The Thirty Years' War was one of the longest and most destructive conflicts, as well as the deadliest European religious war in history. It took place in Central Europe between 1618 and 1648 and resulted in eight million casualties.
Initially a war between various Protestant and Catholic states in the fragmented Holy Roman Empire, it gradually developed into a more general conflict involving most of the great powers. These states employed relatively large mercenary armies, and the war became less about religion and more of a continuation of the FranceHabsburg rivalry for European political pre-eminence.
The war was preceded by the election of the new Holy Roman Emperor, Ferdinand II, who tried to impose religious uniformity on his domains, forcing Roman Catholicism on its peoples. The northern Protestant states, angered by the violation of their rights to choose that had been granted in the Peace of Augsburg, banded together to form the Protestant Union. Ferdinand II was a devout Roman Catholic and relatively intolerant when compared to his predecessor, Rudolf II, who ruled from the largely Protestant city of Prague. Ferdinand's policies were considered strongly pro-Catholic.
These events caused widespread fears throughout northern and central Europe, and triggered the Protestant Bohemians living in the then relatively loose dominion of Habsburg Austria to revolt against their nominal ruler, Ferdinand II. After the so-called Prague Defenestration deposed the emperor's representatives in Prague, the Protestant estates and Catholic Habsburgs started gathering allies for war. They ousted the Habsburgs and elected the Calvinist Frederick V, Elector of the Rhenish Palatinate as the new king of the Kingdom of Bohemia. Frederick took the offer without the support of the Protestant Union. The southern states, mainly Roman Catholic, were angered by this. Led by Bavaria, these states formed the Catholic League to expel Frederick in support of the Emperor. The Empire soon crushed this perceived rebellion in the Battle of White Mountain, executing leading Czech aristocrats shortly after. The Protestant world condemned the Emperor's action.
After the atrocities committed in Bohemia, Saxony finally gave its support to the union and decided to fight back. Sweden, at the time a rising military power, soon intervened in 1630 under its king Gustavus Adolphus, transforming what had been simply the Emperor's attempt to curb the Protestant states into a full-scale war in Europe. Spain, wishing to finally crush the Dutch rebels in the Netherlands and the Dutch Republic, intervened under the pretext of helping its dynastic Habsburg ally, Austria. No longer able to tolerate the encirclement of two major Habsburg powers on its borders, Catholic France entered the coalition on the side of the Protestants in order to counter the Habsburgs.