Lieber Institute White Paper: Responding to Malicious or Hostile Actions under International Law

by | Apr 26, 2022

Response under International Law

Recent consultations with senior legal advisers have highlighted the need for a clear map of response options available to States facing hostile or malicious actions, whether attributable to another State or a non-State actor. After all, to respond effectively to such actions, States, including those operating collectively, need to grasp the full range of responses allowed by international law, including the legal limits on their use. The resulting flow chart and my summary of the options are reproduced here for Articles of War readers. They are my views on the subject and do not necessarily represent those of the United States Military Academy or any other U.S. government entity.


International law allows for a wide array of lawful responses to hostile or malicious actions. They range from arbitration and judicial settlement to action authorized or mandated by the Security Council under Chapter VII of the Security Council, an option that may even include uses of force. But in general international law, four response options loom large—retorsion, countermeasures, necessity, and self-defense. In this post, I will outline their key requirements and limitations. The flow chart below, which Wolff Heintschel von Heinegg, Liis Vihul, and I developed for Cyber Law International’s capacity-building courses, sets forth an approach to thinking through these responses. I have slightly modified it for use in the non-cyber context. The original and an armed conflict companion chart, together with a comprehensive explanation of both, can be found here.

Peacetime chart Revised


 Acts of retorsion are unfriendly but lawful reactions by one State to another. Examples (in the absence of a treaty prohibition to the contrary) include, inter alia, diplomatic protests, economic or trade sanctions, closing ports to ships flagged in the target State, declaring diplomats persona non grata, closing diplomatic facilities, recalling diplomats, calling off State visits, reducing or canceling development or military aid, withdrawing from international organizations, imposing limits on entry into the State’s territory by nationals of the target State, and withdrawing from or terminating a treaty according to its terms.

The sole requirement for qualification as an act of retorsion is that it not breach any international law obligations, such as those found in international human rights law or a treaty bearing on the matter. Because they are by definition lawful, acts of retorsion are always available in response to another State’s action. Indeed, they are even, but not only, available when the State against which they are directed has not violated international law. Further, there is no requirement for notice (unless imposed by a treaty bearing on the matter) before engaging in retorsion, nor is there any evidentiary standard with respect to the action to which the retorsion responds. Finally, acts of retorsion are not subject to any requirement of proportionality, and there is no requirement that an act of retorsion be likely to cause the target State to desist.

As with any response, acts of retorsion are subject to the obligation of peaceful settlement of disputes discussed below. In most cases, though, that obligation will not affect the decision to resort to retorsion because acts of retorsion are generally unlikely to endanger “international peace and security.”


Countermeasures are responses that would be unlawful under international law, but for the fact that they respond to another State’s unlawful action and are designed to put an end to it and/or secure reparations for harm suffered. To take a simple example, if a State’s warships in another State’s territorial waters engage in espionage against the coastal nation, those ships are violating the law of the sea’s “innocent passage” regime that permits transit through territorial waters so long as vessels do nothing adverse to the coastal State. In response, the coastal State would be entitled to, for instance, take the countermeasure of closing its territorial sea to all ships flagged in the first State until the warships desist.

The right to take countermeasures is found in customary international law. With regard to identifying the rules, the International Law Commission’s Articles on State Responsibility are considered a generally reliable restatement of the customary rules governing countermeasures (ASR, arts. 22, 49-54). Like necessity and self-defense (see below), countermeasures are “circumstances precluding wrongfulness” under international law (ASR, ch. V).

Because they allow an “injured State” (the State against which the unlawful activity was originally directed) to engage in otherwise unlawful actions against the “responsible State” (the State that engaged in the unlawful conduct), countermeasures are subject to strict limitations. Key among these are the following.

