In which year did the Thirty Years' War begin?

The Thirty Years' War began in 1618.

The conflict began in the Holy Roman Empire amid deep religious and political tensions between Protestant estates and Catholic Habsburg rulers. The immediate crisis centered on Bohemia, where Protestant nobles resisted efforts by the Catholic Habsburg government to restrict their religious rights and political privileges.

The war is conventionally dated from 1618, especially after the Defenestration of Prague on 23 May. Bohemian rebels rejected Habsburg authority, and the uprising soon spread into a broader struggle involving German princes, Denmark, Sweden, Spain, and France. Although religion helped spark the conflict, dynastic rivalry, territory, trade, and the balance of power increasingly shaped its course.

The war lasted until the Peace of Westphalia in 1648. Some historians trace its background to earlier disputes, including the War of the Jülich Succession beginning in 1609, but 1618 is the standard and fair quiz answer. The conflict therefore lasted roughly three decades, although its phases and participants changed substantially over time.