The Thirty Years' War was one of the longest and most destructive conflicts in European history, lasting from 1618 to 1648. Fought primarily in Central Europe, an estimated 4.5 to 8 million soldiers and civilians died as a result of battle, famine, or disease, while parts of present-day Germany reported population declines of over 50%. Related conflicts include the Eighty Years' War, the War of the Mantuan Succession, the Franco-Spanish War, the Torstenson War, the Dutch-Portuguese War, and the Portuguese Restoration War.

The war was traditionally viewed as a continuation of the religious conflict initiated by the 16th-century Reformation within the Holy Roman Empire. The 1555 Peace of Augsburg attempted to resolve this by dividing the Empire into Catholic and Lutheran states, but over the next 50 years the expansion of Protestantism beyond these boundaries destabilised the settlement. However, while differences over religion and Imperial authority were important factors in causing the war, most contemporary commentators suggest its scope and extent were driven by the contest for European dominance between Habsburg-ruled Spain and Austria, and the French House of Bourbon.

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. Although Imperial forces quickly suppressed the Bohemian Revolt, Frederick's participation expanded the fighting into the Palatinate, whose strategic importance drew in the Dutch Republic and Spain, then engaged in the Eighty Years' War. Rulers like Christian IV of Denmark and Gustavus Adolphus of Sweden also held territories within the Empire, giving them and other foreign powers an excuse to intervene. The result was to turn an internal dynastic dispute into a broader European conflict.