The Protestant Reformation had far reaching political consequences for Western Europe, perhaps more far reaching than were its religious consequences.
Germany, where the Reformation began was part of the Holy Roman Empire. The Holy Roman Emperor considered it his mandate to protect the Roman Catholic Church and ensure its supremacy. With the outbreak of the Reformation, a number of German princes determined that they could separate themselves from the Emperor's control by allying with the Protestant movement, and did so. In France, civil wars frequently broke out between Calvinist (Huguenot) forces and Catholic forces loyal to the King. As a result, a number of catastrophic wars broke out in Europe, primarily the Wars of Religion and the Thirty Years War. Both were fought over a combination of religion and politics. In France, Huguenots were lured to a meeting on St. Bartholomew's day with Catholics to discuss a peace settlement and told not to bring their weapons. They complied and were slaughtered in the St. Bartholomew's Day Massacre.
Interestingly, the Wars of Religion ended with an agreement that the ruler of each area would determine that area's religion.