The Thirty Years' War and Westphalia

In 1618, Protestant nobles in Bohemia threw two Catholic officials out a window in Prague, rejecting a Catholic king imposed on them by the Habsburgs. That act, the Defenestration of Prague, triggered a revolt that the Holy Roman Emperor could not contain, because the Empire was a patchwork of hundreds of mostly autonomous territories, roughly half Protestant, half Catholic, all armed and anxious about who would dominate. The war moved through four overlapping phases named for who joined to fight the Habsburgs: Bohemian, Danish, Swedish, and French. Each time one Protestant power was beaten back, another stepped in, because a Habsburg victory threatened the whole balance of the Empire.

The strangest turn was France. Catholic and ruled by a cardinal, Richelieu, France still feared being encircled by Habsburg power in Spain and Austria. So France funded Protestant Sweden and eventually sent its own troops against Catholic Austria, proving the war had stopped being about religion and become about Habsburg power versus everyone else. The fighting devastated central Europe. Armies lived off the land, plague and famine followed them, and some German regions lost a third to half their population between 1618 and 1648.

The Peace of Westphalia in 1648 ended the war by granting each prince within the Empire the right to set his own territory's religion, free of outside interference, and by recognizing the practical independence of hundreds of German states alongside the Dutch Republic and Switzerland. This did not just end one war. It retired the idea that Europe needed religious uniformity to have peace, replacing it with a system of formally sovereign states negotiating their interests, the balance-of-power diplomacy that shapes the centuries after 1648.