The United States’ Practical Approach to Identifying Customary Law of Armed Conflict

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The following post is part of a symposium based on a conference panel that discussed issues of customary law of armed conflict, at the 4th Israel Defense Forces (IDF) Military Advocate General (MAG) Conference on the Law of Armed Conflict, held in Herzliya, Israel, during May 8-10, 2023. The post is based on the author’s presentation in the panel.

For government attorneys, identifying, interpreting, and applying customary international law is rarely an exercise in applying black letter rules.  This is particularly true with the customary international law of armed conflict (LOAC). As Sir Hersch Lauterpacht, former judge on the International Court of Justice, quipped: “if international law is, in some ways, at the vanishing point of law, the law of war is, perhaps even more conspicuously, at the vanishing point of international law.” 

Nevertheless, all international lawyers begin with the understanding that customary international law exists and that to identify and define its rules, we must look to discover the general and consistent practice of States that they follow from a sense of legal obligation, or opinio juris.  Accordingly, it is States that make and identify custom as international law.

But this simple rule, that States make and identify custom as international law, can pose considerable difficulties in application.  As former State Department Deputy Legal Adviser Mike Matheson said 35 years ago:

“[T]here is no clear line drawn in the dust for all to see between those principles that are now customary law and those which have not yet attained the degree of acceptance and observance that might make them customary law.  Instead, there are degrees of acceptance and degrees of observance, and the judgment as to what degree of each is sufficient for establishment as customary law is inherently subjective and hard to define precisely.  In addition, it may be possible in many cases to say that a general principle is an accepted part of customary law, but to have considerable disagreement as to the precise statement of that general principle.[1]

Government lawyers are accustomed to operating in this environment and typically focus on defining LOAC as it has been deemed applicable to their own armed forces.  The United States has long worked to publish the specific rules applicable to its forces, beginning with the Lieber Code during the American Civil War.  Since that time 160 years ago, the Army, Navy, Air Force, and Marine Corps have prepared and updated numerous manuals and guidance documents on the legal rules for conducting armed conflict, many of which are available to the public, including the Department of Defense (DoD) Law of War Manual.  The State Department Office of the Legal Adviser has also published dozens of volumes cataloguing U.S. practice in international law, including statements of opinio juris.  Treaties are the handiest form of definitive international rules, and the United States has participated actively in negotiations to codify the rules of warfare, including the Hague Regulations, the Geneva Conventions and Additional Protocols, as well as the Convention on Certain Conventional Weapons and its protocols, and has acceded to many of them.

Various States, academic commentators, and national and international courts and tribunals have expressed a desire for additional authoritative guidance and are anxious to find easily accessible, clear, and definitive customary LOAC rules. The International Committee of the Red Cross (ICRC) has made a substantial contribution to this effort with its Study on Customary International Humanitarian Law.  In the Study, the ICRC proffered its own set of 161 customary rules of LOAC with a compendium of citations to State practice and opinio juris that it believes support the existence of those rules.

The United States and some other governments have complimented the ICRC for the effort represented by the Study and the valuable compendium of practice it assembled, which the ICRC continues to update, but have criticized the method by which the ICRC analyzed the material it gathered and how it used that material to identify CIL rules. In a joint letter to ICRC President Kellenberger in 2006, then State Department Legal Adviser John Bellinger and Defense Department General Counsel Jim Haynes argued, based on an analysis of four of the proposed rules, that the Study lacks necessary rigor in its approach to assessing State practice and opinio juris, for example:

Not relying on State practice “sufficiently dense to meet the ‘extensive and virtually uniform” standard for demonstrating the existence of a customary rule;

Relying too little on States’ operational practice and experience from armed conflict and too much on written materials such as manuals and guidelines largely reflecting treaty obligations and policy directives, and on non-binding resolutions of the UN General Assembly;

Failing to properly take into account negative practice by States;

Failing often to give adequate weight to the practice of specially affected States with considerable depth of experience participating in armed conflict;

Eliding too often the requirement of showing opinio juris with the explanation that it can be inferred from State practice;

Giving undue weight to statements by NGOs and the ICRC itself, which is not State practice.

