Over-the-Horizon Operations – Part I: When May Force Be Used?

by | Dec 16, 2021

Over-the-Horizon Strikes I

Responding in early July to concerns about the withdrawal of troops from Afghanistan after two decades of war, President Biden announced, “We are developing a counterterrorism over-the-horizon capability that will allow us to keep our eyes firmly fixed on any direct threats to the United States in the region, and act quickly and decisively if needed.” Major Steve Szymanski and Captain Mike Marchman have authored an excellent Articles of War piece summarizing possible legal justifications for launching such strikes against al Qaeda and ISIS-K in Afghanistan. In this three-part series, I build upon their fine work to delve more deeply into the legal criteria for conducting such operations. The series benefits from a workshop on Afghanistan and international law hosted by the Naval War College’s Stockton Center for International Law this month.

An over-the-horizon (OTH) operation is one launched from outside the State into which it is conducted. This may involve: 1) a kinetic strike from outside the territorial State, as in a cruise missile attack; 2) an operation involving platforms that penetrate national airspace or territorial waters, like a remotely piloted aircraft (RPA) attack; or 3) the insertion of forces into the State, as in a capture-or-kill mission against a high-value target. The analysis that follows does not extend to remotely conducted cyber operations, as they raise numerous distinct legal issues.

Three fundamental questions must be answered about OTH operations. The first is whether the attacking State has the right to use force in the first place. If it does, the question of where that right may be exercised arises. Finally, there is the matter of how the attacker must conduct the operation, for, depending on the circumstances, different bodies of law can govern OTH operations. Post-withdrawal Afghanistan will serve as the analytical crucible, but the discussion applies mutatis mutandis to any OTH operation.

 In this first installment in the series, I examine the right of States to use force. Article 2(4) of the UN Charter and customary international law prohibit the use of force. Despite that prohibition, there are numerous justifications for engaging in forcible operations. For the OTH operations under consideration, three are central: Chapter VII authorization or mandate, self-defense, and a request for assistance. The United States set forth its position on these bases for the use of force in a 2016 White House Report.

Chapter VII Authorization

The clearest legal basis for engaging in a use of force is pursuant to an authorization or mandate adopted by the Security Council under Chapter VII of the UN Charter. “Authorization” denotes a situation in which no particular entity or state has been designated to act. “Mandate” refers to one in which the Security Council names a specific entity to conduct operations. The authorization or mandate comes in the form of a resolution for which nine of the fifteen Council members have voted, and that no permanent member has vetoed (P5 – China, France, Russia, United Kingdom, United States).

To permit the use of force on either basis, the Security Council must, under Article 39 of the Charter, “determine the existence of any threat to the peace, breach of the peace, or act of aggression.” Then, by Article 41, the Council may decide on non-forcible measures to address the situation. If those measures prove unsuccessful, or should the Council determine they are unlikely to work, it can authorize the use of force under Article 42 to accomplish the objective outlined in the resolution. In the broadest sense, that purpose must “be necessary to maintain or restore international peace and security.” It is a purpose that encompasses, for instance, transnational counterterrorism operations like those envisioned by the Biden administration. The resolution will typically use the phrase “all necessary measures” to indicate that it is authorizing or mandating the use of force.

If the Security Council adopted a resolution permitting forcible action against the Taliban or groups such as al Qaeda or ISIS, the United States could lawfully launch OTH operations, so long as 1) it qualified as an entity permitted to act under the terms of the resolution and 2) OTH operations contributed to achieving the resolution’s objective. It is somewhat unlikely that the Security Council will adopt such a resolution given Russia’s and China’s veto power. Still, it might not be unthinkable if the terrorist group against which action is to be taken poses a threat to those States or Afghanistan.


A more likely basis for using force in Afghanistan lies in the right of self-defense found in Article 51 of the UN Charter and customary international law. Article 51 provides, “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.” The key to maturation of the right is the existence of an “armed attack.”

Unfortunately, the precise threshold at which an attack qualifies as an “armed attack” is unsettled. The prevailing view, reflected in the International Court of Justice’s (ICJ) Paramilitary Activities judgment, is that an armed attack is the “most grave form” of a use of force (para. 191). By this view, there may be a situation in which a State is the target of unlawful force but does not have a right to self-defense because the use of force does not fall into the “most grave” category. For instance, in its 2003 Oil Platforms judgment, the ICJ questioned whether the mining of a single warship would amount to an armed attack, taking no position on the matter (para. 72).

