The Evolution and Identification of the Customary International Law of Armed Conflict.

• Author: Wood, Michael
• Position: Special Issue: The Law of Armed Conflict

TABLE OF CONTENTS I. INTRODUCTION 727 II. THE METHODOLOGY FOR IDENTIFYING CIHL 729 III. WHOSE PRACTICE COUNTS? 731 IV. SPECIALLY AFFECTED STATES 733 V. INTERNATIONAL HUMANITARIAN LAW AND 734 INTERNATIONAL HUMAN RIGHTS LAW VI. CONCLUSION 735 I. INTRODUCTION

Despite the many widely ratified treaties on the law of armed conflict (LOAC, also referred to as international humanitarian law (IHL)), customary international law remains of great importance in this branch of international law. So far as concerns international armed conflicts, customary international humanitarian law (CIHL) is of special importance in connection with states not party to Additional Protocol I of 1977. (1) So far as concerns non-international armed conflicts, CIHL is of crucial importance for all states, since, for the most part, treaty provisions are rudimentary. The International Court of Justice has also had occasion to state that "a great many rules of humanitarian law applicable in armed conflict... are to be observed by all states whether or not they have ratified the conventions that contain them, because they constitute intransgressible principles of international customary law." (2)

The present contribution concerns the evolution and identification of CIHL. It does not deal with the substance of that law, on which there is a good deal of material, including in the impressive study under the auspices of the International Committee of the Red Cross (ICRC). (3) It addresses in particular the relevance for CIHL of the UN International Law Commission (ILC)'s recent work on the topic Identification of customary international law. (4) While working on the topic, the Commission had LOAC very much in mind and tried to take into account lessons learned in the field of CIHL.

Identification and evolution of the law are obviously not the same thing, but as the ILC noted when it changed the topic's title from Formation and evidence of customary international law to Identification of customary international law, in that context they are in many ways closely related. (5) In particular, the requirement--for identification of customary international law--to ascertain "a general practice" that is "accepted as law" reflects the fact that rules of customary international law evolve through a general practice and opinio juris. In other words, the two constituent elements of customary international law are also the twin criteria for its identification. The ILC's commentary to draft conclusion 1 on the topic, adopted on first reading in 2016, thus notes:

Dealing as they do with the identification of rules of customary international law, the draft conclusions do not address, directly, the processes by which customary international law develops over time. Yet in practice identification cannot always be considered in isolation from formation; the identification of the existence and content of a rule of customary international law may well involve consideration of the processes by which it has developed. The draft conclusions thus inevitably refer in places to the formation of rules; they do not, however, deal systematically with how rules emerge, or how they change or terminate. (6) II. THE METHODOLOGY FOR IDENTIFYING CIHL

The ILC took up the topic identification of customary international law in 2012, as there was felt to be a need for some reasonably authoritative guidance on the methodology to be employed in identifying rules of customary international law and their content. This need resulted largely from the difficulties sometimes encountered by national courts in this regard, as well as from the potential confusion to which the theories propounded by various writers might give rise.

When the ILC began its work on the topic in 2012, almost the only recent and reasonably detailed statements by states on how rules of customary international law were to be identified were those stimulated by the ICRC study on CIHL. The very fact that the ICRC had produced its study gave rise to important statements on how to identify rules of customary international law, not only by certain governments, (7) but also by the ICRC itself and by individual experts. This has also been the effect of the ILC's own work on the topic; but at its outset, the Commission benefitted greatly from the debate concerning the methodology referred to by the ICRC authors. That methodology also has much in common with the methodology set out by the ILC in its draft conclusions.

To begin with, an important conclusion of the ILC, as of the ICRC, is that the same basic approach--the two-element approach--applies to the identification of the existence and content of rules of customary international law in all fields of international law. This is confirmed in the practice of states and in the case law. It is also consistent with the unity and coherence of international law, which is a single legal system and is not divided into separate branches, each with its own approach to sources. (8) This includes CIHL, just as it includes customary international human rights law.

At the same time, the ILC's commentary acknowledges that the application in practice of the basic approach may well take into account the particular circumstances and context in which an alleged rule has arisen and operates. (9) The ILC's draft conclusion 3 similarly recognizes the need for such flexibility, stating, in part, that:

In assessing evidence for the purpose of ascertaining whether there is a general practice and whether that practice is accepted as law...