In light of these criticisms, Bellinger and Haynes advised those who would consult the Study not to accept the stated rules without independent analysis.

These criticisms of the ICRC Study are important to consider. All practitioners must remain faithful to the foundational rule of international law under which customary international law is created, or the law we are charged with respecting loses its authority. States cannot be bound to international law without their consent. Their consent must be derived either from treaties to which they are party, or through the customary international law that they create through their extensive and virtually uniform practice undertaken out of a sense of legal obligation. 

Such caution is particularly important with respect to customary LOAC. As professors Mike Schmitt and Sean Watts have written:

“[F]ew outside the ambit of States’ defense ministries and armed forces fully appreciate the operational challenges, demands, and limitations of combat so essential to fairly striking the delicate balance between military necessity and humanity that infuses IHL and informs its interpretation and evolution.

In the 17 years since the Bellinger-Haynes letter to the ICRC, the United States has taken steps to explain U.S. views on customary LOAC and to promote the crystallization of those rules.  This article offers three examples: the customary international law status of Article 75 of the First Additional Protocol (AP I) to the Geneva Conventions; the jus ad bellum right of States to use force in self-defense against non-State actors on the territory of another State; and the DoD Law of War Manual.

Article 75

In 2011 the White House announced several “New Actions on Guantanamo and Detainee Policy,” including enhanced procedural protections for law of war detainees and those facing prosecution in military commissions, as well as an announcement that the U.S. government will:

choose out of a sense of legal obligation to treat the principles set forth in Article 75 [of AP I] as applicable to any individual it detains in an international armed conflict, and expects all other nations to adhere to these principles as well.

In a communication to the Senate, State Department Legal Adviser Harold Koh explained:

As a matter of international law, the administration’s statement is likely to be received as a statement of the U.S. Government’s opinio juris as well as a reaffirmation of U.S. practice in this area.  The statement is therefore also likely to be received as a significant contribution to the crystallization of the principles contained in Article 75 as rules of customary international law applicable in international armed conflict. . . . The U.S. statement, coupled with sufficient density of State practice and opinio juris, would contribute to the creation of the principles reflected in Article 75 as rules of customary international law, which all States would be obligated to apply in international armed conflict.”

The United States thereby sought publicly to make its practice and opinio juris clear with respect to the minimum “fundamental guarantees” in Article 75 related to detention and urge other States to contribute in the same way to the establishment of the principles of Article 75 as CIL.

Jus ad Bellum

The United States has been engaged in a non-international armed conflict against al-Qaeda and associated forces, including ISIS, for many years. In facing armed attacks and the threats of imminent armed attacks from these groups, which are dispersed and operating in various countries around the world, the United States has exercised its inherent right, as reflected in Article 51 of the UN Charter, to use force in individual or collective self-defense. Most military operations are conducted in countries that have provided their consent; however, in some cases – particularly in Syria – consent was not provided.

The United States has therefore found it important to clarify the legal basis under which it has used force, in particular its view of the right of States to use force in self-defense in certain circumstances on the territory of another State against non-state actors. In the U.S. view, a State may lawfully use force against a non-state actor on the territory of another State only where it is authorized by a resolution of the UN Security Council acting under Chapter VII, with the consent of the territorial State, or in individual or collective self-defense against an imminent or actual armed attack where the territorial State is unable or unwilling to prevent the use of its territory. 

In line with this view, and following strikes against ISIS in Syria, the United States submitted a letter dated September 23, 2014, to the UN Secretary-General notifying the Security Council of the strikes and stating:

States must be able to defend themselves, in accordance with the inherent right of individual and collective self-defence, as reflected in Article 51 of the Chater of the United Nations, when, as is the case here, the government of the State where the threat is located is unwilling or unable to prevent the use of its territory for such attacks.

