geran-2 drone flew into Romanian airspace, where two Romanian Air Force F-16s tracked it for nearly an hour before it departed. The Geran-2 (an Iranian-designed Shahid-136) is a loitering munition, also known as a kamikaze drone. The incident followed the breaching of Polish airspace by more than 24 drones, including Geran-2s and Gerbera decoy drones, for a period of seven hours on September 11. Polish F-16s and Dutch F-35s intercepted them, shooting down four, while German Patriot missiles in Poland went on high alert. NATO promptly launched Operation Eastern Sentry to enhance its defenses on its Eastern flank.
In response to Russia’s aggressive operations, NATO’s North Atlantic Council (here) and the UN Security Council discussed the Russian actions last week. And both U.S. President Donald Trump and Poland’s Prime Minister Donald Tusk have asserted that Russian military aircraft entering NATO airspace should be shot down.
The operational purpose of the Russian actions is perhaps clear. Moscow is apparently testing NATO’s readiness, including command and control, defensive protocols, reaction times, force disposition, and defensive gaps or weaknesses. Such probes are classic indications and warnings (“I & W”) of escalating adversary intentions, or even preparation for attack. And Russian forces continue to probe NATO defenses, most recently by flying combat aircraft along Latvia’s border last Friday, sparking a scramble by Hungarian interceptors.
This article surveys the international law governing such operations. Analysis begins with an examination of the rules that such operations may breach. I identify three possibilities – violation of territorial sovereignty, prohibited use of force, and breach of the inviolability of neutral territory. Discussion then turns to using forcible measures to expel, or even shoot down, manned or unmanned military aircraft that have penetrated national airspace without the territorial State’s consent. Such shootdowns have occurred with surprising frequency in the past (see, e.g., Pedrozo, p. 855), including during a 2015 incident in which NATO member State Turkey downed a Russian Su-24 Fencer (a manned aircraft) following repeated warnings. I proffer three possible grounds for taking such forcible responses: law enforcement, self-defense, and neutrality rights and obligations.
Russian Actions: Violations of International Law
Sovereignty: As sovereign territory, national airspace is inviolable. This customary international law principle is reflected in Article 1 of the 1944 Convention on International Civil Aviation (Chicago Convention): “[E]very State has complete and exclusive sovereignty over the airspace above its territory.” Although generally inapplicable to military aircraft, the Convention cautions, “No state aircraft [which includes drones] of a contracting State shall fly over the territory of another State or land thereon without authorization” (art. 3; see also DoD Law of War Manual, § 14.2.1.1). Indeed, in Paramilitary Activities, the International Court of Justice found that U.S. high-altitude reconnaissance flights over Nicaragua violated that State’s sovereignty (para. 292(5)).
Of relevance to the Estonian airspace incident, Article 2 of the UN Convention on the Law of the Sea confirms the inviolability of national airspace over the territorial sea. In that regard, there is no equivalent to maritime innocent passage through the territorial sea in the superjacent airspace (LOSC, art. 17; Commander’s Handbook on the Law of Naval Operations, § 1.3.2). Presence there requires consent.
Finally, Russian claims regarding the penetration of Polish airspace by its military drones have ranged from denial to mistake. However, even if one or more of the intrusions was unintended, a non-consensual penetration of another State’s airspace violates that State’s territorial sovereignty (Tallinn Manual 2.0, p. 24).
That said, the principle of “force majeure” (an uncontrollable force) may preclude wrongfulness on the part of a State, including when its aircraft breaches another State’s territory. The force majeure legal defense, however, does not include “situations brought about by the neglect or default of the State concerned, even if the resulting injury itself was accidental and unintended.” (ILC, Articles on State Responsibility Commentaries art. 23 (3)-(4)). Note the ILC includes, by way of example, references to “accidental intrusion” and cases such as U.S. military aircraft entering Yugoslavia’s airspace in 1946. In that case, Acting Secretary of State William Clayton sent a diplomatic note stating, “The Yugoslav Government has already received assurances from the United States Government that the United States planes will not cross Yugoslav territory without prior clearance from Yugoslav authorities, except when forced to do so by circumstances over which there is no control, such as bad weather, loss of direction, and mechanical trouble” (UN Secretariat Yearbook 1978, vol. II (Part One), A/CN.4/315, para. 144 (emphasis in original)).
Given the record, there is essentially no doubt that the non-consensual intrusions by Russian military aircraft into NATO member States’ airspace violated their territorial sovereignty.
Use of force: Article 2(4) of the UN Charter and customary international law prohibit the “threat or use of force” by one State against another, except in certain circumstances (e.g., self-defense) that are inapplicable to Russia’s grounds for its actions. This raises the question of when an intrusion by a military aircraft that does not then conduct an attack nevertheless amounts to a threat of unlawful force against the subjacent State or qualifies as a use of force that is underway.