The United States has long rejected the premise of a gap between uses of force and armed attacks. In its view, all uses of force are armed attacks that open the door to forcible defensive responses (see Soafer, 1988 ASIL Proceedings, p. 422). The issue is relevant in the counterterrorism context because a terrorist attack that is a use of force might not be considered by some States to qualify as an armed attack, as in the case of an attack on a single individual or one that causes minor damage. Thus, in theory, the United States would face criticism for responding in self-defense to a terrorist action that other States do not believe is of the requisite severity. But, presumably, the OTH operations contemplated by the Biden administration would be limited to those that plainly respond to armed attacks against the United States, including U.S. nationals and their property abroad.

Should an ongoing use of force not rise to the level of an armed attack, the victim State’s remedies would be limited primarily to: 1) taking countermeasures (Articles on State Responsibility, art. 22); acting pursuant to the plea of necessity under the law of State responsibility (ASR, art. 25); 3) resort to the Security Council in search of a Chapter VII authorization or mandate; or 4) acts of retorsion, which are always available.

Self-evidently, the right of self-defense includes response to armed attacks mounted by another State’s armed forces. As noted in the Paramilitary Activities judgment, a State will also be deemed to have engaged in an armed attack should a non-State group use force at the armed attack level “by or on behalf” of that State (para. 195). “Substantial involvement” in a non-State group’s operation having the requisite degree of severity likewise qualifies as an armed attack by the State concerned.

This raises the question of whether an attack involving the Taliban, either directly or through involvement in a non-State group’s operation, would provide the legal basis for an OTH response in self-defense on the grounds that the operation qualifies as that of a State. To date, the Taliban has not been recognized by any State as the legitimate government of Afghanistan. That said, it is increasingly the policy of States, including the United Kingdom and United States, to refrain from formally recognizing governments (reflecting the Estrada Doctrine). More to the point, analysis of the simultaneous U.S. operations against al Qaeda and the Taliban in October 2001 did not focus on the recognition issue. Instead, the question was whether the requisite nexus between al Qaeda’s attacks and the Taliban existed, for it did not appear to critics as if they were conducted by or on behalf of the Taliban, or with its substantial involvement.

Nevertheless, during the Stockton Center workshop discussions, the Taliban’s status loomed large. Before turning to recognition of the Taliban, note that recognition of States must be distinguished from recognition of governments. The former refers to acknowledgment that the territory concerned has the status of a State in international law (and enjoys associated rights and obligations); the latter denotes acceptance that a group is governing a territory. Recognition of a government may be de jure or de facto. De jure recognition opens the door, for instance, to diplomatic relations and representation in international organizations. For our purposes, though, it is de facto recognition that matters.

The commentary to the International Law Commission’s Articles on State Responsibility (ASR) notes, “A general de facto Government … is itself an apparatus of the State, replacing that which existed previously. The conduct of the organs of such a Government is covered by article 4” (p. 49). Article 4 addresses attribution to a State of conduct by its “organs,” such as the armed forces. In practice, the comment means that a de facto government’s actions are considered those of the State, in this case, Afghanistan.

This has long been the case. For example, in 1923, Supreme Court Chief Justice and former President William Taft served as arbitrator in the Aguilar-Amory and Royal Bank of Canada arbitration between Great Britain and Costa Rica, better known as the Tinoco Concessions Arbitration. The case, which the ASR’s commentary cites, involved Costa Rica’s cancellation of contracts with British nationals that had been executed by the government of General José Tinoco Granados, who seized and held power from 1917-1919. Numerous countries, including the United States, repeatedly objected to the Tinoco government. Indeed, it was not allowed to sign the Treaty of Versailles on behalf of Costa Rico although that State had declared war on Germany.

Taft held that the key to resolving the dispute was de facto control by the Tinoco government.

The question is, has it really established itself in such a way that all within its influence recognize its control, and that there is no opposing force assuming to be a government in its place? Is it discharging its functions as a government usually does, respected within its own jurisdiction? (p. 382).

This is precisely the situation in Afghanistan today. Indeed, Secretary of State Blinken labeled the Taliban as Afghanistan’s “de facto government” in Congressional testimony on September 13. Accordingly, the best reading of the law is that the United States would have a right to respond in self-defense against either a Taliban attack or one by a terrorist group launched by or on behalf of the Taliban, or with its substantial involvement, on the basis of having been attacked by another State.

However, if the argument that the Taliban should be treated as a de facto government acting for Afghanistan is rejected, or if a non-State group conducts operations against the United States without the requisite Paramilitary Activities relationship to the Taliban, the question of whether the right of self-defense extends to operations at the armed attack level by non-State actors arises. The issue remains unsettled.

In the immediate aftermath of the 9/11 attacks, the international community treated them as triggering the right of self-defense, even though no state pointed the finger directly at Afghanistan. On the contrary, suspicion followed by confirmation quickly settled on a non-State group, al Qaeda. The UN Security Council adopted Resolutions 1368 and 1373 in the weeks following the attacks. Both confirmed the right to self-defense applied. Lest their actions be characterized as emotive reactions to the horrific attacks, the Council repeatedly reaffirmed them in the following months. Moreover, nearly 50 states offered assistance to the United States in collective self-defense.

