The Thirty Years' War (1618–1648) was fought primarily in what is now Germany, and at various points involved most countries in Europe. It was one of the longest and most destructive conflicts in European history. The conflict lasted, unceasing, for 30 years, making it one of the longest continuous wars in modern history.

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.