A question from Yahoo! Answers:
What was the Thirty Years War fought over?
The Thirty Years’ War began as a conflict between Protestants and Catholics, but the real reasons were much more prosaic.
Spain (or, more precisely, the Hapsburg dynasty) sought to restore its hegemony over the Netherlands and possibly expand it into German states to unite most of Europe under its rule.
France (or, more precisely, the Bourbon dynasty) was surrounded (and felt threatened) by two countries (Spain and the Holy Roman Empire) controlled by the Hapsburg dynasty, and would side with anyone who wanted to weaken the Hapsburgs, never mind their religion; hence, France’s alliance with Protestants.
Sweden and Denmark each sought to expand south by gaining control over northern German states along the Baltic coastline. Sweden’s ambitions were partially necessitated by the fact that Swedish king Sigismund (who was also the king of Poland) was deposed in Sweden in 1599 by his uncle, Charles IX, but remained the king of Poland and continued his attempts to reclaim the Swedish throne until his death in 1632 (the series of wars originally started by Sigismund continued well into 1660s), so Sweden had to retaliate.