Earlier this month, Senator Tim Kaine made public a letter he wrote to Secretary of Defense Jim Mattis asking for clarification on the meaning of “collective self-defense of partner forces” as a legal basis for U.S. military action. In the letter, Kaine expresses concern that the executive branch is using this justification to use force against adversaries that are not covered by existing congressional authorizations for use of the military, de facto bypassing Congress. While Kaine appears to be mainly concerned with the authority to act under domestic law, he also specifically asks the Department of Defense for its “international legal rationale for use of U.S. military force for collective self-defense against combatants who pose no direct threat to the U.S., its […] Armed Forces or persons […].”

For international lawyers the phrase “collective self-defense” refers primarily to the well-established UN Charter right of States to defend other States. This right pertains to the jus ad bellum, that is, the law that governs when a State may use force against or in the territory of another State. In the U.S., however, the terminology of collective self-defense appears in the Standing Rules of Engagement (SROE) applied by U.S. forces and is defined as “[t]he act of defending designated non-U.S. citizens, forces, property, and interests from a hostile act or demonstrated hostile intent.” It may be thought of as an extension, not of self-defense of nation states, but self-defense of military units (a form of self-defense specific to military forces) and is sometimes characterized as “tactical self-defense.” In 2016 for instance, the Obama administration invoked the collective self-defense of African Union Mission in Somalia (AMISOM) to justify a strike against an Al-Shabaab camp in Somalia. A statement by U.S. Africa Command released last week invokes the same legal basis in relation to the latest U.S. strike against the group. The DoD statement quoted in Kaine’s letter, however, reflects an extended interpretation of this notion, covering not only the defense of state forces (like AMISOM or Somalian forces) but also non-state forces (“irregular partner forces or persons”). The Trump administration has indeed invoked the collective self-defense of Syrian Democratic Forces (SDF) in June 2017 to justify the downing of a Syrian regime jet, and again in February 2018 to justify strikes against Syrian pro-regime forces.

Is the collective self-defense of non-state partners a valid legal basis for the use of force under international law? The source for unit self-defense and its “collective” extension, as well as how the different versions of self-defense – national, collective, unit, individual – relate to each other is a matter of controversy. It is impossible to address here all aspects of this controversy. Recent scholarly works have contributed to this discussion and in fact reflect a wide array of approaches, for instance on the need to harmonize or not different types of self-defense (see for instance here, here, here, and here).

The main point in my analysis is that a State using force abroad must comply with two distinct legal regimes: first, the jus ad bellum and second, rules governing how much force may be used against specific persons or objects (more below). Invoking collective self-defense alone is not sufficient to establish legality under international law. Hence, the DoD must clarify whether its claims pertain to the first or second part of the legal test. If the first, claims of collective self-defense of non-statepartners are problematic because they have no basis under jus ad bellum. If the second, it is unclear whether the U.S. interpretation of collective self-defense conforms to the body of legal rules applicable to that part of the test. In order to make this determination, it is urgent that other States share their practice and legal views in this respect.

The international law governing the use of force: a two-layered framework

When a State uses force beyond its borders the lawfulness of its activities is a function of two cumulative conditions: first, compliance with the jus ad bellum and second, compliance with the jus in bello, otherwise called the law of armed conflict (LOAC), or other rules regulating the type and degree of force states may use against specific individuals or objects.

Under the jus ad bellum enshrined in the UN Charter a State may use force in the territory of another State only in three circumstances: with the territorial state’s consent, with UN Security Council authorization or in the exercise of its right to individual or collective self-defense in conformity with Art. 51 of the Charter. Thus, as pointed out above, the phrase “collective self-defense” does appear in the UN Charter. There, it refers to States’ right to use force in the defense of otherStates.

As far as the second part of the legal test is concerned, the applicable law is less straightforward. For many international lawyers, there are only two such applicable laws: LOAC or human rights law (HRL). But the practice of States complicates this dichotomy as they rely extensively on their soldiers’ right to self-defense – even, to the surprise of LOAC lawyers, when LOAC is applicable and would provide a more natural legal basis for action.

In classical scenarios where a State uses force against another State, the second part of the legal test would imply considerations of LOAC since any use of force against a State would immediately trigger an international armed conflict (IAC). As a result, the intervening State would have to comply with the jus ad bellum and the LOAC. When a State carries out strikes against a non-state actor as is most often the case nowadays, the applicability of LOAC is less obvious and immediate – confrontations against a non-state actor reach the level of a non-international armed conflict (NIAC) triggering LOAC applicability only when a certain threshold of intensity is crossed. When it is not the case – for instance, if U.S. forces engage an armed group for the first time or target viaa drone an individual who does not belong to a party to conflict – LOAC does not apply (yet) and therefore, the second legality condition must be evaluated under other international rules.

