The Thirty Years' War, fought primarily in Central Europe between 1618 and 1648, was one of the most destructive conflicts in European history. An estimated 4.5 to 8 million soldiers and civilians died from battle, famine, or disease, while parts of 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 had its origins in the 16th-century Reformation, which led to religious conflict within the Holy Roman Empire. The 1555 Peace of Augsburg attempted to resolve this by dividing the Empire into Catholic and Lutheran states, but the settlement was destabilised by the subsequent expansion of Protestantism beyond these boundaries. Combined with disagreements over the limits of imperial authority, religion was thus an important factor in starting the war. However, its scope and extent was largely the consequence of external drivers such as the French–Habsburg rivalry and the Dutch Revolt.
Its outbreak is generally traced to 1618, when the Catholic Emperor Ferdinand II was replaced as king of Bohemia by the Protestant Frederick V of the Palatinate. Although Ferdinand quickly regained control of Bohemia, Frederick's participation expanded fighting into the Palatinate, whose strategic importance drew in the Dutch Republic and Spain, then engaged in the Eighty Years' War. In addition, the acquisition of territories within the Empire by rulers like Christian IV of Denmark and Gustavus Adolphus of Sweden gave them and other foreign powers an ongoing motive to intervene. Combined with fears the Protestant religion in general was threatened, these factors turned an internal dynastic dispute into a European conflict.
The period 1618 to 1635 was primarily a civil war within the Holy Roman Empire, which largely ended with the Peace of Prague. However, France's entry into the war in alliance with Sweden turned the empire into one theatre of a wider struggle with their Habsburg rivals, Emperor Ferdinand III and Spain. Fighting ended with the 1648 Peace of Westphalia, whose terms included greater autonomy for states like Bavaria and Saxony, as well as acceptance of Dutch independence by Spain. The conflict shifted the balance of power in favour of France and its subsequent expansion under Louis XIV.
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