Hey, I made a post a while back touching on this. I'll paste the relevant parts.

There is great writings about this very topic by a German mercenary by the name of Peter Hagendorf. He wrote a great account of his time fighting as a mercenry in the 30 years war. While all wars are in some way said to bring out the worst in mankind, the Thirty Years war, as Historian Peter Wilson writes “became a benchmark of brutality for centuries”. Considered by Germany as worse than both World Wars and the Black Death as the greatest disaster to ever hit their country – It goes without saying that the Thirty Years war holds a high watermark as to the sheer disregard for human life and the amount of blood shed. We can see this reflected throughout the Writings of the Mercenary, Peter Hagendorf. Written with a sense of disassociation not uncommon in military men who have seen their fair share of cruelty on the battlefield, he describes the scene in which a village of what he estimates to be a thousand peasants burnt to the ground “The peasants in the churchyard there resisted so forcefully that we could achieve nothing without cannons. Thus we went back, for there were one thousand in there. Yet we set fire to their village and let it burn” - This was not, as we can see from other historical accounts, an isolated event, as by the end of the war, Swedish armies alone would have laid waste to 2,000 castles, 18,000 villages and 1,500 towns in Germany – One third of all German towns that existed at the time.

This raw violence could perhaps, at least in part, simply be due to the magnificent vastness of the armies involved in the fighting. In one of Hagendorf’s accounts, he describes a battle in which the French army, 70,000 men strong arrived to meet his, which in turn stood at 120,000 thousand. It is evident then, with such epic fielding of armies rarely seen before this time, at least with this amount of frequency, how 25-40% of the population in the German territories were wiped out over the course of the war. This violence naturally had a significant effect on the psyche of the men who fought in these monumental armies. Again, taking from Hagendorf’s writings, many soldiers took to alcohol to, at least temporarily, forget about the death and destruction occurring on a daily basis around them. In the writings of the Mercenary, barring the mentions of interactions with his family, consumption of alcohol is probably mentioned more than anything else the soldier did in his spare time. Instances of “Getting well and truly drunk”, being too hungover to keep up with his unit along with a general infatuation with wine are very evident in the document. One can deduce then that, alcohol was a major factor in allowing the men of these armies to hold their nerve, or at least, dissipate the natural feelings of panic, fear and guilt one would most likely suffer in the conditions of this war.

This is not to say that Hagendorf did not partake in some of the actions that makes this war stand out as being notably brutal. He accounts of entering villages after successfully sacking them to take ‘booty’ – Such as in the Spring of 1634 where his spoils were “A pretty Maiden and 12 talers’ worth of old, clothing and linens” and in October of 1634 wherein he “Led away a young girl, but I let her go again […] Which I often regret, because I had no Woman at this point” Hagendorfs plundering was by no means exclusive to him, as it was the standard for armies attack a town/village to plunder, rape and generally take what they wanted, but it is through Hagendorf’s accounts we can see the mindset of the average soldier partaking in this pillaging, where it seems that the victorious soldiers held a sense of entitlement to anything or anyone in the unfortunate village being ransacked, and this was their reward for a hard fought battle.

Edit: Formatting.

The 30 years war in my opinion is one if the most fascinating events in world history from how it changed western politics, how religion was used as pretext rather than a motive (I could go on) but, for how it killed so many people? I just want to zero in on a few characters. Gustavus Adolphus and Albrecht von Wallenstein. I would love to just give a full account of the battle of lutzen right now but, that's for another time. Both men had genius of a kind, Adolphus in the means and tactics of death and Wallenstein in the funding of it.

Wallenstein had the idea that war was unfair to the loser's so in order to spread the cost around to be fair he decided to charge what he called 'war taxes' on allied towns, while pillaging enemy towns was common place. This lead to Wallenstein being feared and hated not just by his enemies but by his peers and lords. He was actually dismissed by King Charles at one point for fear he and his army would establish an independent state. None the less he was far to effective and was eventually called back to the conflict.

Adolphus (Thank Hitler for ruining a perfectly good name) was a battlefield genius and his core of men was one of the most effective fighting forces around at the time. One of his innovations was fighting with mixed units combining infantry, calvary, and small caliber artillery in discrete mobile units. This made his force much more agile than any other army on the field. Even his death on the field did not ruin his army, their discipline in the midst of battle allowed them to win the battle of lutzen even after he had fallen.

Of course these are just two commanders from the middling years of the war. They were both responsible for 10's of thousands of casualties and my brief synopsis of them does neither real justice. I just wanted to interject a bit of the human element when trying to understand a conflict with such an absurd and catastrophic effect on Europe.

Is it true that the catholic church accepted polygamy so the birth rate could get up after such a reduction in population numbers?