The answer depends on the effects of the action. Analogies can be drawn to cyber operations, where States have issued numerous position papers on whether non-destructive/injurious acts can qualify as a use of force. The prevailing view is that physical effects are not a condition precedent to qualification, with NATO taking the position that the determination in these situations is multi-factored (AJP 3.20, ¶ 3.7). In this regard, the military nature of the act in question is a key consideration. For instance, France would treat the penetration of “military systems to compromise French defense capabilities” as a use of force. The same case-by-case, multi-factored approach applies by analogy to intrusions by military aircraft that do not cause immediate physical harm.
As an effects-based assessment, the central issue is the extent to which an intrusion by another State’s military aircraft undermines the territorial State’s security. Relevant considerations include, inter alia, the nature of the aircraft, whether they are armed; whether the weaponry is air-to-air or air-to-ground; the location in which they are flying, whether they respond to attempts to communicate and demands to leave national airspace; their tactics (e.g., speed, altitude, evasive maneuvers); the current political, geopolitical and security context; and whether express or implied threats had been made prior to the penetration.
Considering the penetrations’ contribution to enhancing Russia’s ability to mount military operations against the affected States, and in light of Russia’s persistent saber-rattling, it is reasonable for those States to treat them as uses of force. The Estonia penetration is the clearest case, as it involved high-performance combat aircraft entering the airspace of the NATO member State that is most vulnerable to Russian aggression. And with respect to the Romanian airspace intrusion, an hour-long unconsented-to flight through national airspace by a drone capable of being armed would likely be seen by most States as crossing the use of force threshold, especially in light of the earlier Russian drone flights over Poland. The number and duration of penetrations into Polish airspace by drones, some of which are capable of being armed, likewise support styling that incident as a use of force.
Qualification as “threats” of force is even more clear-cut. Threats can be express or implied, and there is no requirement that a demand accompany the threat (Wood). Through the intrusions, Russia appears to be signaling its military capacity and readiness to employ force against NATO members. Perhaps the goal is to weaken the resolve of NATO member States that have pledged to act in the collective defense of those members most likely to be targeted by Russian aggression (especially the Baltic States). Or maybe the intrusions are designed to cause NATO to waver in ramping up support for Ukraine. In either case, the flights impliedly threaten an unlawful use of force. They are patently unlawful.
Neutrality: Although it might be suggested that certain NATO States have become “belligerents” by virtue of their substantial support for Ukraine, the prevailing stance in the international community is that the affected NATO States have neutral status in the Russia-Ukraine international armed conflict. Consequently, they enjoy neutral rights and shoulder neutral obligations.
Early treaty law on neutrality is found in the 1907 Hague Conventions, specifically Convention V and Convention XIII, which address the land and maritime domains, respectively. Although these instruments predated aerial warfare, they are often looked to by analogy as a reflection of customary neutrality law (DoD Law of War Manual, § 15.1.4), including that governing aerial operations. In this regard, Article 1 of Convention V provides that “the territory of neutral Powers is inviolable,” while Article 1 of Convention XII requires respect for the “sovereign rights” of neutral Powers in “neutral territory or neutral waters.” The non-binding 1923 Hague Rules of Air Warfare, which are also considered to generally reflect contemporary customary international law, distilled these rules in the aerial context: “Belligerent military aircraft are forbidden to penetrate into the jurisdiction of the neutral State” (Art. 40).
The 2013 HPCR Manual on International Law Applicable to Air and Missile Warfare (AMW Manual), a modern expert restatement, reinforces the principle of neutral airspace inviolability. Rule 170(a) provides, “Any incursion or transit by a belligerent military aircraft (including a UAV/UCAV) [unarmed or armed drone] or missile into or through neutral airspace is prohibited.” Rule 172 similarly emphasizes that “[b]elligerent military aircraft may not enter the airspace of Neutrals,” except in limited circumstances (e.g., aircraft in distress, capitulation, transit passage).
All military manuals addressing the issue are in accord. For instance, the DoD Law of War Manual provides, “Belligerent military aircraft are forbidden to enter neutral airspace, subject to certain exceptions,” none of which apply to the Russian intrusions (§ 15.10.2; see also § 15.3.1.1). Unquestionably, the Russian penetrations violated the neutrality of the States concerned.
Permissible Responses: Using Force Against the Intruding Aircraft
As discussed, there is little question that the Russian aircraft intrusions were unlawful. But on what legal grounds may force be used to expel the Russian drones and other aircraft, or shoot them down, as was done with several drones that entered Polish airspace? There are three potential grounds for taking such action, depending on the circumstances.
