The Principle of Proportionality in Modern Ius Gentium

The principle of proportionality refers to the criteria for fair and optimal balancing of interests. It is widely applied to international disputes and has gained institutional and scholarly acceptance in the field of international law. This paper aims to explore the longue durée of the principle, drawing on an interdisciplinary perspective on international law. It affirms the traditional role of proportionality in international legal sphere and values its familiar role in introducing flexibility in law, remaining close to its conventional interpretation. However, the paper also questions its contemporary ethos, as it is based historically on its relation to equity. To this end, it examines the historical roots of the principle as part of the early modern law of nations, as well as how such a general principle should be seen as applicable to private relationships. The aim is therefore to re-think the principle of proportionality in modern ius gentium as based on how public and private law principles need to be interpreted relative to each other and continue to be shaped continuously as an extension of their shared history. It is in this sense that we can examine the need for equity in the international sphere, which will be demonstrated concretely for three distinct areas where proportionality predominates: the law of war, the law of maritime delimitation and international human rights law.