Editor’s Note: This is the second post in a two-part series discussing the legal justifications various countries have put forth related to airstrikes in Syria. You can find the first part here.

With nearly a dozen States carrying out airstrikes in Syria against ISIL, my previous post set out the various legal reasons many of those States have used to justify their attacks. In this post, I’ll set out the general rules of international law that regulate when a State can engage in such airstrikes and will reflect on some (but certainly not all) of the emerging legal patterns, trends, and challenges.

To recap: Canada, Turkey, the UK (here and here), and the US invoked the doctrines of both individual and collective self-defense under Article 51 of the UN Charter, which enshrines States’ rights to engage in either type of self-defense “if an armed attack occurs.” Australia was the only country to have invoked only the doctrine of collective self-defense. France’s letter to the UN was ambiguous, though it highlighted both general threats that ISIL poses and noted ISIL’s specific threat to France (a claim that has been made all the stronger in the wake of the Paris attacks). Russia stands alone in justifying its airstrikes on the grounds that Syria has granted permission for them.

Additionally, Australia, Canada, Turkey, and the US referenced the controversial “unwilling or unable” test in their letters to the UN. The UK and France did not, but the UK Prime Minister has made passing reference to the test elsewhere in relation to Syria. Also worth noting is that Australia, Canada, Turkey, the UK, and the US all said their actions in Syria would be “necessary and proportionate.”

What Does International Law Say About These Justifications?

As a general rule, States are prohibited from carrying out armed attacks on the territory of other States. This rule is found in Article 2(4) of the Charter of the United Nations, which requires states to refrain 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.” When this prohibition is not followed, the offending State has breached a fundamental rule of international law, one that serves to maintain a baseline of equality amongst States and that was intended to reduce inter-State conflict.

All of the States mentioned in the first post (the United States, Canada, Turkey, the United Kingdom, Australia, France, and Russia) engaged Article 2(4)’s general prohibition by using force in Syria. The question then becomes whether the strikes were covered by one of the exceptional circumstances in which international law permits States to use such force abroad.

Those exceptions to Article 2(4) are:

  1. Consent of the host country
  2. Individual and collective self-defense
  3. UN Security Council authorization

1. Consent

Russia claims that its strikes in Syria clearly fall under this first exception to Article 2(4). This is because a State, such as Syria, may allow another State to carry out attacks on its territory. Consent, in this context, effectively washes away any claim Article 2(4) has been violated. Russia’s invocation of this exception is similar to the United States claiming that its drone strikes in Somalia, Yemen, and Pakistan are justified because those governments granted the United States the right to do so. These examples, however, expose complications that can arise when it is unclear whether consent was indeed given, who gave consent to whom, and whether the consent was coerced. To this end, some have questioned Syrian President Bashar al-Assad’s legitimacy as the representative of Syria and challenge the claim that he has authority to grant consent to Russian airstrikes.

2. Individual and Collective Self-Defense

All of the States conducting airstrikes in Syria aside from Russia claim their strikes are covered by the doctrines of individual and/or collective self-defense. Article 51 of the UN Charter recognizes that a State has an inherent right to individual self-defense when it has been subjected to an armed attack. The Charter also recognizes an inherent right to collective self-defense, whereby one State can ask another State for its assistance to act in self-defense against an armed attack, which is exactly what Iraq did in September 2014. But, as discussed below, that isn’t the end of the story.

3. UN Security Council Resolution

The UN Security Council is permitted to authorize the use of force against a State when the Council makes a determination of the existence of a “threat to the peace, breach of the peace, or act of aggression.” Under this power, the Security Council has on several occasions authorized States and peacekeeping missions to respond to armed attacks with mandates that permit the use force.

To date, the Security Council has not granted explicit authorization for armed attacks against ISIL in Syria. But, in response to ISIL attacks in Sousse, Ankara, the Sinai Peninsula, Beirut, Paris, and elsewhere, the Security Council passed Resolution 2249 on November 20, 2015, which gets very close. For a broader discussion of that issue, I’d recommend Dapo Akande and Marko Milanovic’s EJIL: Talk! post from November.

So, Can States Defend Themselves Before Being Attacked?

The UN Charter recognizes an inherent right to self-defense “if an armed attack occurs.” This language makes it clear that an attack either must have already taken place or be in progress, to invoke the right to self-defense. Of the countries noted above, only Turkey had come under attack at the time of its first strikes in Syria, thus prompting its justification for acting in individual self-defense. So, what justified Canada, France, the UK, and the United States making similar claims?

There is a widely held belief that under international customary law States also have a right to self-defense prior to an attack occurring, but only when the “necessity of that self-defense is instant, overwhelming, and leaving no choice of means, and no moment of deliberation.” In other words, there must be an imminent threat of attack.

The Bush administration went far beyond any notion of using self-defense in response to an imminent threat, instead claiming a right to use force “before attacks occur, even if uncertainty remains as to the time and place of the enemy’s attack” (sometimes referred to as preemptive or anticipatory self-defense). The Obama administration meanwhile has whittled away at the notion of an “imminent threat” to the point that it “does not require the United States to have clear evidence that a specific attack on U.S. persons or interests will take place in the immediate future.”

It’s less clear how other countries interpret the term “imminent threat.” And they should certainly be asked!

