The ius gentium or jus gentium (Latin for “law of nations”) is a concept of international law within the ancient Roman legal system and Western law traditions based on or influenced by it.
The ius gentium is not a body of statute law nor a legal code, but rather customary law thought to be held in common by all gentes (“peoples” or “nations”) in “reasoned compliance with standards of international conduct”.
Following the Christianization of the Roman Empire, canon law also contributed to the European ius gentium. By the 16th century, the shared concept of the ius gentium disintegrated as individual European nations developed distinct bodies of law, the authority of the Pope declined, and colonialism created subject nations outside the west.
Quoted in Laurens Winkel, “The Peace Treaties of Westphalia as an Instance of the Reception of Roman Law”, in Peace Treaties and International Law in European History, p. 225.
Cicero, Partitiones oratoriae 37.130
Source; https://en.m.wikipedia.org/wiki/Jus_gentium
sources for further reading & research
Ius Gentium was primarily used to govern the rules of peace and war, diplomatic exchanges, extradition and issues regarding natural boundaries:
The Rules of Ius Gentium:
1. Ius Gentium states that no country shall attack other nations unless a declaration of war has been affirmed or a situation calls for an aggressive action.
2. All nations, under Ius Gentium, must honor peace treaties, satisfy the agreements conducted in truce negotiations and respect the boundaries of nations.
3. Ius Gentium states that all wrecked ships are protected and the individuals on these ships may not be killed by the other nation in a time of war.
4. Acts of piracy shall be deemed illegal and those who engage in piracy shall be prosecuted to the fullest extent under Ius Gentium
5. All prisoners of war, under Ius Gentium, shall be treated decently and awarded basic human necessities such as food and water. Any mistreatment of a prisoner of war that results in death will be prosecuted under Ius Gentium.
6. All embassies and diplomats of foreign countries will be protected. Any violent action placed on another country’s embassy or an acting diplomat will be prosecuted under Ius Gentium.
7. Ius Gentium states that extradition treaties must be honored by those participating countries. Failure to adhere to the agreements latent in an extradition treaty will be prosecuted under Ius Gentium.
8. Ius Gentium prohibited slavery and the trading of slaves—in the modern era, the enslavement of those defeated in war under specific conditions was not deemed contrary to the laws of Ius Gentium
https://common.laws.com/ius-gentium
Additional reading:
Randall Lesaffer, introduction to Peace Treaties and International Law in European History from the Late Middle Ages to World War One (Cambridge University Press, 2004), pp. 5, 13
R.W. Dyson, Natural Law and Political Realism in the History of Political Thought (Peter Lang, 2005), vol. 1, p. 127.
Brian Tierney, The Idea of Natural Rights(Wm. B. Eerdmans, 2002, originally published 1997 by Scholars Press for Emory University), pp. 66–67; Dyson, Natural Law and Political Realism, p. 236
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