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The ratification of the Treaty of Münster, part of the Peace of Westphalia that ended the Thirty Years' War.
The field of international relations dates from the time of the Greek historian Thucydides.
The official portraits of King Władysław IV dressed according to French, Spanish and Polish fashion reflects the complex politics of the Polish–Lithuanian Commonwealth during the Thirty Years' War.
Empires of the world in 1910
NATO International Security Assistance Force in Afghanistan
The United Nations Secretariat Building at the United Nations headquarters in New York City
The World Bank headquarters in Washington, D.C.
NATO E-3A flying with USAF F-16s in a NATO exercise

The principle underlies the modern international system of sovereign states and is enshrined in the United Nations Charter, which states that "nothing ... shall authorize the United Nations to intervene in matters which are essentially within the domestic jurisdiction of any state."

- Westphalian sovereignty

This makes the establishment of the modern state system the natural starting point of international relations history.

- International relations
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The historic town hall of Münster where the treaty was signed

Peace of Westphalia

Collective name for two peace treaties signed in October 1648 in the Westphalian cities of Osnabrück and Münster.

Collective name for two peace treaties signed in October 1648 in the Westphalian cities of Osnabrück and Münster.

The historic town hall of Münster where the treaty was signed
Dutch envoy Adriaan Pauw enters Münster around 1646 for the peace negotiations
Sebastian Dadler undated medal (1648), Christina of Sweden, portrait with feathered helmet right. Obverse
The reverse of this medal: Christina of Sweden as Minerva holding an olive branch in her left arm and grasping the tree of knowledge with her right hand.
A map showing European borders in 1648
The Holy Roman Empire in 1648
Allegory of the Peace of Westphalia, by Jacob Jordaens.

Scholars of international relations have identified the Peace of Westphalia as the origin of principles crucial to modern international relations, collectively known as Westphalian sovereignty, though other historians argue that this is largely a myth invented after the fact.

The Hittite version of the Treaty of Kadesh, among the earliest extant examples of an international agreement.

International law

Set of rules, norms, and standards generally recognized as binding between nations.

Set of rules, norms, and standards generally recognized as binding between nations.

The Hittite version of the Treaty of Kadesh, among the earliest extant examples of an international agreement.
Hugo Grotius' De jure belli ac pacis, is considered one of the foundational texts of international law. (Pictured is the title page from the second edition of 1631).
A portrait of the Dutch jurist Hugo Grotius (alias Hugo de Groot)
Sir Alberico Gentili is regarded as the Father of international law.
The First Geneva Convention (1864) is one of the earliest formulations of international law

International law differs from state-based legal systems in that it is primarily—though not exclusively—applicable to countries, rather than to individuals, and operates largely through consent, since there is no universally accepted authority to enforce it upon sovereign states.

As D. W. Greig notes, "international law cannot exist in isolation from the political factors operating in the sphere of international relations".