The Use of Force and the International Legal System
The editors of Articles of War kindly invited me to submit a short post regarding the book I co-authored with Kinga Tibori Szabó on the use of force which was recently published by Cambridge University Press under the abovementioned title. I will give a brief overview of the structure and general content of the book and then highlight one or two aspects that characterize the approach we have taken in setting out the role of the jus ad bellum in the international legal system and how it relates to and interacts with other legal regimes within the international legal landscape.
I should point out that although my co-author is not in a position to contribute to this post together with me due to her present engagement as a legal officer in the UN war crimes investigation unit in Iraq, the book is a joint effort and reflects our common approach to the law.
General Coverage
The book consists of five parts. Part I is a general and historical introduction. It traces the development of the law governing the use of force from its philosophical roots grounded in the just war tradition through the positivism of the 18th and 19th centuries and into the first half of the 20th century. That period saw the gradual development of the jus ad bellum during the interbellum and the negotiations during the Second World War leading to the adoption of the UN Charter and the rules contained in it on the prohibition of the use of force.
Part II sets out the contemporary jus ad bellum and examines the scope and content of the prohibition and discusses the two recognized legal grounds for using force in some detail. The UN Security Council’s envisaged role in the maintenance of peace is discussed against the background of developments in the way the UN collective security system has functioned through different periods since 1945. The right of self-defence is given particularly detailed attention as it is the only recognized basis for the unilateral use of force by States under contemporary international law. This includes attention to the conditions for its exercise and modalities of its application, including in relation to attacks by non-State entities.
Part III discusses a number of recurring and newer controversies relating to the use of force, ranging from humanitarian intervention and armed reprisals to the impact of new technologies for weapons and methods of warfare on the jus ad bellum.
Part IV is discussed below, while Part V contains the synthesis and conclusions and applies the various parts of the book to the current war between Ukraine and Russia as an illustration of the relevance of the different topics covered throughout the book.
Why We Wrote
There are plenty of books on the use of force and most of them are of good quality so why add to the list of publications one might ask? We felt there was room for our views on a number of controversial issues in the law. We have adopted a somewhat different approach to the topic than is used in most of the literature, which is reflected in the title.
We take the view that international law forms a coherent legal system which shares a common set of foundational principles and a common set of sources and methodology in the way it should be interpreted and applied. That means that the rules arising from the different branches of international law continue to apply and interact alongside and with each other in any given situation in which international law is relevant, including when armed force is used by States in their international relations.
In some cases the rules will apply in tandem and generate parallel legal obligations that operate alongside each other. In other cases the rules will directly interact and influence each other and the way they apply in a given situation. Where genuine conflicts of obligation arise, the rules relating to harmonization, complementarity of obligations, and conflict resolution between legal norms will operate to maintain the coherence of the system and determine how the rules will interact and, if necessary, take precedence in relation to each other.
This includes, but is by no means limited, to the lex specialis principle. In short, lex specialis is neither a “get out of jail free” card as it is sometimes seen, so as to exclude entire legal regimes from consideration any time an armed conflict is deemed to exist, nor is it a throwback to a bygone legal era when international law was seen as a binary system in which one set of rules applied in peacetime and another applied in war. It is simply a conflict resolution tool in legal methodology alongside other tools. For more in depth discussion on the way legal methodology operates to maintain the coherence of the legal system see here.
Illustrating Legal Interrelations
To illustrate how we have integrated this approach into our book I will briefly discuss the chapters in Part IV of the book in which we discuss the relationship of the law governing the use of force with other rules within the international legal system.
In Chapter 12, we examine the use of force as an attribute of State sovereignty, particularly in relation to “intervention by consent or invitation.” Consensual intervention by a State on another State’s territory against a non-State armed group or to assist the consenting State in maintaining or restoring internal stability does not fall within the jus ad bellum since it does not involve a use of force in contravention of the prohibition of the use of force in international relations. But it is widely practiced both in the context of multilateral peacekeeping and in the context of ad hoc consent by a State to one or more other States to conduct a military operation against a non-State actor on its territory and is governed by international law. The chapter examines the inherent conditions arising from international law for any consensual intervention and the more procedural conditions relating to granting lawful consent that are reflected in Article 20 of the Articles on State Responsibility (see here). The chapter also devotes attention to the relationship of sovereignty to the “policing of territory” in relation to unauthorized aerial incursion by foreign State aircraft and unauthorized and non-innocent passage by foreign naval vessels in national airspace or territorial waters and the remedies available to address such activity, including the use of force under international law in certain situations.