More recently, on March 27, 2023, following strikes in Syria in response to a series of attacks against U.S. personnel and facilities perpetrated by Iranian Revolutionary Guard Corps-backed militia groups the United States submitted a similar letter, stating in part:

This military response was taken after non-military options proved inadequate to address the threat, with the aim of deescalating the situation and preventing further attacks. As the United States has noted in prior letters to the Security Council, States must be able to defend themselves, in accordance with the inherent right of self-defense reflected in Article 51 of the Charter of the United Nations, when, as is the case here, the government of the State where the threat is located is unwilling or unable to prevent the use of its territory by non-State militia groups responsible for such attacks. This action was conducted together with diplomatic measures.

The United States published a report in 2016 on the “Legal and Policy Frameworks Guiding the United States’ Use of Military Force and Related National Security Operations.” The report identifies places where the United States has used military force and the international law basis for doing so, such as in Syria against ISIS in the collective self-defense of Iraq and other States.

The 2016 Report explained that the inherent right of self-defense as reflected in customary international law is not restricted to threats posed by States, and pointed to the Caroline incident, which involved the use of force by the United Kingdom in self-defense against a non-State actor located in the United States.  The Report also explained that a State may use force under the jus ad bellum in response to imminent attacks before they occur and laid out certain factors that the United States considers for determining whether an attack is imminent for these purposes, which include those identified by former UK Foreign and Commonwealth Office Principal Legal Adviser, Sir Daniel Bethlehem in a paper published in 2012.

Finally, the Report also explained that:

Under international law, States may defend themselves, in accordance with the inherent right of individual and collective self-defense, when they face actual or imminent armed attacks by a non-State armed group and the use of force is necessary because the government of the State where the threat is located is unable or unwilling to prevent the use of its territory by the non-State actor for such attacks.

At the same time, the Report underlined that:

“The unable or unwilling standard is not a license to wage war globally or to disregard the borders and territorial integrity of other States.

. . . .

“[A]pplying the standard ensures that force is used on foreign territory without consent only in those exceptional circumstances in which a State cannot or will not take effective measures to confront a non-State actor that is using the State’s territory as a base for attacks and related operations against other States.”

These documents constitute a useful record of State practice and opinio juris by a specially affected State and contribute to the elucidation and establishment of customary international law.

DoD Law of War Manual

At nearly 1200 pages, the DoD Law of War Manual represents a sweeping and detailed compendium of official DoD views on treaty and customary international law that applies to the jus in bello for the United States.  Unlike the Army Field Manual, which is directed to Army and Marine Corps personnel and their conduct during times of war, the Law of War Manual is heavily footnoted with references to sources materials in treaties and customary international law, case law, academic writing, as well as prior U.S. statements.   

In certain areas where the United States does not agree with the ICRC study, such as on the ICRC’s proposed blanket prohibition of expanding bullets (Rule 77), the Law of War Manual explains why practice and opinio juris do not support the ICRC’s rule.  As such, the Law of War Manual serves as a ready source of evidence of existing U.S. practice and opinio juris, and its sources and analysis can be paired with evidence of consistent practice.

Fostering State Practice

Given that international law is established by States, whether by treaty or custom, what else can government lawyers do to improve the identification and development of customary LOAC?  One important way is to provide opportunities for militaries to meet, particularly experts knowledgeable in operations and operational law. 

In November 2022, in Dublin, at the conference adopting the Political Declaration on Strengthening the Protection of Civilians from the Humanitarian Consequences Arising from the Use of Explosive Weapons in Populated Areas (“EWIPA”) the U.S. Ambassador to Ireland said:

We must now turn our efforts to implementation.  To have a lasting impact, this Declaration will need robust implementation by each State and active follow-on exchanges among States.  We want to see militaries from around the world learning from each other and sharing practical and realistic measures to strengthen their implementation of international humanitarian law and improve the protection of civilians.

Efforts are underway to plan the EWIPA Declaration’s follow-on activities, and governments should look forward to the opportunity to participate. By exchanging best practices and understandings of applicable law, they can influence the continuing development of consistent and uniform State practice, and the establishment of customary LOAC.

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