On 31 October 2018, responding to the movement of migrants from Central America toward the southern US border, President Donald Trump tweeted: “The Caravans are made up of some very tough fighters and people. Fought back hard and viciously against Mexico at Northern Border before breaking through. Mexican soldiers hurt, were unable, or unwilling to stop Caravan. Should stop them before they reach our Border, but won’t!” (emphasis added). Later, in February 2019, President Trump declared a national emergency on the border with Mexico. The words used by President Trump and the circumstances around them are worryingly similar to the recent invocation by some States, including the United States, of the so-called “unwilling or unable” standard to justify the use of military force against non-state actors in other State’s territory as though such conditions satisfy Article 51 of the UN Charter.

While the application of the “unwilling or unable” standard to international law on the use of force has generated discussion amongst academics and commentators, less well known is how controversial this doctrine remains amongst States. This is partly because of the procedural secrecy and inefficiency in the way the UN Security Council addresses these issues. In this essay, I explain some of these problems from the perspective of a UN delegate, and outline an initiative put forward by Mexico to help all States to engage and participate in debates of such fundamental importance.

Prohibition on the use of force and the so-called “unwilling or unable” standard

Since 2014, in accordance with Art. 51 of the UN Charter, at least thirteen States have informed the United Nations Security Council of military action taken on the basis of individual or collective self-defense to combat non-state actors, particularly terrorist groups, without the consent of the State in which the operations were executed. Most of these situations relate to the fight against ISIL in Syria, where countries like the United States of America, Canada, Turkey, and Australia, have argued that such action is permitted since the Syrian government is “unwilling or unable” to prevent the use of its territory for such attacks. Others like Germany and Belgium, avoided an express reference to the “unwilling or unable” standard, but indicated that the threat of ISIL came from a territory where Syrian authorities did not “exercise effective control.”

These propositions are controversial from a legal point of view, to say the least. The point of departure for a legal analysis is Art. 51 of the UN Charter, which reads:

“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. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.”

Despite the fact that there is no explicit reference whatsoever in this provision to an “unwilling or unable” standard, some continue to argue that actions under this interpretation of Art. 51 are legal. This position is highly questionable given the scarce practice, which is not uniform nor consistent, and the lack of opinio juris (Denmark, for example, invokes Art. 51 but makes no reference to the “unwilling or unable” standard). This makes such claims an insufficient justification for the development of a customary rule. Notably, Syria has also sent letters to the Security Council formally complaining about the way in which self-defense has been invoked, making reference to the collaboration with Russia in the fight against ISIL. Furthermore, as explained by Tom Ruys, “a failure to condemn should not automatically be considered evidence of the legality of the use of force.”

The debate regarding the legitimacy and relevance of the “unable or unwilling” standard to use force against terrorist groups has so far been largely dominated by a few western States. Until recently, most UN Member States have been quiet about this issue, perhaps surprisingly given its fundamental importance to the international order. And silence can be tricky. The conclusions of the International Law Commission on the Identification of customary international law, adopted last year, indicate that “failure to react over time to a practice may serve as evidence of acceptance as law (opinio juris), provided that States were in a position to react and the circumstances called for some reaction” (conclusion 10.3). However, the International Law Commission made the following commentary regarding this conclusion:

“For such a lack of open objection or protest to have this probative value, however, two requirements must be satisfied […] First, it is essential that a reaction to the practice in question would have been called for […] Second, the reference to a State being ‘in a position to react’ means that the State concerned must have had knowledge of the practice (which includes circumstances where, because of the publicity given to the practice, it must be assumed that the State had such knowledge), and that it must have had sufficient time and ability to act. Where a State did not or could not have been expected to know of a certain practice, or has not yet had a reasonable time to respond, inaction cannot be attributed to an acknowledgment that such practice was mandated (or permitted) under customary international law. A State may also provide other explanations for its inaction.” (pp. 141-142)

If silence were to be taken as acceptance on the law, the question is whether all States are indeed truly in a position to react to such practice.

