The Evolution of International Law [Lecture notes]

Lecture on the evolution of international law by Prof. Philipp H. Fluri. The lecture is part of the master’s course International Development and NGO Management, an elective offered by the Department of International Affairs. This post presents lecture notes based on the class.

From Empire to Nation: Independent States Emerge

Medieval Europe was imagined as one Christian order under the Pope and the Holy Roman Emperor, with Latin as the language of church, courts and universities. By the 16th century that order was fraying: kingdoms such as France, England, Spain and Portugal acted as the highest authority inside their own borders, and the northern Netherlands renounced the Spanish king in 1581. To make independence last, states built nations through a shared language, a national legal system and a common culture, and, as an ideal, a community of people rather than of feudal lords. The aristocracy kept real power for centuries, but royal governments steadily reduced its independence.

The Printing Press and the Bible in National Languages

The printing press turned the Bible into a tool of national identity. Until Gutenberg’s movable type arrived around 1450, every Bible was copied by hand, mostly in Latin, which made it slow to produce and largely a clergy possession, even though its sources are Hebrew and Greek. Print then carried it into everyday languages, from Luther’s German New Testament (1522) to Tyndale’s English one (1526), spreading standard written languages, uniting readers and weakening the Church’s Latin monopoly, so that language and religion together fed national identity.

The School of Salamanca and the Colonial World

Spanish expansion in the Americas after 1492 forced European lawyers to ask who owned the lands and peoples there. Jurists at the University of Salamanca, led by Francisco de Vitoria, argued around 1539 that Indigenous peoples had their own political communities and property rights, so conquest could not rest on discovery or papal grant alone. Working from ius gentium, the law common to all peoples, they helped establish that states, not princes, hold rights and duties, and Domingo de Soto and Francisco Suárez later developed these ideas. Portugal, which divided its claims with Spain in the Treaty of Tordesillas (1494), built its own overseas empire, and Portuguese universities such as Coimbra trained jurists and theologians in the same tradition.

Nations Become Legal Persons

Out of the Salamanca debate and Grotius’s work came the idea that states, not princes, are the bearers of rights and duties in international law. Jurists of the time drew on the vocabulary of Roman law, preserved in the corpus iuris, with its persons, property and contracts, and applied it to nations. A nation could now own territory, make treaties, grant rights and concessions to other nations and receive them in return, and be held to its promises as a single legal person, whoever sat on the throne.

Grotius, Westphalia and State Sovereignty

Hugo Grotius and the Peace of Westphalia turned ius gentium into a law between sovereign states. Grotius (1583–1645) was a Dutch jurist who lived through the Dutch struggle for independence, and he made two distinct arguments. In Mare Liberum (1609) he argued for freedom of the seas, so that no state could claim ownership of the high seas. In De iure belli ac pacis (1625) he argued that states are bound by law in war and peace and that promises between them must be kept, the idea behind pacta sunt servanda. The Peace of Westphalia of 1648 ended the Thirty Years’ War, with the Protestant side negotiating in Osnabrück and the Catholic side in Münster. It confirmed each ruler’s authority over his own territory and religion and recognised the Dutch Republic and the Swiss Confederation.

The result was a shift from a medieval order with emperor and pope above kings, through Salamanca’s common ius gentium, to formally equal territorial states with no superior above them, which we call state sovereignty. Historians caution that the “Westphalian system” is partly a later simplification, but Westphalia remains the symbol of the move from empire to a society of equal states and the start of classical international law.

Latin Legal Terms in the History of International Law, with Basic Latin Grammar

Five Latin terms from the lecture mark stages in this history, and each doubles as a small grammar lesson.

• Ius gentium: “law of peoples”
  • Grammar: ius is nominative singular neuter; gentium is the genitive plural of gens, an i-stem noun, so the phrase means “law of peoples”.
  • It was Roman law common to all peoples, and Salamanca and Grotius used it for law between communities.
• Do ut des: “I give so that you may give”
  • Grammar: do is the 1st person singular present indicative of dare; ut introduces a purpose clause; des is the 2nd person singular subjunctive.
  • It expresses reciprocity, the logic of exchange and of treaties between states.
• Corpus iuris: “body of law”
  • Grammar: corpus is nominative singular neuter; iuris is the genitive singular of ius, so the phrase means “body of law”.
  • It names Justinian’s compilation of Roman law from the 6th century, studied in European universities from about 1100.
• Pacta sunt servanda: “agreements must be kept”
  • Grammar: pacta is nominative plural neuter; sunt means “they are”; servanda is a gerundive of servare that expresses obligation, so “are to be kept”.
  • Grotius made it the basis for keeping promises between states, and Article 26 of the Vienna Convention on the Law of Treaties restates it.
• Ius cogens: “compelling law”, a peremptory norm
  • Grammar: ius is nominative singular neuter; cogens is the present participle of cogere (“to compel”) and agrees with ius.
  • It describes norms no state may set aside, defined in Article 53 of the Vienna Convention on the Law of Treaties.

Three grammar rules explain most of the table. A genitive means “of”, as in ius gentium and corpus iuris. Adjectives and participles agree with their noun in case, number and gender, as in ius cogens. And a verb ending carries the subject, so do alone means “I give”.

From Renaissance Ideas to the UN Charter

The UN Charter of 1945 turned the Renaissance ideas of sovereignty and law between states into five working principles. The first is the non-use of force (Article 2(4)), which bars states from threatening or using force against each other apart from self-defence and Security Council action; the lecture’s example was how land was acquired from Native American nations by the US government in the name of peace and development. The second is non-intervention in internal affairs (Article 2(7)), whose reach depends on how a question is classified, as when Beijing calls the Taiwan question a purely domestic affair while Taiwan’s government and others dispute that framing. The third is self-determination of peoples (Article 1(2)), which drove decolonisation after 1945, and the fourth is good faith (Article 2(2)), pacta sunt servanda in modern form, repeated in Article 26 of the Vienna Convention on the Law of Treaties. The fifth is non-derogable rights, or ius cogens (from cogere, “to compel”): rules that must be respected under all circumstances and that no treaty can override, defined in Article 53 of that Convention, with the bans on genocide, slavery and torture as usual examples.

Conclusion

International law grew from a medieval order under emperor and pope into a society of sovereign states, and the Renaissance supplied the conditions for that change: independent kingdoms, national languages, the printed Bible, the colonial debate at Salamanca and the sovereignty of Grotius and Westphalia. The Latin terms record the path, from ius gentium, the law common to all peoples, through pacta sunt servanda, the duty to keep one’s word, to ius cogens, the modern limit that no state may override. How far these rules are enforced remains open. The Nuremberg Trials (1945–46) prosecuted leading Nazi officials for crimes against peace, war crimes and crimes against humanity and established that individuals, not only states, answer to international law. The United States, by contrast, never faced an official, state-backed international war crimes trial for its actions during the Vietnam War.