The long, savage series of conflicts that followed the Protestant Reformation's splitting of Western Christianity, in which Catholics and Protestants slaughtered one another across Europe from roughly the 1520s to 1648 — the German wars of the Reformation, the French Wars of Religion, the Dutch Revolt, and the catastrophic Thirty Years' War. The killing ended not in the victory of one faith but in exhaustion. Many historians argue that out of that exhaustion came a gradual loosening of religious uniformity and the principle of state sovereignty, partly settled at the Peace of Westphalia in 1648.
For more than a century, Europe spent itself killing over how to worship God. When the Reformation shattered the unity of Western Christianity in the early 1500s, religion became a reason — and an excuse — to go to war. Catholics and Protestants fought across the German lands, across France, across the Low Countries, and finally in the all-consuming Thirty Years' War. In Paris in 1572, on St. Bartholomew's Day, thousands of Protestants were murdered in a few weeks. In the German lands, war, famine, and plague killed a large share of the population. The wars ended not because one faith won, but because both sides were too exhausted to keep fighting. Out of that exhaustion, many historians argue, came a bitter, world-changing lesson: rulers found there were limits to forcing one faith on everyone. The Peace of Westphalia in 1648 did not create modern religious freedom, but it normalized limited coexistence among recognized Christian confessions and became a symbol of state sovereignty.