For a century after Luther, Europeans fought over religion, though rarely over religion alone. Dynastic ambition, noble factions, taxes and the balance of power were always tangled in. Yet contemporaries understood the wars as religious, and they killed and died believing that truth itself was at stake.

Augsburg 1555

In the Holy Roman Empire, Lutheran princes formed the Schmalkaldic League in 1531. Charles V defeated it in 1547 but could not restore religious unity. The Peace of Augsburg of 1555 let each ruler in the empire choose Lutheranism or Catholicism for his lands; subjects who disagreed could emigrate. Lawyers later summed up the rule as cuius regio, eius religio, "whose realm, his religion," a phrase coined decades afterward. The peace kept the empire largely calm for over sixty years, but it had a gap: Calvinists, the Reformed Protestants, were left out of the Peace of Augsburg, even as Reformed churches grew in the empire.

France

In France, Reformed Protestants, called Huguenots, grew to perhaps a tenth of the population by the 1560s, including many nobles. From 1562 to 1598 France endured eight rounds of civil war, driven by rival noble houses as well as by faith. The worst moment came in August 1572. Paris had gathered for the wedding of the Protestant Henry of Navarre to the king's Catholic sister, a match meant to seal a peace. On Saint Bartholomew's Day 1572, Catholic crowds and soldiers massacred thousands of Huguenots in Paris, and killings spread to other cities. Catholic Paris celebrated; Protestant Europe recoiled. Both sides produced martyrs, propaganda and assassins: two French kings, Henry III in 1589 and Henry IV in 1610, were killed by Catholic zealots.

A group of Catholic officials and writers, nicknamed the politiques, drew a different lesson. The politiques argued that civil peace mattered more than forcing religious uniformity. Henry of Navarre, heir to the throne from 1589, converted to Catholicism in 1593 to win his capital. Henry IV's Edict of Nantes of 1598 granted Huguenots freedom of conscience and limited rights of public worship, along with access to offices and a number of fortified towns. It was a truce, not an embrace of pluralism, and in 1685 Louis XIV revoked it, driving perhaps 200,000 Huguenots abroad.

The Netherlands

In the Low Countries, ruled by Philip II of Spain, heavy taxes, attacks on local privileges and the persecution of Protestants provoked a revolt in 1568. The northern provinces, in the Union of Utrecht of 1579, promised that each person should remain free in religion and that no one should be investigated over it. The Dutch Republic that emerged kept an official Reformed church but tolerated Catholics, Mennonites and Jews in practice, and became a refuge for printers and dissenters. Spain recognized Dutch independence only in 1648.

The Thirty Years' War

The empire's uneasy calm broke in Bohemia. The Thirty Years' War began in 1618 when Protestant nobles in Prague threw two royal officials and a secretary from a castle window, an act of defiance against their Catholic Habsburg king. All three survived the fall. The Bohemian revolt was crushed at the White Mountain in 1620, but the war widened as Denmark, then Sweden under Gustavus Adolphus, entered on the Protestant side. Gustavus Adolphus saw himself as a Protestant champion, but he also wanted control of the Baltic coast and its trade. Then came the clearest sign that faith was not the only motive: Catholic France, led by Cardinal Richelieu, allied with Protestant Sweden against the Catholic Habsburgs, putting reason of state ahead of confession. Armies lived off the land, and plague and hunger followed them. Many regions of Germany lost a third or more of their people, though estimates vary widely.

The treaties signed in Westphalia in 1648 ended the war. On religion they extended the Augsburg system to include the Reformed churches and fixed 1624 as the normative year for deciding which confession held each church property and territory, which limited rulers' power to force their subjects to change faith. Religion did not leave European politics, but after 1648 major wars were rarely fought to impose it.

Worked example: one city, two faiths

The German city of Augsburg itself shows the arrangement at work. After 1648 it was one of a handful of imperial cities run on parity: Catholic and Lutheran citizens shared public offices equally, and each group kept its own churches, schools and holidays. The historian Étienne François found that marriages across the line were rare and that the two communities lived side by side yet largely apart, divided by what he called an invisible border. Was this toleration? It was peace without much affection, built on careful legal rules rather than goodwill. Yet it held for generations, and it suggests a lesson for divided societies today: rules that guarantee each side's security can come first, and mutual respect, if it comes at all, can grow later.