Thirty Years' War

The Thirty Years' War (1618–1648) began with the Defenestration of Prague in May 1618, when Bohemian Protestant nobles threw imperial regents from a castle window in protest against Habsburg Catholic policies. What started as a confessional revolt within the Holy Roman Empire escalated into a continent-wide struggle drawing in Denmark, Sweden, Spain, France, the Dutch Republic, and various German principalities.

Historians typically divide the war into four phases: the Bohemian (1618–1625), Danish (1625–1629), Swedish (1630–1635), and French (1635–1648) periods. Key turning points include the Battle of White Mountain (1620), the Edict of Restitution (1629), the intervention of Swedish king Gustavus Adolphus and his death at Lützen (1632), and France's open entry against the Habsburgs in 1635 under Cardinal Richelieu — a Catholic power siding with Protestants, signaling that raison d'état had overtaken confessional loyalty.

The war devastated Central Europe. Mortality estimates for the German lands range from roughly 15% to 30% of the population, through combat, famine, and disease compounded by mercenary armies that lived off the land.

It ended with the Peace of Westphalia in 1648, comprising the Treaties of Münster and Osnabrück. Westphalia is conventionally credited (though historians debate the extent) with establishing core principles of the modern state system:

• territorial sovereignty and cuius regio, eius religio extended to include Calvinism;
• legal equality among sovereign states;
• non-intervention in domestic affairs.

For IR students, the war is the standard reference point for the "Westphalian system" — a shorthand in realist and English School theory for a world of sovereign, territorially defined states. Constructivists and revisionist historians such as Andreas Osiander have challenged this narrative, arguing the "Westphalian myth" was largely a 19th- and 20th-century construction.