    • States may only take countermeasures in response to an “internationally wrongful act,” which requires attribution (factual and legal) to a State and breach of an obligation that State owes the injured State. Acts by non-State individuals or groups do not open the door to countermeasures unless they are legally attributable to a State (but see the possibility of action against non-State actors based on the territorial State’s failure to comply with its due diligence obligation). The most likely basis for legal attribution is that the non-State actor is acting pursuant to the “instructions or direction or control” of a State (ASR, art. 8).
    • Only States are entitled to take countermeasures; private entities may act on behalf of a State in executing countermeasures, but the State will be responsible for the conduct under the law of State responsibility.
    • An injured State may not take countermeasures for any purpose other than causing the responsible State’s unlawful activity to cease or securing reparations that are due. Retribution, retaliation, or punishment are not legitimate purposes of countermeasures (or of any other response option). Because countermeasures are designed to terminate the responsible State’s unlawful conduct and secure reparations, they may not be escalatory or unlikely to succeed.
    • Countermeasures are not available until the wrongful conduct by the responsible State has commenced. They must be terminated once the wrongful conduct ceases. The sole exception is, as noted, taking or continuing countermeasures to secure reparations still due to the injured State.
    • An injured State should notify the responsible State of its intention to take countermeasures and afford the latter an opportunity to cease its unlawful conduct and/or provide reparations due. However, this requirement is excused when the need for countermeasures is “urgent,” a position taken by most States that have spoken to the issue in the cyber context (e.g., France, Netherlands, Norway, United Kingdom, United States).
    • Countermeasures cannot involve the “use of force” (U.N. Charter, 2(4)). A forceful response is only permissible in self-defense against an armed attack (see below) or pursuant to Security Council authorization or mandate under Chapter VII of the UN Charter. Countermeasures should not be confused with belligerent reprisals, an extremely limited response option during international armed conflicts.
    • Countermeasures must be “commensurate with the injury suffered” and consider the gravity of the wrongful act and the right concerned (ASR, art. 51). In other words, the injured State’s countermeasure must be proportionate to the scale and nature of the original unlawful action by the responsible State. This differs from proportionality in the law of self-defense, where proportionality is assessed by reference to what is required to end the armed attack to which it responds (see below).
    • International law is unsettled whether “collective countermeasures” are permissible. They involve one State assisting another’s countermeasures or acting on its behalf (see analysis here).

Despite these limitations, several aspects of countermeasures facilitate their use. Importantly, countermeasures need not be in kind, either in terms of the international law rule involved or the nature of the countermeasure. For example, an injured State may respond to a breached treaty obligation with a countermeasure that would otherwise violate a customary international law rule and vice versa. Consider the non-innocent passage example above. The injured coastal State could respond by, for instance, denying landing rights provided for in a bilateral agreement to aircraft registered in the responsible State. Or it could conduct cyber operations against private entities in the responsible State that would otherwise violate the responsible State’s sovereignty. This is an important aspect of countermeasures, for injured States may lack the capacity to respond in kind.

As these examples illustrate, the injured State does not have to direct a countermeasure at the entity that engaged in the initial unlawful conduct. For instance, assume the responsible State’s intelligence agency is involved in unlawful cyber operations, such as causing damage to the injured State’s government systems. The injured State may direct countermeasures at the intelligence agency’s cyber infrastructure, that of uninvolved government agencies, or even private-sector systems.

This example illustrates a crucial aspect of the response. Countermeasures may terminate the unlawful conduct to which they respond either directly or indirectly. In the previous example, operations disabling the intelligence service’s systems would directly end the responsible State’s unlawful conduct. However, because those systems would likely be secure, the injured State might opt to pressure the responsible State by targeting other systems, thereby indirectly forcing it to desist.


Like countermeasures, the “plea of necessity” is a circumstance precluding wrongfulness (ASR, art. 25). Accordingly, it allows a State to engage in what would otherwise be unlawful conduct. Necessity is available as the basis for an otherwise unlawful response only in extreme circumstances. Yet, the plea of necessity has several advantages that may make it available in cases where countermeasures are not.

There are four cumulative conditions precedent to engaging in an otherwise unlawful response pursuant to the plea.