Issues with UN Security Council Procedure on the Use of Force in Self-Defense – and Why This Matters

In order to understand properly this “silence,” it is crucial to know the circumstances, context, and procedure by which the letters invoking self-defense and the “unable or unwilling” standard are sent and dealt with inside the United Nations. In my view, the silence of many UN Members States does not reflect a lack of interest and cannot be considered as acquiescence regarding any novel legal interpretations of Art. 51 of the UN Charter on the use of force against terrorists in a third country. This is for several reasons related primarily to the lack of transparency in UN Security Council procedure.

The way in which this issue is handled by the Security Council is extremely opaque, and not transparent to most Member States. None of the Article 51 letters I reference above have triggered a public debate in the Security Council to evaluate their content, validity, and adherence to the UN Charter. Neither has the Security Council taken any measure to maintain or restore international peace and security in response to those letters. Based on my own experience, it seems that the prevailing practice of the Council is to limit itself to receiving and distributing the letters among its Members, which gives some the mistaken impression that, as long as a State covers the formality of reporting its actions, it can then claim a de facto carte blanche to use force without further scrutiny or control by the Council. This is very troubling given that the Security Council has the primary responsibility for the maintenance of international peace and security.

It is even harder to identify what views most individual States hold on these issues—but again I do not believe this is because of lack of interest or concern. After more than three years serving as a delegation’s legal adviser to the UN, it has become clear to me that the core issue is the lack of information, publicity, and transparency. Even when Art. 51 letters are in principle public documents, the truth is that it is very difficult to get a hold of them, especially for those delegations not in the Security Council—a small group of 5 permanent members and 10 rotating members who serve for 2 years at a time. Here are some of the reasons why:

1. Despite being documents of profound interest to all States, relating to the core prohibition on the use of force under Art. 2(4) of the UN Charter, these letters are not circulated to all UN Members. The stated purpose of the United Nations is the maintenance of international peace and security and the Security Council acts on behalf of all UN Members States, yet there is not even a link on the Council’s webpage to find these documents.

2. In these circumstances, delegates have to find Art. 51 letters themselves, and this is a very difficult and circular task: To find these documents in the UN Official Document System you basically need the detailed information (official document symbol, title of the document, or relevant state) you can only get from the letter itself.

3. Alternatively, delegates may refer to the Repertoire of the Practice of the Security Council. However, the Repertoire currently has a two-year backlog: Its latest update covering 2016-17 was published earlier this year in English only. While the backlog remains, all letters sent after 2017 are still not available in the Repertoire and will continue to be unavailable for the next two years. General Assembly resolution 686(VII), which mandated the creation of the Repertoire in 1952, is entitled “Ways and means for making the evidence of customary international law more readily available.” This highlights how problematic the Repertoire’s backlog is: by the time a State wants to react to the information contained in the Repertoire, it is far too late. Moreover, since there are almost no reactions to these letters recorded in the Repertoire, the information can be misleading in terms of “evidence of customary law.”

4. A final option is to subscribe to the system eSubscription to United Nations Documents. By doing so, the system sends daily emails with the documents issued by the Security Council, but is a very inefficient mechanism: The documents in the email are only referenced by their official UN symbol without including their title, while many others have very general titles like “Letter dated 12 march 2019 from the Permanent Representative of the Sudan to the United Nations addressed to the President of the Security Council,” which offer no indication to the subject matter of the document. Therefore, the only practical way of finding an Art. 51 letter through the emails sent by eSubscription is to open daily each and every one of the links to the documents, noting that they are not circulated on the same date they were issued, to verify their content—something beyond the capacity of most delegations given their heavy workload and limited resources.

At the end of the day, the most effective way to obtain these letters is through comradery: asking colleagues to kindly share either the letters sent by their own delegations or those they have obtained from others. It goes without saying that this is far from a full-proof or fair mechanism. No Member State should have to go through this trouble to get a hold of a public UN document related to the UN’s core function of maintaining international peace and security.

