The Thirty Years War was indeed a very long conflict involving the great majority of European nations ( except Russia and England which however worked with the Protestant nations and France against the Habsburg Empire and its allies ).

The war was long, terrible and of course very expansive. At some points, many nations used mercenaries to fulfill their goals but were unable to pay them.

An army ( regular or of mercenaries) that is not paid is unable to properly equip themselves and feed themselves. Their only solution was to take themselves their food and payment. At first it may stealing the crops but come winter they looted the barns and granaries, stole people. Most of these men had been unpaid and on the road for a long time, I imagine they were quite enraged and so horrors were committed on the population. First those unprotected in the countryside but soon even the cities were vulnerable to these groups of men.

Add to that some severe winters and catastrophic harvests.

In May 1631, around 24,000 men from the Catholic League lead by Gottfried Heinrich von Pappenheim arrived at the cities of Magdeburg in actual Saxony-Anhalt, Germany. The city was defended by a bit more than 2,000 men but at the end of the day, maybe more than 20,000 civilians were butchered. Some regions, especially in Germany but also in actual France have been devastated by the passage of troops. Rarely this had been seen before as the point when you captured regions was to preserve them the best you could for it to be useful in your economy later. However, there the point was not to capture territory but it was about large armies and mercenaries roaming Europe sacking, butchering, raping and looting.

Months ago, I heard a radio show in French which was about the Franche-Comté region in actual France. I can't remember which city it was but one of the historian guest mentioned that the city lost some 80% of its population in less than a year due to sacking and starving.

Antoine Jacquot, priest of the Val d'Ajol who was a witness of the war wrote : " In this year 1635, from Easter to the Saint Remy, the whole country ( region ) was full of German, Hungarian, Croatian and other soldiers pursuing the French and Swedes coming back from Germany. The region was so devastated that only some people of the parish remained while the others wandered in the forests and impenetrable woods. "

This is a part of Abbot Paul Idoux's book published soon before the First World War named " The Annals of the society of the Vosges " . Very sorry about the translation of the book's name but I can't really find how to translate that ( original title : " Les Annales de la société d'émulation du département des Vosges " )

Idoux adds that : " The Swedes, ferocious allies of the French crushing the Lorraine, left a horrific memory. There are no abominations they have not committed. To their ferocity, they added a deep anti-Catholic fanaticism which lead them to commit every sin. They strove against the churches and religious symbols. For them, it was less a French war against Lorraine than a war of religion where the Reformed could devastate a Catholic land. "

I hope this helps you. While doing my genealogy, I found a document stating the presence of Swedish troops then Imperial soldiers near Dijon, Burgundy, France and they committed quite horrible actions there. Many German regions were completely depopulated ( killed, starved or fled ) such as parts of the Palatinate. So far as I know, this war ( or series of wars ) was one of the first where populations were so much targeted to that extent and in so many places at the same time.

Sorry for my English mistakes and maybe the more or less poor translation I made of the quotes.