The Thirty Years’ War from 1618 to 1648 produced an amazing amount of casualties. It raged between small fiefdoms of Protestants and Catholics swirling around the Holy Roman Empire or Western Rome. Eventually major powers entered the war, Spain, Sweden, Dutch forces, Germany, Saxony, Bavaria, and France all poured resources and soldiers into this war, and it as fought by people who were grimly determined that they had a monopoly on the truth. To many it began to look like a reprisal of the France Hapsburg Wars. Ferdinand I tried to accommodate communion for Catholics in Germany and Bohemia, but he failed even to keep these two combatants apart.
The fact of the matter was complex. There were thousands of fiefdoms loosely allied with either the Roman Catholic Church or the Protestant Reformers. All of these principalities were under the suzerainty of the Hapsburgs in Austria. The Thirty Years War was about independence, it was about the freedom to worship outside of the official religion of a state. Certainly the word protest is the root of Protestantism and this was a long violent protest against the monarchs in the pocket of the Pope.
So much territory was purloined by Protestant forces and purges of Catholicism. When the Catholic forces prevailed, they took control of Protestant territories and tried to undo years of policy with mandates made on the fly. What was left behind this tapestry of governance was a mess that would be managed only when the Protestant movement grew less violent and the newly emerging Lutherans became the alternative to Catholicism. The Lutherans, unlike the Protestants, were willing to compromise.
It is one of the most complex wars ever fought and it resulted at the low estimate in 4 million deaths. At the high end, it killed 12 million people mostly through disease and famine. The Thirty Years War had lasting effects as it killed as many people, industry suffered and so did all the infrastructure, such as it was, in Western Europe.
Plague was carried by soldiers from one nation into others. Famines resulted from lack of man power. So many had deid from war and disease, infrastructure suffered. What made the Thirty Years War particularly lethal was that plague that ravaged the countryside of Western Europe, and the famines that starved millions simultaneously.