The Thirty Years' War was a war fought primarily in Central Europe between 1618 and 1648. One of the most destructive conflicts in human history, it resulted in eight million fatalities not only from military engagements but also from violence, famine, and plague. Casualties were overwhelmingly and disproportionately inhabitants of the Holy Roman Empire, most of the rest being battle deaths from various foreign armies. In terms of proportional German casualties and destruction, it was surpassed only by the period January to May 1945; one of its enduring results was 19th-century Pan-Germanism, when it served as an example of the dangers of a divided Germany and became a key justification for the 1871 creation of the German Empire.
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 European great powers. These states employed relatively large mercenary armies, and the war became less about religion and more of a continuation of the France–Habsburg 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, which had been granted in the Peace of Augsburg, banded together to form the Protestant Union. Ferdinand II was a devout Roman Catholic and much more intolerant than his predecessor, Rudolf II, who ruled from the largely Protestant city of Prague. Ferdinand's policies were considered strongly pro-Catholic and anti-Protestant.
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 (and also with the Holy Roman Empire) to revolt against their nominal ruler, Ferdinand II. After the so-called Defenestration of Prague deposed the Emperor's representatives in Prague, the Protestant estates and Catholic Habsburgs started gathering allies for war. The Protestant Bohemians 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 the perceived Protestant rebellion in the Battle of White Mountain, executing leading Bohemian aristocrats shortly after. Protestant rulers across Europe unanimously condemned the Emperor's action.
After the atrocities committed in Bohemia, Saxony finally gave its support to the Protestant 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. Habsburg Spain, wishing to finally crush the Dutch rebels in the Netherlands and the Dutch Republic (which was still a part of the Holy Roman Empire), 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.
The Thirty Years' War devastated entire regions, resulting in high mortality, especially among the populations of the German and Italian states, the Crown of Bohemia, and the Southern Netherlands. Both mercenaries and soldiers in fighting armies traditionally looted or extorted tribute to get operating funds, which imposed severe hardships on the inhabitants of occupied territories. The war also bankrupted most of the combatant powers.
The Dutch Republic enjoyed contrasting fortune; it was removed from the Holy Roman Empire and was able to end its revolt against Spain in 1648 and subsequently enjoyed a time of great prosperity and development, known as the Dutch Golden Age, during which it became one of the world's foremost economic, colonial, and naval powers. The Thirty Years' War ended with the Treaty of Osnabrück and the Treaties of Münster, part of the wider Peace of Westphalia. The war altered the previous political order of European powers. The rise of Bourbon France, the curtailing of Habsburg ambition, and the ascendancy of Sweden as a great power created a new balance of power on the continent, with France emerging from the war strengthened and increasingly dominant in the latter part of the 17th century.