Was reading an IR book called "How the World Works" by Russell Bova, and he says:
The cost of that war was very high. As many as 400,000 soldiers and millions of civilians perished. One estimate is that the population of Germany fell 69 percent, from 13 million to 4 million, and of 35,000 villages in Bohemia, only 6,000 remained after the war.
While I realize this may be reductionist in that "Germany" was hard to define, I also realized I don't know much about the cause of the war, the belligerents, or what the casualties were. Help?
Also, he sources "Peace and War: Armed Conflicts and International Order 1648-1949" by Kalevi J. Holsti for the claim.
/u/bemonk ´s explanation on how the Bohemian Revolt started is mostly correct, but the reason for the escalation is wrong, but we have to start earlier to explain that.
Bohemia elected a king when it declareded independence, they chose Friedrich of the Palatinate, one of the seven electors. Ferdinand of Habsburg, up to that point King of Bohemia and most likely candidat for the Kaiser title was elected Kaiser at the same time (even with Friedrichs vote). So Friedrich now tried to ursup Bohemia from the Kaiser himself.
Ferdinand was now at war, but he was completly broke and Austria was plagued by revolts. He turned to Maximilian of Bavaria, who agreed to supply him with money and troops through the Catholic League, in return for later payment either through money or land (Maximilan was fully aware that the Kaiser could never repay him, so it had to be land).
After the Bohemian revolt was crushed, Ferdinand had to find a way to repay Maximilan. He tried to give him the Palatinate, but the HREs constitutin forbid this. Ferdinand could not care, broke the constitution and repayed Maximilian through land. This blatant violation would evoke new found sympathies among the protestant rulers, who now feared for their lands, not to mention France, who feared a complete surround by Habsburg.
The war could have been over after Bohemia was defeated, but the stuborness of the 3 main actors, Friedrich, who refused to give up his claim for Bohemia, Ferdinand, who broke the constitution and Maximilan who lusted for an electorate, made the war escalate from regional conflict to a European war.
Souces:
Wegdwood, Veronica, The Thirty Years War
Lectures on the war
Many peripheral causes, but I'd argue the main one were the events in Bohemia (modern day Czech Republic).
All of Europe was by this time split between Catholic and Protestants. One or the other group being highly discriminated against in almost all of Europe.
Bohemia, in contrast was quite tolerant of both. The Austrian aristocracy was mostly Catholic, while most of the population was Protestant. But in a way Bohemia was a microcosm of the rest of Europe in a tiny space.
A minor uprising of Protestants turned into a 2 year rebellion. (the famous 2nd Defenstration of Prague in 1618) With Bohemian noblemen (protestants) trying to get independence from Austrian rule (all still under the Holy Roman Empire at this point, but the very Catholic Hapsburgs already in charge).
The Austrians finally squashed the rebellion at the battle of White Mountain (near Prague) in 1620. After some months of the leaders being in dungeons they were very publicly mutilated and executed in 1621. Representatives from most major kingdoms in Europe were present, their heads were hung from one of the towers of Charles Bridge. (very game of thrones)
The problem was that the leaders were also noblemen, and for the other Protestant countries of Europe this was enough to go to war.
So the scope of the war was all of Europe, with most of the fighting being in central Germany and northern/western Bohemia.
There's a reason this was the last great mercenary war. Many of Europe's armies congregated in the center of Europe for the war, they were far from home and didn't really care about atrocities (since they were far from home, and so were their enemies), and that goes double for mercenaries.
Whole villages were massacred, raped, plundered to the last grain. Which is why the population as a whole fell, not just the fighting men, but also women and children.
The figures I've heard (I don't have my books in front of me, and I'm not going to bother with wikipedia, since someone will come in with better numbers I hope) is an overall population reduction
Mercenaries became much less important after that war.
Sorry about the lack of sources, this is basically the narrative I tell on the Bohemican podcast and as a former tour guide.. I hope I didn't simplify things too much.