During the Reformation, the many German-speaking states within the Holy Roman Empire were plunged into religious wars. Some princes remained loyal to the Roman Catholic Church. Others accepted the teachings of Protestant reformers like Martin Luther or John Calvin. By the early 1600s, war raged across much of the Holy Roman Empire.
At the Battle of White Mountain in 1620, forces led by the Holy Roman emperor destroyed the Bohemian army.
By early modern times, as the French philosopher Voltaire later observed, the Holy Roman Empire was neither holy, nor Roman, nor an empire. Instead, by the seventeenth century it had become a patchwork of several hundred small, separate states.
In theory, these states were ruled by the Holy Roman emperor, who was chosen by seven leading German princes called electors. In practice, the emperor had little power over the many rival princes. This power vacuum contributed to the outbreak of the Thirty Years' War.
Religion further divided the German states. The north had become largely Protestant, while the south remained Catholic.
The Thirty Years' War was actually a series of wars. It began in Bohemia, the present-day Czech Republic. Ferdinand, the Catholic Hapsburg king of Bohemia, sought to suppress Protestants and to assert royal power over nobles.