Thirty Years' War
The 1618–1648 catastrophe that began as a religious quarrel, killed perhaps a third of Germans, and ended in the Peace of Westphalia.
The Thirty Years' War was the deadliest religious conflict in European history and, by its end, barely a religious war at all. It ran from 1618 to 1648, began as a Catholic-Protestant quarrel inside the Holy Roman Empire, and metastasized into a continental struggle that drew in Denmark, Sweden, France, Spain, and the Dutch. In the German lands where most of it was fought, roughly a third of the population died. It ended with the Peace of Westphalia, leaving the Empire a hollow shell and Spain a spent great power.
A shove out a window
It started, literally, with a defenestration. On May 23, 1618, Protestant nobles in Prague, furious at Habsburg violations of their religious liberties, threw two Catholic imperial governors and their secretary out of a castle window some seventy feet up. All three survived the fall; Catholics credited angels, Protestants noted the dung heap below. The Second Defenestration of Prague lit the Bohemian Revolt. The rebels deposed the Habsburg Ferdinand II as king of Bohemia and elected the Calvinist Frederick V of the Palatinate in his place. Ferdinand crushed them at the Battle of White Mountain in 1620, and what might have stayed a local revolt instead pulled in outside powers one after another.
Four wars in one
Historians break the conflict into phases named for whoever carried the Protestant cause. The Bohemian phase (1618-1625) gave way to the Danish (1625-1629), when Christian IV of Denmark intervened and was beaten back by imperial armies. Then came the Swedish phase (1630-1635): Gustavus Adolphus, the formidable Protestant king of Sweden, landed in Germany, won a stunning victory at Breitenfeld in 1631, and was killed leading a cavalry charge at Lutzen in 1632. Finally the French phase (1635-1648), the longest and bloodiest, turned the war openly into a contest for power.
That last turn is the war's central irony. Catholic France, run by Cardinal Richelieu, first bankrolled Protestant Sweden and then, in 1635, declared war on Catholic Spain and the Catholic emperor outright. The point was no longer faith but raison d'etat: bleeding the Habsburgs who encircled France mattered more than confessional solidarity. Religion had become the pretext; the balance of power was the prize.
The killing machine
The armies that fought were mostly mercenaries, and the greatest of the entrepreneurs was Albrecht von Wallenstein, a Bohemian who raised private armies of tens of thousands and funded them by extorting "contributions" from whatever lands they occupied. War fed on war. When Wallenstein grew too powerful and too independent, the emperor had him assassinated in 1634.
The signature atrocity came at Magdeburg. In May 1631 imperial and Catholic League forces stormed the Protestant city; fire and massacre killed around 20,000 people, most of its population, and the sack became a byword across Europe for total destruction. But the real killers were not swords. Marauding armies spread famine and disease, typhus, plague, and dysentery, across a countryside they stripped bare. Whole regions lost half their people. The German population fell from perhaps 18 to 20 million to around 12 million.
What it left behind
By 1648 everyone was exhausted, and the Peace of Westphalia ratified the result: a decentralized Empire whose princes were now nearly sovereign, an independent Dutch Republic and Switzerland, and a France ascendant. Spain fought France alone for another eleven years before conceding at the Pyrenees in 1659; its long decline dates from here. The war became Europe's benchmark catastrophe, the disaster later generations reached for whenever a conflict of belief threatened to consume a whole region. It is remembered less for its battles than for what it taught: that holy wars have a way of curdling into naked power politics, and that the bill is paid by the people on the ground.