In the century after Luther's theses, Europe killed its way through the question of whose Christianity was correct. The German Peasants' War of 1525, which Luther himself urged the princes to crush. The French Wars of Religion — eight successive civil wars between 1562 and 1598, punctuated by the 1572 St. Bartholomew's Day Massacre. The eighty-year Dutch Revolt against Spain. The Thirty Years' War (1618–48), which killed perhaps a third of the German-speaking population and reduced some regions to famine and cannibalism. By the time the smoke cleared at Westphalia, Europeans had concluded — exhaustedly, not philosophically — that living under a different denomination from your prince was preferable to extermination.
The wars of religion are the canonical case for the proposition that modern liberal toleration of religious difference is a political solution to a military problem, not an ethical achievement. The Reformation had shattered the assumption that one realm meant one church, and into that fracture poured a century of armed confessional ambition: Habsburg attempts to re-Catholicise the Empire, Calvinist princes defending the Palatinate, a French monarchy torn between Guise and Bourbon, Spain bleeding itself white in the Low Countries. After the Thirty Years' War, no major European power ever again committed itself to wholesale war for confessional reasons; the costs — depopulated provinces, bankrupt treasuries, armies under condottieri like Wallenstein that devoured the lands they were sent to defend — had been too obviously unsupportable. Toleration entered first as pragmatic mutual recognition between Catholic and Protestant princes, codified in formulas like the 1555 cuius regio, eius religio, which let a ruler choose his territory's faith but said nothing of his subjects' conscience; only later, slowly, did dissent become a positive right rather than a grudged exception. France produced the most explicit instance: in 1598 Henri IV, who had converted from Calvinism to end the wars, issued the Edict of Nantes, granting the Protestant Huguenots a measure of toleration and fortified safe havens — and the truce's fragility was confirmed when Louis XIV revoked it in 1685, driving the Huguenots into exile. The sack of Magdeburg in 1631, where some twenty thousand died, the witchcraft persecutions, the long Catholic-Protestant border violence in the British Isles — these are the events that secular liberalism is, in retrospect, an insurance policy against. The structural lesson outlasted the theology: a polity that cannot agree on ultimate ends can still agree to stop killing over them, and that minimal agreement is the seed of the secular state. People who think religious toleration is fragile in the modern West are correct, but the fragility is built in: it was an exhausted truce, not a metaphysical settlement, and a truce can be unlearned.
Every contemporary debate about religious accommodation, secular public space, and the rights of minorities sits on a substrate that was built out of a century of European blood. The substrate is not as durable as we sometimes assume; it depends on a living memory of what the alternative costs. Sectarian violence has not disappeared from history; it has only relocated — to partitions, to civil wars, to the places where one community's certainty about salvation still outranks its neighbour's right to exist.