In international relations, reciprocity describes an environment in which States support one another for short- or long-term advantage through the balancing of rights, duties and interests. This book examines reciprocity in the context of international law. It considers the role reciprocity plays in the creation and development of international law as well as in the interpretation and application of international law. The book illuminates the reciprocal framework of international law and international relations by examining the role reciprocity plays in different types of States’ obligations, including bilateral, bilateralisable multilateral, non-bilateralisable multilateral and obligations erga omnes. The book examines how reciprocity is intertwined with the principle of equality, as the rights and obligations of States are equal irrespective of size and economic or military strength, and the beneficial effects of reciprocity in creating stability and cooperation amongst States.

There is a common perception of reciprocity as a concept that is opposed to the communitarian interests that characterise contemporary international law, or merely a way of denoting reactions to unfriendly or wrongful conduct. This book disputes this approach, and highlights how reciprocity is instead linked to the structural characteristic of sovereign equality of States in international law. This book carries out an in-depth analysis of the concept of reciprocity and the elements that characterise it, before examining the various roles and articulations of reciprocity in a number of fields of public international law: the law of treaties, the treatment of individuals, the execution of international law, and the jurisdiction of international courts and tribunals. In all these areas, it analyses both more traditional and more contemporary examples, to demonstrate how reciprocity is closely linked to the very structure of public international law.

The world is changing. The era of globalization – where the states have been eager to cooperate towards common aims – might seem to be disappearing. Russia expresses a contempt for international law, China makes claim to power, while the United States, at least temporarily, shows disregard to international institutions they themselves created (in particular, the WTO and the WHO). The growing phenomenon of nationalistic and authoritarian leaders may seem to hamper the development of community interests. Some states may be more inclined to resort to reciprocal behaviour as certain events indicate (e.g., the exchange of prisoners-of-war in the Russia-Ukraine conflict and an exchange of economic sanctions – tit-for-tat or tat-for-tat). The field of international law appears to be returning to first principles in the light of multiple world crises. Thus, reciprocity – as a classic concept of international law – may “come back from the shadows,” be reinvented and reinforced to form one of the basic pillars in the relations among states. Given the above, the purpose of the book is to rediscover and identify the position and role of reciprocity in contemporary international law.

An evaluation of the importance of reciprocity in considering states' legal obligations in armed conflicts.