    • The plea of necessity is only available in situations that present a “grave and imminent peril” to the State. International law does not define the term grave, but the Tallinn Manual 2.0 experts agreed that a “peril is grave when the threat is especially severe. It involves interfering with an interest in a fundamental way, like destroying the interest or rendering it largely dysfunctional” (Rule 26 commentary).
    • Grave peril that is imminent or ongoing must be posed to an “essential interest” of the State. Like “grave peril,” the term essential interest lacks a definitive definition in international law. Still, the Tallinn Manual 2.0 experts “agreed that an essential interest is one that is of fundamental and great importance to the State concerned. The determination of whether an interest is essential is always contextual. Essentiality of a particular interest is also, to an extent, likely to vary from State to State.” The term surely would include the health of the population; the economic and financial systems; the national food, power, and water supplies; and national security. Services provided by most critical infrastructure, such as the power grid, would qualify as essential, although designation as critical is not required to qualify as essential.
    • The grave peril the State’s essential interest is facing must be “imminent.” This includes situations in which the harm is occurring or impending. In other words, necessity allows for anticipatory responses (see also the discussion below on imminency in self-defense).
    • A fourth requirement is that the measure being considered to end the grave and imminent peril to an essential interest must be the only way to safeguard that interest. From a practical perspective, this does not mean that the action being considered has to be the only conceivable way to deal with the situation. Instead, it must be the only reasonably feasible means of addressing it. There is no requirement to attempt speculative or unlikely measures.

In addition to the four conditions, acting based on necessity is prohibited if the action would affect the essential interest of another State. Additionally, a State may not look to the plea of necessity when it has contributed to the situation, as with Russia’s unlawful aggression against Ukraine motivating cyber attacks against Russian targets.

Despite these limitations, the plea of necessity offers two advantages over countermeasures. First, there is no requirement that the peril be caused by the breach of any international law rule. For example, there is uncertainty over the threshold at which a cyber operation violates a State’s sovereignty, amounts to intervention in a State’s internal affairs, or rises to the level of a use of force. Because necessity need not respond to an unlawful act, such uncertainty would not preclude taking measures based on the plea of necessity.

Second, and relatedly, countermeasures are only available in response to actions attributable to other States. This condition is sometimes an obstacle to a response either because of the difficulty of factual attribution or because the requisite nexus between a non-State actor’s action and a State is insufficient to legally attribute it to a State. However, the plea of necessity is available in the face of action by non-State actors that cannot reliably be attributed to a State and when the action’s originator is unknown. In both cases, the State may respond even though its response may violate an international law obligation owed another State.

To illustrate, consider hostile cyber operations into a State that is disrupting critical infrastructure. The State’s cyber force has the technical capability to conduct disabling operations against the attacker but is uncertain as to its identity or location. Nevertheless, the plea of necessity would likely justify the response.


The fourth major response option available to States facing hostile actions is acting pursuant to the right of self-defense, which is a circumstance precluding wrongfulness (ASR, art. 21). The United Nations Charter provides for the right of self-defense in Article 51, which reflects, in significant part, customary international law. As a response operation, it is unique, for it allows the State to use force, which is generally prohibited pursuant to Article 2(4) of the UN Charter and customary law. The only other clear legal basis for a response at the use of force level is Security Council authorization or mandate under Chapter VII of the Charter. Article 51 provides that self-defense may be exercised collectively and requires uses of force in self-defense to be reported to the Security Council.

There are four conditions precedent to responses based on self-defense.

    • First, according to Article 51, self-defense is only available in the face of an “armed attack.” Although a significant military attack qualifies, there is relative uncertainty as to when else the right of self-defense is triggered. This is especially so with cyber operations that can severely disrupt another State without causing physical damage or injury, as in the case of a significant cyber operation against the target State’s economy. The law is unsettled on this matter, although some States have taken the position that, depending on the “scale and effects” of the consequences, non-destructive and non-injurious cyber operations can qualify as an armed attack (e.g., France and Singapore). France is of the view that a very severe economic cyber attack could amount to an armed attack.

The United States has adopted a relatively liberal approach to the meaning of “armed attack” by equating it to the “use of force” (see, e.g., here). Most other States, including NATO allies, that have taken a position on the matter, characterize armed attacks as, in the words of the International Court of Justice in the Paramilitary Activities case, the “most grave forms” of the use of force. The practical effect of this disagreement is that the United States would label an action as an armed attack meriting a response in self-defense at a lower level than that of most other States. As NATO (Brussels Summit) and individual States (e.g., France and Singapore) have observed, multiple related actions by the same actor may be aggregated to reach the armed attack level.

