The Thirty Years War occurred for a number of reasons, although it is principally a religious conflict between protestants and catholics. The Defenestration of Prague, although relatively trivial in itself, was to become a defining moment. The will for the preservation of the Hapsburg empire is also central.
The election of the catholic zealot, Ferdinand, Archduke of Styria, as King of Bohemia caused the Bohemian protestants to fear for their religious freedom and in May 1618, at Hradcany castle, two catholic councillors were thrown from the castle windows. Soon the Bohemian conflict erupted in the entirety of Greater Bohemia, effectively Bohemia, Silesia, Lusatia and Moravia, which was already riven by conflict between catholics and protestants. This confrontation was to find many facets and mirrors across the continent of Europe with the involvement of France, Sweden, inter alia.
The Battle of the White Mountain, near Prague in 1620 was a serious blow to protestant ambitions in the region. The rebellion effectively collapsed and widespread confiscations of property and suppression of the pre-existing Bohemian nobility ensured that country would return to the Catholic fold after more than a century of Hussite and other heresy.
Had the Bohemian rebellion remained a purely Eastern European affair, the Thirty Years War would have been over in fewer than thirty months. Unfortunately, the weakness of both Ferdinand and of the Bohemians themselves led to the spread of the war to Western Germany.
Ferdinand had been compelled to call on his cousin, King Phillip IV of Spain for assistance. The Bohemians had called on the Calvinist Frederick IV of the Palatinate to be their King. The Spanish, seeking to outflank the Dutch in preparation for the soon-to-be-renewed Eighty Years War took Frederick's lands, the Palatinate, along the Rhine.
..to be continued..