Chapter 13 explores the relationship of the jus ad bellum with the law governing the use of the international commons (high seas, international airspace and outer space) and other areas subject to international conventions (exclusive economic zone, international straits and Antarctica). It also discusses how the rules relating to the rights and obligations of States that are set out in the relevant conventions applicable to those areas relate to and impact upon the use of force. Examples discussed include the distinction between maritime law enforcement and the use of force at sea, permissible measures of control in air defence identification zones, and in the enforcement of “no-fly zones” and the strict limits on the use of force against civil aircraft in any situation.
Another topic discussed in the chapter is the meaning of the “peaceful purposes” clauses in respectively the 1982 UN Convention on the Law of the Sea, the 1959 Antarctic Treaty System, and the 1967 Outer Space Treaty (OST). With respect to the OST, we discuss the different applications of “peaceful purposes” in orbital space and in interplanetary space. We argue for an interpretation that balances the present use of orbital space for military and security purposes alongside commercial and other purposes and the exclusion of military activities on celestial bodies that in our opinion extends to outer space beyond the narrow band of orbital space on the basis of the object and purpose of the OST and the lack of any feasible relevance or use of interplanetary space for military purposes.
Chapter 14 examines the relationship between the jus ad bellum and the law of armed conflict (LOAC), including the law of neutrality and the traditional law of naval warfare. It also discusses the respective roles of the jus ad bellum, LOAC and international human rights law (IHRL) in governing the application of force.
We argue that the separation of the jus ad bellum and the jus in bello does not mean that the former ceases to apply once force is resorted to and that they both are relevant to how, where and how long force must be applied, albeit from different perspectives and without prejudice to their respective functions.
We also discuss the relevance of IHRL to the use of force and how the question of (ill)legality of the use of force under the jus ad bellum relates to the notion of “arbitrary deprivation of life” under IHRL. Likewise the relationship of the rules relating to the application of force under LOAC and IHRL receives attention with the former being relevant to the conduct of hostilities in an armed conflict and the latter governing the application of force outside the hostilities paradigm, inside or outside the context of armed conflict.
With respect to the law of neutrality we argue that it has lost much—if perhaps not all—of its relevance in light of the development since its adoption of the contemporary law governing the use of force. This extends, we argue, beyond situations in which the UN Security Council has adopted enforcement measures under Chapter VII of the UN Charter, as is illustrated in our opinion by the current conflict in Ukraine. The same applies in our view to much of the traditional law of naval warfare with many of its rules dating back to the era prior to the First World War being hardly compatible with the rest of contemporary international law—including the jus ad bellum—as well as the rest of LOAC and IHRL.
Chapter 15 examines the application of the law of international responsibility to the use of force and gives detailed attention to contemporary developments in which economic sanctions of various types under the rubric of retorsions and countermeasures have come to play such an important role in inducing compliance or exacting reparation for unlawful uses of force. It also examines the criminal liability of individuals for the crime of aggression in a separate subsection within the chapter.
Concluding Thoughts
We have aimed the book at three distinct audiences. Firstly as a contribution to the academic discussion of the scope and interpretation of the jus ad bellum for our academic colleagues. We think the book makes an original and hopefully worthwhile contribution to the literature on the use of force. Secondly as an attempt to provide usable guidance on complex questions to practitioners engaged in application of the law in a practical setting. We are both “practice oriented lawyers” and have regularly provided advice on these and related questions to the armed forces, international tribunals, and other governmental departments and international bodies engaged in applying this branch of the law and hope this book will be of use to them.
Finally and not least, we have also aimed the book at (post) graduate level students and PhD candidates interested in this area of the law as a supplementary textbook and research tool. Consequently, we have tried to keep the book as accessible as possible and have used numerous examples and summaries to illustrate the arguments relating to the interpretation of the law on the use of force.
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Terry D. Gill is Professor Emeritus of Military Law at the University of Amsterdam, having held the chair from September 2001 until September 2020.
Photo credit: Pexels