    • A forcible response in self-defense must be “necessary.” Necessity in the self-defense context means that the State facing the armed attack must use measures at the use of force level of severity to defeat or otherwise terminate the armed attack. If non-forcible measures, such as countermeasures, would suffice, the victim State may not use force in self-defense.
    • A third requirement is temporal; it also derives from the principle of necessity. The armed attack to which the defensive use of force responds must be underway or imminent. International law is unsettled as to whether imminence denotes harm that is about to happen or also refers to situations in which the “last window of opportunity” to counter the armed attack effectively is about to close (see, e.g., the S. and Australian positions). Further, because self-defense is only available in the face of an imminent or ongoing armed attack, a State that has been the victim of such an attack may not respond in self-defense once the attack is over. However, if the operation in question is but one in a series of attacks, the right of self-defense remains intact until that campaign is over.
    • The final self-defense requirement is proportionality. As understood in the law of self-defense, proportionality limits a State acting in self-defense to those actions that are required to head off the imminent armed attack or defeat an ongoing one. This may limit the defensive response to less severe action than the State is experiencing, but which is nevertheless likely to defeat the armed attack. But, applying the same logic, a defensive response can satisfy the proportionality criterion even though its severity exceeds that of the armed attack, so long as a response at that level is needed to defeat the armed attack.

There are several ongoing debates regarding the exercise of the right of self-defense. Two stand out. The first involves whether an operation at the armed attack level of severity by a non-State actor qualifies as an armed attack for the purposes of the law of self-defense. In its Paramilitary Activities judgment, the International Court of Justice held that only attacks by non-State actors on behalf of a State or with its substantial involvement triggered the right of self-defense; the Court appears to have maintained this position in other cases (Armed Activities judgement and Wall advisory opinion). However, numerous States have adopted the position that self-defense is available in response to non-State actor attacks lacking a relationship to a State. For instance, this is the view, for instance, of the United States, United Kingdom, Germany, and the Netherlands (see here).

The second debate concerns where defensive actions may occur. Undeniably, they are permissible in the territory of a State engaged in an armed attack, the victim State’s territory, and the commons (international waters and airspace, and outer space). The disagreement is about whether a State may take forcible defensive measures into the territory of a State that is not responsible for the armed attack. Some States argue that doing so is impermissible and would violate, at a minimum, the sovereignty of the State into which the operation is conducted. The United States and numerous other States take the position doing so is allowed when the territorial State is “unwilling or unable” to put an end to the hostile operations emanating from its territory (or under other exceptional circumstances).

Peaceful Settlement of Disputes

It must be cautioned that any response must be consistent with international law obligations to settle disputes peacefully. There are two. First, according to Article 33(1) of the UN Charter, the States involved in a dispute must “first of all, seek a solution by negotiation, inquiry, mediation, conciliation, arbitration, judicial settlement, resort to regional agencies or arrangements, or other peaceful means of their own choice” if it is likely to “endanger the maintenance of international peace and security.” In other words, States involved in a dispute that risks breaking out into an international armed conflict or that might involve the use of force have an affirmative duty to try settling the matter peacefully. Only once peaceful options fail, or are sure to fail if attempted, does the possibility of using force arise under the right of self-defense or Security Council action.

Second, while there is no obligation to try to settle a dispute unless it risks hostilities or other uses of force, should an attempt be made to do so, the attempt must be carried out “by peaceful means in such a manner that international peace and security, and justice, are not endangered” (UN Charter, art. 2(3)). The concept of “peaceful means” excludes non-forceful responses that are likely to escalate to a use of force.


Hostile and malicious actions against or into States are all too common. Fortunately, as illustrated in this white paper, international law usually leaves the door open to a practical and effective response. However, because some response options involve conduct that would otherwise be unlawful, it imposes strict and nuanced limitations on when and how they may be conducted. States considering a response to hostile or malicious actions can be confident that a response option is available but should navigate through them carefully.


Michael N. Schmitt is the G. Norman Lieber Distinguished Scholar at the United States Military Academy at West Point. He is also Professor of Public International Law at the University of Reading and Professor Emeritus and Charles H. Stockton Distinguished Scholar-in-Residence at the United States Naval War College.



Photo credit: U.S. Air Force Tech. Sgt. Alexandre Monte