What these other international rules are is controversial. At least when ground forces are involved, it is uncontroversial (except for the United States!) that HRL applies. For instance, when U.S. or French forces in the Sahel use force for a reason unrelated to an armed conflict – such as to defend themselves from civilian looters – they are bound by HRL. HRL prohibits the arbitrary deprivation of life; and self-defense is obviously admitted as a valid, non-arbitrary justification for the taking of human life. From this perspective, the right to self-defense of State agents is a subset of, and constrained by, HRL. By contrast, the U.S. seems to consider that there exists a separate “law of self-defense” (proper to military forces?) with its own legal authority to use force, unconstrained by HRL. The burden of proof that such law exists at the international plane lies with the United States or any State making such claim. Indeed, as there is clearly no treaty on individual/unit self-defense, the source for such law can only be customary international law. This requires evidence of state practice and opinio juris. Therefore, it would be very important for other States to share their position in this respect.

In sum, irrespective of whether it forms part of HRL or is a distinct legal regime, self-defense at the individual level – and perhaps unit level – pertains to the second part of the legal test (and is distinct from State/national self-defense, a jus ad bellum notion). However, there are many uncertainties as to the contours of this right to self-defense. According to State practice and soft law instruments designed to guide the application of HRL, lethal force may be used in self-defense against an imminent threat to life. But there remain points of controversy. One of them is whether this right extends to the defense not only of one’s unit but also of others such as military partners (what the U.S. SROEs identify as “collective self-defense”). As pointed out in the San Remo ROE Manual not all States recognize such a right. In other words, States have not yet agreed on whether, under international law, their forces can defend their military partners when they are not themselves (or their unit) threatened and the adversary is not a legitimate target in an armed conflict (again, irrespective of jus ad bellum considerations). Other controversial aspects include whether individual and unit self-defense are available only against unlawful violence (as is the case of the criminal law defense of self-defense in many domestic systems) and whether they extend to the defense of property.

To conclude, under the general international law framework governing the use of force, a State using force must comply first, with the jus ad bellum – which includes an authority to use force in the collective self-defense of other states – and second, with other rules governing how much force may be used against which individuals and objects, the source and nature of which depend on the circumstances (LOAC or HRL). This second set of rules also provides for a right to self-defense (under HRL or separately) though not at State level but individual (and arguably unit) level; however it is controversial whether this right extends to the collective self-defense of military partners.

Applying this framework to U.S. claims of collective self-defense

In relation to this well-established two-part test, the United States adopts a controversial position: successive administrations seem to consider that self-defense alone is sufficient to comply with international law. This idea appears in the Obama administration’s “civilian casualties report” when it considered that an individual may be lawfully targeted by a drone because he is targetable under the LOAC “or” in the exercise of “U.S. national self-defense” (footnote a). U.S. national self-defense refers to the jus ad bellum right to self-defense. The document therefore suggests that a jus ad bellum authority would be sufficient for these strikes to comply with international law, without the need to consider the additional application of LOAC, HRL, or (if distinct from HRL) the law of individual/unit self-defense. Or, at a minimum, the report is silent on which law would apply in addition to the jus ad bellum, if LOAC does not apply.

Now, recent official statements indicate that the U.S. government also considers collective self-defense as another “umbrella justification.” When the DoD invokes collective self-defense in relation to the 2016 al-Shabaab camp attack, or the 2017and 2018 attacks on Syrian regime and pro-regime forces, it seems to suggest that this is enough to justify those actions at the international level. However, it is unclear whether it views this collective self-defense as originating from the jus ad bellum or not.

It is suggested that this position – considering national or collective self-defense, however they are defined, as a sufficient international legal basis for the use of force – would contradict the position of U.S. allies, including NATO ones. The United States must critically question its position and actively engage with them in this respect.


The use of force against Syrian regime and pro-regime forces must comply with this two-layered framework. Let us examine first whether the DoD claim of collective self-defense of the SDF could provide a valid legal basis under the jus ad bellum – the first part of the test.

