Excerpts from Books and Wikipedia
Fought primarily in Germany; [National Geographic Almanac, p. 137] "The war was preceded by the election of the new Habsburg 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 religious liberty, which had been granted in the Peace of Augsburg, banded together to form the Protestant Union. . . . 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 . . . 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. . . . 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." [Wikipedia]