Which empire was the main battleground of the Thirty Years' War?

The Holy Roman Empire was the main battleground of the Thirty Years’ War.

The conflict began in 1618 with the Bohemian Revolt, when Protestant nobles in Bohemia rebelled against the Catholic Habsburg ruler Ferdinand II. Fighting then spread across the empire’s many principalities, bishoprics, and free cities, which together occupied much of central Europe. The war involved both religious disputes and struggles over imperial authority and international power.

Although Denmark, Sweden, France, and Spain intervened, much of the destruction occurred inside the Holy Roman Empire. Armies repeatedly crossed German lands, besieged cities, requisitioned food, and devastated rural communities. Major campaigns took place in Bohemia, Saxony, Bavaria, the Rhineland, and northern Germany.

The war was not simply a conflict between Catholics and Protestants. Catholic France eventually fought against the Catholic Habsburgs to prevent their dominance, while Protestant and Catholic German rulers pursued their own political interests. The Peace of Westphalia in 1648 ended the fighting and confirmed the autonomy of many imperial states. The Ottoman and Spanish empires were involved in the wider balance of power, but neither was the principal battlefield.