Law Enforcement: Every State enjoys the right to control entry into its territory. This includes taking forcible measures that are consistent with any applicable international law restrictions, most notably those found in international human rights law (IHRL).
There is some disagreement over whether a State may lawfully use force against a crewed aircraft that merely enters its territory. During the Tallinn Manuals deliberations, some of the experts were of the view that a “legal basis for forceful actions to expel a foreign State aircraft conducting cyber operations, separate from that of self-defence, is that the aircraft has violated the State’s territorial integrity” (p. 264).
The majority, however, would generally require the engagement to comply with the law of self-defense (but see also the discussion of neutrality below). I agree. In such circumstances, the crew would enjoy the “right to life” under IHRL. Under that body of law, “[t]he use of potentially lethal force for law enforcement purposes is an extreme measure that should be resorted to only when strictly necessary in order to protect life or prevent serious injury from an imminent threat” (General Comment 36, ¶ 12). Of course, there may be circumstances that satisfy this requirement (e.g., if NATO member States’ pilots sent to engage the Russian aircraft came under fire); however, in most such cases, the right to use force in self-defense would also be triggered (see below).
Since drones do not enjoy IHRL protection, States may employ forcible measures to expel or destroy them, consistent with their domestic law. It was on this basis that the United States destroyed a Chinese high altitude balloon in 2023 (von Heinegg) and that Iran shot down a U.S. drone, claiming it was over Iranian waters (Deeks and Anderson; but see Helal).
The fact that State aircraft, like the Russian military aircraft and drones, are “sovereign immune platforms” exempt from the enforcement jurisdiction of other States might seem to muddy the waters (AMW Manual, p. 47; Tallinn Manual 2.0, p. 71). It does not. Sovereign immune platforms that abuse their status, as was done in the Russian case, risk losing it, thereby opening the door to these forcible actions (von Heinegg).
Finally, one State may assist another’s law enforcement operations. Thus, to the extent a State is entitled to use force as a tool of law enforcement against intruding drones, other States can not only provide support but also employ that force on behalf of the former. It must be cautioned that such support is strictly limited to that which is consented to by the territorial State. For instance, if that State limits forcible measures against drones to those entering sensitive airspace, any engagement outside that area would violate the consenting State’s sovereignty.
Self-Defense: Article 51 of the UN Charter, which reflects customary international law, provides, “Nothing in the present charter shall impair the inherent right of individual or collective self-defense 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.” This right applies to both armed attacks that are underway and those that are “imminent.”
The threshold question in every self-defense analysis is whether the action to which the defensive use of force responds qualifies as an “armed attack.” There are two views regarding this threshold. The United States takes the position that all “uses of force” are equally “armed attacks” (DoD Law of War Manual, §1.11.5.2; Taft, p. 300). By this interpretation, the assessments above regarding the qualification of Russian intrusions as uses of force would determine whether the territorial States concerned enjoyed the right of self-defense against them.
I do not share this view. Instead, I concur with the International Court of Justice that an “armed attack” is the “most grave form” of “the use of force” (Paramilitary Activities, para. 191). So do most States. But despite the more demanding standard, it would have been reasonable for the affected States to treat the three incidents (and future intrusions by Russian combat aircraft and armed-capable drones) as armed attacks.
The penetration of Estonian airspace at a time of high tensions by multiple armed military aircraft that fail to depart upon demand justifies treating the Russian action as an armed attack that may be unfolding, especially given the previous Russian violations of NATO airspace by attack-capable platforms. Similarly, the extensive intrusions into Polish airspace by combat-capable drones, as well as decoy drones designed to confuse air defenses, merit the same characterization. The incident involving the single drone over Romania is the most challenging case to make. Still, as it involved a loitering munition that has been widely used to attack Ukrainian targets, it is likely that most States in similar circumstances would feel entitled to consider such an intrusion an armed attack triggering their right of self-defense. Whether there was any intent on the part of Russia to launch an attack or not, in circumstances in which Russian aggression against NATO States, especially the Baltic States, has become highly plausible, it is only fair that the State rattling the saber – Russia – bear the risk of miscalculation or misinterpretation. Consider, for example, the understandable nature of Turkey’s actions in that regard.
All exercises of self-defense by States facing an imminent or ongoing armed attack are subject to the conditions of “necessity” and “proportionality” (Paramilitary Activities, paras. 194, 237; Nuclear Weapons, para. 41; Oil Platforms, paras. 43, 73-74, 76; DoD Law of War Manual, § 1.11.5). Proportionality limits the degree of force used to that required to defeat an imminent or ongoing attack. Since downing an aircraft is usually the minimum viable degree of force that can effectively terminate its presence in national airspace, proportionality is not at issue here.
