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Establishment of Westphalian system

The developments of the 17th century culminated at the conclusion of the Peace of Westphalia in 1648, which is considered the seminal event in international law. The resulting Westphalian sovereignty is said to have established the current international legal order characterised by independent nation states, which have equal sovereignty regardless of their size and power, defined primarily by non-interference in the domestic affairs of sovereign states, although historians have challenged this narrative. The idea of nationalism further solidified the concept and formation of nation-states. Elements of the naturalist and positivist schools were synthesised, notably by German philosopher Christian Wolff (1679–1754) and Swiss jurist Emer de Vattel (1714–1767), both of whom sought a middle-ground approach. During the 18th century, the positivist tradition gained broader acceptance, although the concept of natural rights remained influential in international politics, particularly through the republican revolutions of the United States and France.

Until the mid-19th century, relations between states were dictated mostly by treaties, agreements between states to behave in a certain way, unenforceable except by force, and nonbinding except as matters of honour and faithfulness. One of the first instruments of modern armed conflict law was the Lieber Code of 1863, which governed the conduct of warfare during the American Civil War, and is noted for codifying rules and articles of war adhered to by nations across the world, including the United Kingdom, Prussia, Serbia and Argentina. In the years that followed, numerous other treaties and bodies were created to regulate the conduct of states towards one another, including the Permanent Court of Arbitration in 1899, and the Hague and Geneva Conventions, the first of which was passed in 1864.