The Thirty Years' War was a religious war principally fought in Germany, where it involved most of the European powers. The conflict began between Protestants and Catholics in the Holy Roman Empire, but gradually developed into a general, political war involving most of Europe. The Thirty Years' War was a continuation of the France-Habsburg rivalry for European political pre-eminence, and in turn led to further warfare between France and the Habsburg powers.

Its outbreak is generally traced to 1618 when Emperor Ferdinand II was deposed as king of Bohemia and replaced by the Protestant Frederick V of the Palatinate in 1619. Although Imperial forces quickly suppressed the Bohemian Revolt, his participation expanded the fighting into the Palatinate, whose strategic importance drew in the Dutch Republic and Spain, then engaged in the Eighty Years' War. Since rulers like Christian IV of Denmark and Gustavus Adolphus of Sweden also held territories within the Empire, this gave them and other foreign powers an excuse to intervene, turning an internal dynastic dispute into a European-wide conflict.

The first phase from 1618 until 1635 was primarily a civil war between German members of the Holy Roman Empire, with support from external powers. After 1635, the Empire became one theatre in a wider struggle between France, supported by Sweden, and Emperor Ferdinand III, allied with Spain.

The war concluded with the 1648 Peace of Westphalia, whose provisions reconfirmed "German liberties", ending Habsburg attempts to convert the Holy Roman Empire into a more centralised state similar to Spain. Over the next 50 years, Bavaria, Brandenburg-Prussia, Saxony and others increasingly pursued their own policies, while Sweden gained a permanent foothold in the Empire.