Which war lasted from 1618 to 1648 and devastated much of central Europe?

The Thirty Years' War, which began as a Protestant-Catholic conflict in Bohemia and ended as a struggle over who would dominate Europe - Catholic France fighting on the Protestant side against Catholic Habsburgs, which tells you how much of it was really about religion by the end.

It killed a larger share of the German population than either world war. Estimates run to a third in some regions and much higher in others, mostly not in battle: armies of the period fed themselves by taking what they walked over, and the combination of requisitioning, displacement and typhus emptied whole districts. Mercenary forces were paid by plunder, which made devastation a payroll mechanism.

The Peace of Westphalia that ended it is usually credited with inventing the modern state system - sovereign states with exclusive authority inside their borders and no superior power above them. Historians have picked at that for decades, since the treaties themselves are mostly detailed territorial and religious settlements rather than a philosophical charter, but the principle that a ruler's religion is not a neighbour's business had to start somewhere, and this is where it is conventionally dated.