The catastrophic conflict that devastated Central Europe from 1618 to 1648 -- beginning as a religious war between Catholics and Protestants and ending as a struggle between European great powers that established the modern system of sovereign states.
It started as a religious dispute in Bohemia and became the first modern war involving most of Europe's major powers. By the time it ended, a third of the population of some German states had died from battle, plague, and famine. What it produced -- the Peace of Westphalia -- became the foundation of the international order that still governs world politics.