Origin and Development of International Law — Public International Law Notes

Origin and Development of International Law

In 1625, in the middle of the Thirty Years’ War — a war so brutal it killed a third of the German population — a Dutch lawyer named Hugo Grotius published De Jure Belli ac Pacis (“On the Law of War and Peace”). His argument was radical: even in war, States are bound by rules, and those rules exist whether or not any ruler commands them, because they flow from human reason itself. That book earned Grotius the title “the father of international law”.

How international law grew — trace it in four visible stages

International law is not a code that was enacted on a single day; it grew. Examiners want the named milestones and dates, so learn it as four stages.

A. Ancient and medieval practice

Even ancient civilisations followed rudimentary inter-community rules — treaties, the sending of envoys, rules on the conduct of war. Ancient India had rules in the Dharmashastra and Kautilya’s Arthashastra on envoys and war; Greek city-states and Rome (the jus gentium, the “law of peoples” applied to foreigners) contributed ideas. But there was no system of equal sovereign States yet, so this is only the pre-history.

B. The Peace of Westphalia (1648) — the birth of the State system

The Peace of Westphalia ended the Thirty Years’ War and is treated as the birth certificate of modern international law. It recognised the sovereign, territorial State as the basic unit, each equal and independent, with no superior (the Pope or Emperor) above it. Once you have many equal sovereigns and no world ruler, you need a law between them — and that is international law.

C. Grotius and the naturalists (17th century)

Grotius’ De Jure Belli ac Pacis (1625) systematised the subject and is why he is the “father of international law”. His school — the naturalists — held that international law is part of the law of nature: a body of rules discoverable by human reason, binding independently of any State’s will. Others of the era: Vitoria and Suárez (Spanish theologians on the law of nations), Zouche, Pufendorf and Bynkershoek.

D. Positivism and the institutional era (18th century onward)

From the 18th century the positivists (Vattel, later Anzilotti and Triepel) shifted the ground: international law binds because States have consented to it, expressly (treaties) or tacitly (custom). The 19th and 20th centuries added institutions: the Hague Peace Conferences (1899, 1907), the Permanent Court of Arbitration, the League of Nations (1919) after the First World War, and — after the Second — the United Nations (1945) and the International Court of Justice, plus the great codifying conventions (VCLT 1969, UNCLOS 1982). Modern international law is thus both consent-based and institutional.

Grotius, De Jure Belli ac Pacis (1625): the law of nations derives from right reason and the social nature of man, and binds sovereigns even in the absence of a common superior.

In Simple Terms: Grotius’ big idea was that rules between nations are not just polite habits — they are real obligations, rooted in reason and in the fact that humans (and States) must live together. That is why he is remembered as the founder of the subject as a discipline.

🧩 WORKED EXAMPLE — a self-contained short note on Grotius

Facts. “Write a short note on the contribution of Hugo Grotius.” (6–8 marks)

Rule. Grotius (1583–1645), Dutch jurist; wrote De Jure Belli ac Pacis (1625) during the Thirty Years’ War.

Apply. State four contributions: (1) systematised international law as a discipline; (2) grounded it in the law of nature / right reason, so it binds even without a world sovereign; (3) developed rules on just war, treaties (pacta sunt servanda) and the treatment of neutrals; (4) influenced the freedom of the seas (Mare Liberum, 1609).

Conclusion. For these he is called the “father of international law”. Naming the book and the year is what earns the marks.

flowchart TD
    A["Development of International Law"]
    A --> B["Ancient/medieval practice<br/>envoys, treaties, jus gentium"]
    A --> C["Peace of Westphalia (1648)<br/>birth of sovereign-State system"]
    A --> D["Grotius (1625) + naturalists<br/>law of nature / reason"]
    A --> E["Positivists + institutions<br/>consent; Hague, League 1919, UN 1945"]
    classDef box fill:#e8f0fe,stroke:#333,color:#111;
    class A,B,C,D,E box;

Case Laws

The Paquete Habana (1900) — traces custom on exempting fishing vessels back through centuries of practice, illustrating how usage hardens into law over time.
S.S. Lotus (France v. Turkey, PCIJ, 1927) — the positivist high-water mark: rules binding on States “emanate from their own free will” expressed in conventions or usages.

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