However, while the origins of the modern system of international law can be traced back 400 years, the development of the concepts and practises that would underpin that system can be traced back to ancient historical politics and relationships thousands of years old.
In East Asia, political theory was based not on the equality of states, but rather the cosmological supremacy of the Emperor of China.
[1] Early examples of treaties include around 2100 BC an agreement between the rulers of the city-states of Lagash and Umma in Mesopotamia, inscribed on a stone block, setting a prescribed boundary between their two states.
The many requirements on how prisoners of war should be treated included, for example, providing shelter, food and clothing, respecting their cultures, and preventing any acts of execution, rape or revenge.
[3] Islamic law under the early Caliphate institutionalised humanitarian limitations on military conduct, including attempts to limit the severity of war, guidelines for ceasing hostilities, distinguishing between civilians and combatants, preventing unnecessary destruction, and caring for the sick and wounded.
The Hanseatic League of the more than 150 entities in what is now Germany, Scandinavia, and the Baltic states developed many useful international customs, which facilitated trade and communication among other things.
The Italian city-states developed diplomatic rules, as they began sending ambassadors to foreign capitals.
The horrors of the Thirty Years' War, meanwhile, created an outcry for rules of combat that would protect civilian communities.
The most important of these was Hugo Grotius, whose treatise De Jure Belli Ac Pacis Libri Tres is considered the starting point for modern international law.
Before Hugo Grotius, most European thinkers treated law as something independent of mankind, with its own existence.
Grotius was no different, except in one important respect: Unlike the earlier thinkers, who believed that the natural law was imposed by a deity, Grotius believed that the natural law came from an essential universal reason, common to all men.
This rationalist perspective enabled Grotius to posit several rational principles underlying law.
According to the Jewish jurist and diplomat Shabtai Rosenne, the 17-century major figures of the law of nations were used to extensively refer to Jewish sources like the Codes of Law, Maimonides, Moses of Coucy, Ibn Ezra, Leon of Modena and Menasseh Ben Israel.
[5] The jurists referring to them were Grotius, Selden, Ayala, Gentili, Zouch, Samuel Rachel and Pufendorf.
[5] The Westphalian treaties of 1648 were a turning point in establishing the principle of state sovereignty as a cornerstone of the international order.
However the first attempts at formulating autonomous theories of international law occurred before this, in Spain, in the 16th century.
This tension between legal norms and political imperatives is well reflected in the century's most important treatise on international law, Emer de Vattel's Du Droit des Gens (1758).
However, as recent research has shown, ius contra bellum (the outlawry of war) has its roots in 19th century legal and political discourse.
The League of Nations, established after the war, attempted to curb invasions by enacting a treaty agreement providing for economic and military sanctions against member states that used "external aggression" to invade or conquer other member states.
Aggression on the part of Germany, Italy and Japan went unchecked by international law, and it took a Second World War to end it.
An evolution of the positivist approach of Grotius, the concept of consent is an element of customary international law.
In Canada, on the other hand, ratification is strictly an executive action, and no parliamentary approval is required before the nation is bound.
The convention's most important and sensible rule is that a treaty should be interpreted according to the plain meaning of its language, in the context of its purpose, and in good faith.