It appeared clear that the international community now interpreted self-defense as extending to actions by non-state actors even when the Paramilitary Activities criteria have not been satisfied. Many scholars, including myself, and numerous States, such as the United States, embrace this position today. For those in this camp, it is the only reasonable interpretation considering the contemporary context in which self-defense is to be applied, including the prospect of non-State groups mounting catastrophic attacks without warning.

However, in its 2004 Wall advisory opinion (para. 139) and 2005 Armed Activities judgment (para. 146), the ICJ may have stepped backward by looking to the Paramilitary Activities criteria as the proper approach to determining whether non-State operations can amount to an armed attack triggering the right of self-defense. While judgments of the ICJ bind only the Parties before the court, they are nevertheless highly influential. I would note that the decisions were not unanimous. In particular, Judges Higgins (U.K.), Kooijmans (Netherlands), and Buergenthal (U.S.) disagreed, convincingly in my estimation, with limiting the right to the circumstances the Court had outlined in Paramilitary Activities.

Should the United States enjoy a right to act in self-defense against an operation at the armed attack level conducted by a non-State group, it would have to comply with the criteria of necessity and proportionality, as those criteria are understood in the law governing the use of force (rather than international humanitarian law and international human rights law) (see, e.g., Paramilitary Activities, para. 176; Oil Platforms, para. 73; Armed Activities, para. 147).

The condition of necessity requires that the State acting defensively has no viable, non-forcible option to address the situation effectively. Necessity also has a temporal component in that a defensive use of force may neither occur too early (imminency) nor too late (immediacy).

“Imminency” allows a State to act in anticipatory self-defense before an armed attack occurs. In the past, imminency reflected the Caroline criteria, by which a State could act when the need for self-defense was “instant, overwhelming, and leaving no choice of means, and no moment for deliberation.” But given that catastrophic armed attacks may occur without warning, an emerging, and contextually sounder, interpretation of anticipatory self-defense is that the defending State may act when its opponent has both the capability and will to attack and failure to act defensively could mean the State has forfeited its opportunity to defend itself effectively. This approach, which numerous other scholars and I (2003, pages 534-535) suggested in the aftermath of the 9/11 attacks, has now been adopted by the United States as the “last window of opportunity” test (2011 DoJ White Paper; 2012 Attorney General Address). By this test, the United States could conduct OTH operations into Afghanistan before being attacked in certain circumstances.

A timing requirement labeled “immediacy” also applies following an armed attack. The right to use force in self-defense extends throughout the period during which there is a reasonable expectation of follow-on attacks. However, if further attacks are unlikely, the right to act defensively is extinguished. In such circumstances, any OTH operation into Afghanistan would constitute unlawful retaliation, at least absent new grounds for acting.

Request for Assistance

Finally, a State may use force at another State’s request when the latter enjoys the right to use force. This is most likely the case when the requesting State uses force against terrorists or insurgents. Any limits on the requesting State’s forcible actions equally limit the assisting State. For instance, if the operations qualify as a non-international armed conflict, the IHL applicable in such conflicts will govern the operations of both parties. But if they do not, both States will be bound by their own domestic and international human rights law obligations, but not IHL. Obligations binding the consenting State will also constrain the assisting State. This is because a State cannot request, or consent to, activities it could not legally conduct.

Therefore, the United States could lawfully conduct OTH operations into Afghanistan against such groups as ISIS-K at the request of the Taliban when the Taliban would be entitled to do so itself. Requesting States are permitted to set limits on the scope of consent granted (ASR, art. 20). This authority derives from the requesting state’s enjoyment of sovereignty over its territory. For instance, the Taliban could place additional limits on U.S. OTH operations, such as geographical constraints.

I must caution that if one takes the position that the Taliban’s de facto control of Afghanistan does not afford it the authority to represent the State of Afghanistan, the group would equally lack the authority to request a U.S. OTH operation into Afghanistan. As noted, however, I contend the Taliban enjoys that authority as a matter of international law.


 In this first installment of this series, I have examined when the United States enjoys a right to use force in the form of OTH operations. But that is only the first step in assessing the lawfulness of any such operation into Afghanistan or other States. In Part II, I turn to the separate legal issue of where that right may be exercised, specifically whether OTH operations may be conducted into Afghanistan. The final installment will examine the law that would govern how the OTH operations must be carried out.


Michael N. Schmitt is the G. Norman Lieber Distinguished Scholar at the United States Military Academy. He is also Professor of Public International Law at the University of Reading, Strauss Center Distinguished Scholar and Visiting Professor of Law at the University of Texas, and Professor Emeritus at the United States Naval War College.