Consequently, if a delegation is not a Security Council member, it is not actively part of a coalition using force to counter terrorism in other countries, and it doesn’t have the time or human resources to track down these letters, it is almost impossible for it to take part of the discussion on the substantive legal terms that this practice entails. Most UN Member States are left in the dark.

Latin American Disquiet with the “Unable or Unwilling” Doctrine

Notwithstanding these procedural issues, there is a growing sense that not all Member States agree with the novel interpretations of Art. 51 advanced by a few States. Despite the many procedural limitations, Mexico has raised its concerns with the “unable or unwilling” standard on several occasions in the context of different UN debates (see here, here, and here, all in Spanish).

The Community of Latin American and Caribbean States (CELAC), which encompasses 33 countries, also addressed this issue last October, indicating the following:

“We take note with concern of the increase in the number of letters to the Security Council under Article 51 of the Charter submitted by some States in order to have recourse to the use of force in the context of counter-terrorism, most of the times “ex post facto”. We reiterate that any use of force which is not in compliance with the UN Charter is not only illegal, it is also unjustifiable and unacceptable. Further consideration should be given in an open and transparent debate on this issue.”

Other Latin American countries, like Brazil, have also been very vocal on this issue too.

Unfortunately, there has not been sufficient attention to these divergent views in academic writings. This is understandably so given that, first, these UN debates are generally unknown to an audience who is not familiar with the UN’s detailed agenda, especially when it comes to debates within the Committees of the General Assembly (even more so when they are available only in languages other than English); and second, because there is currently no UN forum with a specific agenda item that would allow all Members States to address these challenges. Consequently, these references to the interpretation of Art. 51 end up “hidden” in speeches on topics which do not directly address this issue.

Enhancing the Ability of All States to Debate International Law on the Use of Force

In an attempt to open up a forum for all states to engage in a public and transparent debate on these issues, Mexico has proposed an agenda item to tackle this particular question for the Special Committee on the Charter of the United Nations and on the Strengthening of the Role of the Organization. The details of this proposal were spelled out last year in the debate of the Sixth Committee on the report of the Special Committee (see the related document in Spanish). The Special Committee on the Charter is a subsidiary body of the GA and all Member States participate in it. Its mandate, as referenced in General Assembly resolution 70/117, includes to “continue its consideration of all proposals concerning the question of the maintenance of international peace and security in all its aspects in order to strengthen the role of the United Nations.” Hopefully, this proposal may help open a space for States to share information and to shed more light on this topic, offering all States a meaningful opportunity to express their views on this important question. The Special Committee has already taken note of this proposal in the section on Identification of new subjects of its 2018 and 2019 reports, respectively, most recently at its 2019 session last month.

Need for Broader Engagement on Use of Force by All UN Member States

Regardless of the future of this specific initiative, there is an urgent need for States to openly discuss these current challenges to international law which can determine the way in which we understand the collective security system in the future. For that to happen, there has to be more transparency on the way in which States report back to the Security Council in accordance with Art. 51 of the UN Charter.

Even if they are far from becoming law, the precedents that we have today are dangerous and open the door to interpretations that can seriously undermine the rule-based order upon which the UN system is built, particularly in the context of countering terrorism where certain legal margins remain unclear and where the Security Council, being a political organ, has been de facto legislating for several years. The fact that just this month President Trump reportedly declared that he is thinking “very seriously” about labeling Mexican drug cartels as terrorists is one more example of how pressing this question has become. International law should not and cannot be defined by a few; it is vital to take into consideration the views of all States on an issue that concerns the peace and security of the whole international community.


I want to express my deepest gratitude to Naz K. Modirzadeh, Director of the Harvard Law School Program on International Law and Armed Conflict, for offering a compass to navigate in the dark, and to Alex Moorehead, Lecturer-in-Law and Director of the Columbia Law School Program on Counterterrorism, Armed Conflict, and Human Rights, for his invaluable support in this journey. The views expressed are solely in my personal capacity.

Image: UN Photos