The Thirty Years’ War was fought from 1618-1648 (Thirty Years!) in the Holy Roman Empire. It began as a conflict between Catholics and Protestants in Bohemia, but grew to involve Denmark, Sweden, and France. After the French began helping Gustavus Adolphus, the Protestant king of Sweden, the lines became blurry and the war became more about the balance of power in Europe than about religion. The Peace of Westphalia paved the way for France to become the dominant power in Western Europe and for the permanent decline of the Holy Roman Empire as a political institution. If you like this lecture, check out my other lectures for AP European History and Western Civilization!