The condition of necessity, however, is highly relevant. Necessity requires the exhaustion of all feasible non-forcible means of responding to the armed attack before resorting to force. Whether non-forcible means have been exhausted is a case-specific determination. As NATO Secretary-General Mark Rutte has explained, “Decisions on whether to engage intruding aircraft, such as firing upon them, are, of course, taken in real time, are always based on available intelligence regarding the threat posed by the aircraft, including questions we have to answer like intent, armament and potential risk to Allied forces, civilians or infrastructure.”
A key factor in making necessity determinations is timing, for aerial intrusions by military aircraft afford little time to negotiate or take other non-forcible steps to address the situation. With an armed combat aircraft, the necessity condition will generally be satisfied once the intruding aircraft ignores a demand to immediately leave national airspace. In some cases, a demand to depart may not be required at all. For instance, if an aircraft is in an attack profile, engaging that aircraft with lethal force without prior demand to depart national airspace would be lawful. With drones, there may be no way to communicate at all.
In the case of the Estonian airspace incident, Rutte stated that “there was no immediate threat assessed. So, what happened is that Swedish, Finnish, and Italian aircraft were active to make sure that these three MiGs were moved out of Estonian airspace.” In other words, he suggested the necessity criterion was not satisfied. Perhaps not, but it is essential to emphasize that there is no requirement to wait until hostile action has begun to engage aircraft that are reasonably believed to be involved in an imminent or unfolding armed attack.
As is clear from the text of Article 51, a State that is entitled to employ defensive force in the face of an armed attack may seek the assistance of other States in collective self-defense. Accordingly, there can be no doubt that Baltic Sentry and other NATO defensive assets may engage Russian aircraft and drones in collective self-defense of NATO member States, as long as the intrusion in question qualifies as an armed attack, the necessity condition is satisfied, and the territorial State has consented to the engagement.
Enforcement of Neutrality: As noted, the law of neutrality prohibits belligerent military aircraft from entering neutral airspace. But the right of inviolability comes with obligations. One such obligation is to prevent overflight by belligerent military aircraft. It is well-accepted that should one or more aircraft enter its airspace, a neutral “[m]ust use all the means at its disposal to prevent or terminate the violation” (AMW Manual, Rule 170; see also 1923 Air Warfare Rules, art. 42; DoD Law of War Manual, §§ 15.3.2.2 and 15.4.3). As noted in the AMW Manual,
If possible, the intruding aircraft ought to be compelled to land. But, if it does not comply, the Neutral is entitled to shoot it down. Should the aircraft land, as required, the Neutral must intern the aircraft and its crew for the duration of the international armed conflict (p. 393).
This has long been the law, a fact illustrated by the numerous shootdowns of Allied and Axis aircraft by neutral Switzerland during World War II.
Accordingly, neutrality law provides a separate basis for the use of force by neutral States against Russian aircraft and drones, so long as they fail to leave the territory upon demand and the violation is serious. Moreover, the territorial State may request the assistance of other neutrals in responding to the breach of its neutrality and meeting the obligation to expel the belligerent aircraft, thereby justifying, for instance, the employment of NATO air policing/Baltic Sentry aircraft in support of the affected State. This is especially significant in the case of the Baltic States, which do not have combat aircraft.
Concluding Thoughts
The Russian penetrations of NATO airspace constitute unmistakable violations of the sovereignty of the States concerned and of the protection provided to them by the law of neutrality. They can also be characterized as prohibited uses of force. These are not marginal or technical infractions. On the contrary, the intrusions have been deliberate and frequent during a period of extreme geopolitical tension.
Such intrusions open the door to using force on multiple grounds against Russian military aircraft and drones that engage in such operations. States are obligated under the law of neutrality to terminate such aircraft’s unlawful presence in neutral airspace, including by force if necessary. Additionally, forcible actions may be taken against uncrewed systems that penetrate a State’s sovereign airspace, although the use of force against crewed aircraft is less clear-cut in law. In my estimation, the penetrations also qualify as armed attacks triggering the right to use force in self-defense and to seek assistance from other States in doing so, so long as the necessity criterion is satisfied in the attendant circumstances.
The implications of Russia’s unlawful actions are stark. These incidents comprise part of a broader strategy of probing, testing, and undermining Alliance cohesion. It is understandable that NATO leaders have concluded that failure by the Alliance to respond with clarity and forcefulness will only embolden Russia. Indeed, some have expressed the view that a firm response, one that demonstrates NATO’s readiness to intercept and engage intruding military aircraft, will serve as a visible deterrent to further Russian aggression in Europe. International law, in its wisdom, makes such options legally available.