If they interpret the principle as originally intended, then the necessity of their strikes must be “instant, overwhelming, and leaving no choice of means, and no moment of deliberation.” I’m skeptical, however, that they do, and I worry that countries may be sliding towards the US’s far more elastic definition based on the text of their letters to the UN. Frustratingly, the legality of these strikes can only be known through the unlikely disclosure of how each State defines an “imminent threat” and their assessment of the facts as they relate to that definition.

Who Can Carry Out an “Armed Attack?”

It’s widely understood that Article 51 applies when one State carries out an armed attack on another State. But, in this case, Australia, Canada, France, Turkey, the UK, and the United States are all lined up in apparent agreement that non-State groups (such as ISIL) are capable of carrying out an “armed attack” under the Charter’s rules on self-defense. As such, those six States felt permitted to act in self-defense.

The Security Council has indicated in the past (see here and here) that it agrees with such a conclusion, and the text of the Charter does not preclude this interpretation. That said, the Charter’s purpose was to focus primarily on regulating relations between States, not between States and non-State groups. Some legal experts have reflected on this to argue that Article 51’s self-defense exception applies only between States. (See here for a more detailed discussion on this point.)

What Does Self-Defense Against a Non-State Actor Look Like?

Assuming that non-State actors can carry out armed attacks (and thus that States can act in self-defense against those attacks), questions remain as to whether the trigger for an act of self-defense against a non-State actor operates differently than an act of self-defense against a State-led attack. A State that carries out an armed attack is directly responsible for it, so it’s logical that it can be the object of a counter-attack. In contrast, a State may not be directly responsible for the act of a non-State actor operating in its territory. As such, some have stated that an attack that responds to actions of a non-State group must only be done after it is established that the host State is “unwilling or unable” to address the threat.

I find this test operationally problematic, especially when coupled with an expansive interpretation of “imminence.” Others point out that the test is not an established legal doctrine because States have not (yet) granted it a wide consensus. Whatever the case may be, Australia, Canada, Turkey, and the United States referenced the “unwilling or unable” test in their letters to the UN. Whether this is indicative of a broader trend has yet to be fully seen, but if it is, then a considerable amount of work must be done to define its parameters.

What Law Regulates the Use of Force?

If a legal analysis provides a positive answer to the question of whether a given State can use force in Syria, there remains the question of what legal concepts regulate that use of force. Australia, Canada, Turkey, the UK, and the US all said their actions in Syria would be “necessary and proportionate.” But what exactly does that mean? The short answer, as David Kretzmer explains, is that we don’t really know.

Many States, through their actions if not words, seems to believe that any use of force carried out in the name of self-defense should follow either directly, or by analogy, the targeting rules of international humanitarian law (IHL). That is to say, in highly generalized terms, States can target a legitimate military object or individual as long as precautionary measures are taken to avoid civilian harm and so long as the expected incidental harm to civilians and civilian objects is not excessive to the anticipated military advantage.

This should not be assumed however. First, the question of whether use of force under Article 51 is “necessary and proportionate” is a distinct legal question from whether IHL applies, which is itself distinct from questions of whether IHL is followed. Moreover, even if the rules of IHL were to influence this question, remember that IHL’s rules regulating the conduct of hostilities only apply during armed conflict, and that not every act of self-defense takes place as part of an armed conflict. When fighting a non-State actor, an armed conflict begins only under certain conditions, including when the hostilities meet a certain level of intensity. When those conditions are not met (or even when they are), then it may be more appropriate to rely on other concepts relating to necessary and proportionate use of force, such as those found in international human rights law.

Separately, there is the question of how the airstrikes should be legally assessed apart from Article 51 considerations. One the one hand, if a State is joining another State’s pre-existing armed conflict through the doctrine of collective self-defense, then it seems reasonable that any use of force associated with that conflict would be regulated by IHL. This is because an armed conflict already exists. For example, Canada’s letter to the UN says it is acting on behalf of Iraq (at Iraq’s request), so it could be argued that Canada joined Iraq’s ongoing armed conflict with ISIL. Canada’s armed conflict-related actions would then be governed primarily by IHL rather than more stringent international human rights law standards. If, on the other hand, a State is claiming a right to individual self-defense independent of any armed conflict, then it is harder to see how IHL becomes the regulating body of law.

The United States has found its way around this “problem” by claiming that international law permits it to use IHL to follow its enemies wherever they roam. Other States have been far more reluctant to take this approach because it is a trademark of the controversial notion of a “global battlefield.” As a result, while States like Canada, France, the UK, and Turkey, are launching airstrikes under the doctrine of individual self-defense, I have yet to hear a strong explanation from any of them as to what precise rules regulate how those strikes are conducted and why.

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This two-part analysis of how States are justifying their attacks in Syria and how those justifications match up against international law exposes four distinct issues for States to address.

First, letters notifying the UN should be more detailed and could better explain how States distinguish their acts of collective self-defense from acts of individual self-defense. By way of example, the UK provided a relatively precise explanation of its use of individual self-defense related to its August 2015 targeted killing in Syria. At the other end, you have France’s comparatively vague letter.

Second, States need to provide the public with detailed interpretations of what constitutes an “imminent threat.” This is of particular importance because an overly expansive interpretation of the term could seriously jeopardize the raison d’être of the UN Charter.

Third, States should provide the public with detailed explanations of what rules are regulating their use of force in Syria and why. Without them, there is no way to assess the legality of their actions, and accountability will suffer.

Finally, as Ashley Deeks has written about at length, rules on consent for the use of force need further elaboration to ensure that the consent given is not coerced and is appropriate in relation to the aim it seeks to achieve.