The development ladder (five dates minimum): Antiquity's fragments: treaty practice and diplomatic immunity in the ancient Near East, Greece (ius fetiale, amphictyonic councils) and India (Kautilya's inter-state rules) — rules of inter-polity conduct, but no system among sovereign equals. Grotius (1583–1645) and the natural-law era: the Thirty Years' War's wreckage produced Hugo Grotius, De Jure Belli ac Pacis (1625) — the "father of international law": law between states grounded in natural reason, binding even in war. Precursors: Vitoria, Gentilis, Suárez.
Westphalia (1648): the sovereignty settlement — a Europe of territorially sovereign, legally equal states — international law's political matrix. The positivist 18th–19th centuries: Vattel (The Law of Nations, 1758 — sovereign equality's classic); the Congress of Vienna (1815); the Hague Conferences (1899, 1907 — peaceful settlement, laws of war, the Permanent Court of Arbitration).
The 20th century — institutionalization and universalization: the League of Nations (1919); the United Nations (1945) with its force-prohibition; the ICJ; Nuremberg/Tokyo (individual criminal responsibility); decolonization universalizing the law (Nepal's UN admission, 1955, a chapter of this story); the rights and trade revolutions (UDHR 1948; ICCPR/ ICESCR 1966; WTO 1995); international criminal justice (the ad hoc tribunals; the ICC 1998/2002 — Nepal the first South Asian state party, 2006). The contemporary phase: globalization's legalization, fragmentation debates, non-state actors, cyber and AI frontiers, and great-power strain on the Charter order. Importance (why a law student must master it): international law now reaches into every municipal courtroom — treaty obligations (trade, human rights, environment), extradition and mutual legal assistance, the law of armed conflict, and the interpretive rule that statutes are read in conformity with treaties. For Nepal: a small state's primary shield and stage — the Charter's territorial-integrity guarantee, boundary diplomacy, the WTO, and the human-rights treaties are all international law at work.