The Thirty Years War was the result of religious strife between the Catholics and the Protestants (starting in 1618 and lasting "thirty years") because when Ferdinand II, who was a Catholic, "inherited the throne" of the Holy Roman Empire, the Protestant nobility revolted. Most of the fighting occurred in the area now known as Germany, but it spread throughout Europe (so that "almost all the countries of Europe were involved"), and the effects of the war spread as well. This included the massive destruction of farmland which would have destroyed the crops. Soon famine and then disease "arrived." The war and these consequences killed almost a third of the population of Germany.
The war is separated into four "phases," the last one ending with the signing of the Treaty of Munster. The process of a coming up with a treaty took several years, but what was most important with this treaty was that it ended the fighting in 1648.
There were many changes seen because of this war. Hiring mercenaries became less common; countries began to organize and depend upon their own armies. Boundary lines in Europe were more firmly established. Last, it was easier for people to find a separation of church and state in terms of where there loyalties were placed.
This war would then have been responsible for the four soldiers in The Hangman's Daughter by Oliver Pötzsch. As the war was over and mercenaries were not needed for battle, fighting men would still need to find a way to survive if they did not become a part of the state's military. In this way, these men would have been able (or found it necessary) to provide their services to nefarious sorts, like Matthias Augustin. It is also possible that the witch-hunts of seventy years before may well have been brought about due to famine and ensuing illness because of the Thirty Years War: the people would have been looking to blame some thing or, in this case, some one.