It is well known that in 2014, at the start of the counter-ISIL campaign in Syria, the Obama Administration invoked both individual self-defense and the collective self-defense of Iraq in an Art. 51 letter to the UN Secretary General. This was the “classical” version of collective self-defense – in the defense of another State. However, this initial self-defense claim cannot cover the use of force against regime and pro-regime forces: the 2014 letter mentions only military actions against ISIL. Thus, when the U.S. targeted these new adversaries in June 2017 and in February 2018, it had to make a distinct self-defense claim. As pointed out above, the Art. 51 right to collective self-defense is only a right to defend other States, not non-State entities. This flows from the nature of the UN Charter which is a treaty amongst States. This reading of Art. 51 is uncontroversial in academic commentary. There is no prior instance where a State has openly invoked Art. 51 in defense of a non-state entity.

Another possibility would be to frame the defense of the SDF not as collective self-defense but, in fact, as individual (national) self-defense under Art. 51: the U.S. would argue that by targeting the SDF, Syrian regime and pro-regime forces are in fact targeting the U.S, thus triggering the U.S.’ individual right to self-defense. After all, States (in fact, the U.S.) have invoked their right to self-defense in response to attacks which occurred outside their territory and did not target their military personnel or assets. Think of the Oil Platforms case – which involved attacks on U.S. merchant vessels – and the 1998 bombings of the U.S. embassies in Kenya and Tanzania: in both cases the U.S. claimed to have been victim of an armed attack triggering its right to self-defense. However, in the Oil Platform case the U.S. claim was unsuccessful (the Court specifically found that an attack on a ship not flying the U.S. flag, even if owned by the U.S., could “not in itself […] be equated with an attack on that State” – p. 192). Needless to say, claiming that an attack on the SDF – whose personnel are not U.S. nationals (with exceptions) and assets are not currently owned by the U.S. nor identified as such – is an attack on the U.S. itself has little chance of success (space does not allow for a full treatment of this question, for instance whether the notion of control over the group would be relevant). In sum, these U.S. attacks against pro-Syrian forces lack a legal basis under jus ad bellum, and the DoD claim of collective self-defense fails to provide such a basis.

As to the second part of the legal test, the DoD claim of collective self-defense of the SDF would also be controversial and, in fact, unnecessary. As pointed out above, States have not yet agreed on whether their soldiers’ (individual and arguably unit) right to self-defense extends to the defense of others (either subject to HRL or originating from a distinct “law of self-defense”), providing a legal basis for the use of force under that second part of the legal test (note that if the U.S. claims that such right exists and derives from the jus ad bellum right to collective self-defense, it can be invoked only in the defense of State armed forces – see above). It is also possible that this right to individual, unit and collective (?) self-defense is available only against unlawful violence. If unlawfulness is to be assessed under the law of the territorial state, the U.S. cannot invoke self-defense to defend the SDF because under Syrian law it is perfectly lawful for the Syrian regime to use force against the SDF. If unlawfulness turns on an international law test, the result is the same: the use of force by Syria against the SDF is neither unlawful under the jus ad bellum nor unlawful under the jus in bello and the US cannot rightfully invoke the right to defend the SDF. In any case, it is submitted that this controversial claim of collective self-defense is unnecessary in this scenario: to the extent that the targets are Syrian State assets, an IAC with Syria was immediately triggered (or even existed prior to those incidents), together with the applicability of the LOAC. Under the LOAC, a military jet is a military objective by nature in the IAC opposing the United States and Syria, and hence a legitimate target. Therefore, the second part of the legal test is fulfilled without the need to invoke collective self-defense.

However, because of the lack of a jus ad bellum justification, the 2017 and 2018 attacks against regime and pro-regime forces remain unlawful under international law.

As is clear from this essay, the DoD claims of collective self-defense raise a number of international law questions, which must urgently be settled. First and foremost, the U.S must clarify what is the source of this right of collective self-defense — jus ad bellum or another international law regime and if so which one? [As an aside, if SROE collective self-defense refers to a different concept than jus ad bellum self-defense, a change of term should seriously be considered as this clearly contributes to the conceptual confusion.] Clearly, this is not a discussion the U.S. can have alone. U.S. allies operating alongside U.S. forces, States hosting U.S. forces, and other States must share their legal views on this crucial topic. Last April, at the Annual ASIL conference, Brazil expressed its opposition to the “unwilling or unable” test for the use of force against non-state actors. Other states must take advantage of the participatory nature of the international legislative system and share their interpretation of treaty provisions as well as their opinio juris concerning possible customary rules in this important area of state practice. Failing to do so, states that are militarily active continue to take life-and-death decisions on the basis of unchallenged legal views.

Photo of April 14, 2018 Tomahawk missile launched at Syrian targets from the U.S. 5th Fleet by Matthew Daniels/U.S. Navy via Getty Images.