The Thirty Years' War (1618–1648) was one of the most destructive conflicts in European history. The war was fought primarily in what is now Germany, and at various points involved most of the countries of Europe. Naval warfare also reached overseas and shaped the colonial formation of future nations.
The origins of the conflict and goals of the participants were complex, and no single cause can accurately be described as the main reason for the fighting. Initially, the war was fought largely as a religious conflict between Protestants and Catholics in the Holy Roman Empire, although disputes over the internal politics and balance of power within the Empire played a significant part. Gradually, the war developed into a more general conflict involving most of the European powers. In this general phase, the war became more a continuation of the Bourbon–Habsburg rivalry for European political pre-eminence, and in turn led to further warfare between France and the Habsburg powers, and less specifically about religion.
A major impact of the Thirty Years' War was the extensive destruction of entire regions, denuded by the foraging armies (bellum se ipsum alet). Episodes of famine and disease significantly decreased the populace of the German states, Bohemia, the Low Countries and Italy, while bankrupting most of the combatant powers. While the regiments within each army were not strictly mercenary in that they were not guns for hire that changed sides from battle to battle, the individual soldiers that made up the regiments for the most part probably were. The problem of discipline was made more difficult still by the ad hoc nature of 17th-century military financing. Armies were expected to be largely self-funding from loot taken or tribute extorted from the settlements where they operated. This encouraged a form of lawlessness that imposed often severe hardship on inhabitants of the occupied territory. Some of the quarrels that provoked the war went unresolved for a much longer time. The Thirty Years' War was ended with the treaties of Osnabrück and Münster, part of the wider Peace of Westphalia.
Notable battles are as follows: