Pope Leo XIV on International Law

Moral Cosmopolitanism and the Imitation of Jesus

In the opening line of the Declaration of Independence, Thomas Jefferson explains why he is laying out the reasons for the American colonies’ attempt to throw off the authority of King George III: “[A] decent respect to the opinions of mankind requires that [a people] should declare the causes which impel them to the separation.”

Jefferson’s point is not that the rightness of the American cause depends on the support of other nations. Rather, Jefferson is appealing to the idea that there is a moral order among nations, usually referred to as the law of nations (or jus gentium in Latin), and that the use of violence in war or revolution ought to be justifiable by an appeal to our shared reason. In Enlightenment-era jurisprudence, this principle was closely associated with the just-war criterion of “right intention,” and formal declarations of war from that period often included a rationale similar to that given in the Declaration of Independence, exposing the stated reasons for the war to public scrutiny.

The concept of the “law of nations” originated as part of the ancient Roman legal system and was revived by medieval canonists like Gratian and theologians like Thomas Aquinas. The modern sense of the term, however, was developed by the early 16th-century Spanish Dominican Francisco de Vitoria, as well as his students and later scholars at the University of Salamanca, including the Dominican Domingo de Soto and the Jesuits Luis de Molina (more famous for his views on divine foreknowledge and predestination) and Francisco Suárez. Importantly, this development of the notion of the law of nations as a moral order governing the affairs of nations emerged as these scholars grappled with the ethical questions arising from the Spanish conquest of the Americas and the treatment of the indigenous peoples of the Americas. It also, however, came to offer a moral framework for assessing the relations of the great powers on the European continent, as well.

In the first half of the twentieth century, these Catholic scholars—and later Protestant thinkers like Alberico Gentili and above all Hugo Grotius—were sometimes honored as the “founders” of international law. This was no doubt intended to emphasize the Christian roots of the modern system of international law and 20th-century institutions like the League of Nations and United Nations. The law of nations and international law, however, despite their similar names, are distinct concepts. Whereas international law typically refers to explicit rules written in treaties and other documents, the law of nations, as these early modern scholars understood it, refers to moral norms knowable by reason and acknowledge by all, or most, nations. The law of nations is rooted in, although distinct from, the natural law governing human conduct.

As international affairs became increasingly governed by a patchwork of treaties like the 1648 Peace of Westphalia, for example, these treaties came to be understood as expressions of the law of nations, leading to the near identification of the law of nations with international law. Today, international law is embodied in the United Nations Charter and treaties on specific topics like the Hague and Geneva Conventions governing the conduct of war and the 2016 Paris Climate Accords. In more recent times, the notion of a set of shared, unwritten, moral norms governing international affairs lives on in the idea of “customary international law.”

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I’m reviewing this intellectual history because, in remarks offered last week, Pope Leo XIV laments the lack of respect for international law in contemporary foreign affairs. Pope Leo’s comments were made during an address to representatives of the Reunion of Aid Agencies for the Oriental Churches, a network of agencies (mostly in the United States and Europe) that provide aid to the Eastern Catholic Churches. Referring to the ongoing conflicts in Ukraine, Gaza, and the broader Middle East (the address was given only four days after the United States carried out strikes against Iranian nuclear facilities), Leo acknowledges that the lands that many Eastern Catholics call home have been particularly ravaged by violence: “[V]iolent conflict seems to be raging in the Christian East with a diabolical intensity previously unknown.” He also points to those conflicts as examples of the lack of respect for international law.

Pope Leo claims that respect for international law has been replaced by the principle of “might makes right” and an “alleged right to coerce others.” Although it would be naive to think there was ever a time when international law was universally respected and followed, in the post-World War II period, international law at least served as a shared language for assessing the conduct of nations, and even when it was breached, there was often a decent enough respect for the opinions of humankind, to paraphrase Jefferson, that nations felt compelled to at least justify their actions “under the color of international law,” so to speak. Pope Leo is suggesting, then, that there has been a shift in attitudes toward the laws of war. Indeed, nations seem to thumb their nose at the very idea of moral and legal restraints on war. For example, Russia has flagrantly bombed civilian populations in Ukrainian cities since that war began in 2022, Israeli soldiers have fired on crowds of Gazan civilians in line for aid, and the President of the United States has threatened (seriously or not, who’s to say?) to invade peaceful NATO allies Canada and Denmark.