International legal theories

International legal theory comprises a variety of theoretical and methodological approaches used to explain and analyse the content, formation and effectiveness of public international law and institutions and to suggest improvements. Some approaches center on the question of compliance: why states follow international norms in the absence of a coercive power that ensures compliance. Other approaches focus on the problem of the formation of international rules: why states voluntarily adopt international legal norms, that limit their freedom of action, in the absence of a world legislature. Other perspectives are policy oriented; they elaborate theoretical frameworks and instruments to criticize the existing rules and make suggestions on how to improve them. Some of these approaches are based on domestic legal theory, others are interdisciplinary, while others have been developed expressly to analyse international law.

Many early international legal theorists were concerned with axiomatic truths thought to be reposed in natural law. 16th century natural law writer, Francisco de Vitoria, a professor of theology at the University of Salamanca, examined the questions of the just war, the Spanish authority in the Americas, and the rights of the Native American peoples.

Hugo Grotius, a Dutch theologian, humanist and jurist played a key role in the development of modern international law. In his De jure Belli ac Pacis Libri Tres ("Three Books on the Law of War and Peace") of 1625, and drawing from the Bible and from the St. Augustine's just war theory, he argued that nations as well as persons ought to be governed by universal principle based on morality and divine justice. Drawing, though, from domestic contract law, he argued that relations among polities ought to be governed by the law of peoples, the jus gentium, established by the consent of the community of nations on the basis of the principle of pacta sunt servanda, that is, on the basis of the observance of commitments. On his part, Christian von Wolff, contended the international community should be a world superstate (civitas maxima), having authority over the component member states. Emmerich de Vattel rejected this view and argued instead for the equality of states as articulated by 18th century natural law. In Le droit des gens, Vattel suggested that the law of nations was composed of custom and law on the one hand, and natural law on the other.

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