<P> 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 . </P> <P> 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 . 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 this perceived rebellion in the Battle of White Mountain, executing leading Czech aristocrats shortly after . The Protestant world condemned the Emperor's action . </P> <P> 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 . </P> <P> The Thirty Years' War devastated entire regions, with famine and disease resulting in high mortality in 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 . </P>

Who did france support in the 30 years war