I've been reading "The Thirty Years War" by Victoria Wedgwood for a bit now - and it's both eye opening and depressing. Eye opening because the war was somewhat different than I had thought.
My impression had been that it was purely a war of schismatic fanaticism - Catholics and Protestants having at it in central Europe. It looks to me like religion was more of an excuse than a cause in this case. Certainly various rulers imposed (or tried to impose) their religious sensibilities once they took over an areas; that's what Ferdinand (then Holy Roman Emperor) tried to do, stamping out Protestantism and trying to impose Catholicism. However, that was something he did in a secondary fashion - it looks to me, based on this book, that it had a lot more to do with dynastic ambition on his part.
Stir in the dynastic clashes of the Hapsburg (Austria/Spain) and Bourbon (France) families, and you got a mess. Various alliances switched in ways that would make a modern person's head swim, being more familiar with the post-Westphalia nation state system. Then you have to stir in the mercenary armies of the era, something we simply haven't seen in the West in hundreds of years.
It's all very depressing, because - to hear the author tell it - there were various points between 1618 - 1648 where the bleeding could have stopped, but various acts of omission prevented that.