Warmatrix

Civilians ambushed outside a town, by Sebastiaan Vrancx

The Thirty Years' War, ranging from 1618 CE to 1648 CE, was not a single conflict, but a complex series of wars in Europe. On the surface the main cause seemed to be religious strife, though in reality political and economic factors were more important. It was one of the most destructive wars in European history.
The rise of protestantism brought much strife against catholicism, especially in Germany, where the former originated. The Peace of Augsburg in 1555 CE temporarily ended the hostilities, but did not solve underlying problems and bit by bit unraveled. When Ferdinand I, the catholic Holy Roman emperor, tried to suppress protestantism, some of his representatives were thrown out of a window in Prague by angry nobles. This was the start of the Bohemian revolt. Other regions and foreign powers like England and the Dutch republic joined in on the protestant side and most of Austria for the catholics. The Ottoman empire, sensing an opportunity, attacked Austria and next got into conflict with Poland. The protestants suffered a heavy defeat at the Battle of White Mountain in 1620 CE, forcing Bohemia back into the catholic camp.
The fight continued in other theaters. In the Netherlands, the Twelve Years' Truce between Spain and the Dutch republic expired, re-opening the Eighty Years' War. In France there were rebellions by the Hugenots, who after a spell of peace were increasingly repressed by king Louis XIII, who tried to unify France by force. Meanwhile in Germany, which was a hodgepodge of small states, some protestant and some catholic, the fighting continued too on a smaller scale. At this point the war was not as destructive as it would become later.
In 1625 CE Christian IV, king of protestant Denmark, intervened to expand its influence in northern Germany. Ferdinand II, the new Holy Roman emperor, employed Albrecht von Wallenstein, a Bohemian nobleman made rich by war, who raised a vast army. In this year, the total number of soldiers employed in the empire rose from some 80,000 - 100,000 to 160,000 men. Wallenstein conquered his way into Denmark, however the costs started to go beyond the profits and both sides agreed to peace in 1629 CE. One of the conditions was that Denmark would become catholic again.
All countries involved in the war, especially after Wallenstein's rise in 1625 CE, made heavy use of mercenaries, who received little pay and compensated by looting and plundering on a large scale. Many states fought wars of attrition and tacitly encouraged this, hoping to wear their opponents out. This made the Thirty Years' War one of the first total wars, which affected the civilian population as much as the armies. The former vented their anger in revolts, pogroms and witch-hunts, to no avail. The extend of the war was so great that Germany, the main battlefield throughout the conflict(s), lost 15% - 20% of its population, less than is often thought, though some areas suffered much more than others. The murders and hunger were aggravated by several waves of plague and typhus, which were spread by the traveling armies. Disease killed far more people than fighting and plundering, though these contributed to the spread of the former.
After the Danish adventure, both sides were weary and peace was possible. But then Gustavus Adolphus, king of Sweden, took up the protestant cause. His country had suffered from Denmark earlier though had rebuilt and modernized its army. The king landed in Pomerania in 1630 CE, fought his way half through Germany and scored several victories, especially at Breitenfeld and Lützen. However in the latter battle, despite the Swedish being victorious, he was killed. Over the next few years the Swedes were pushed back to the north and in 1635 CE the Peace of Prague was made.
The peace did not suit Louis XIII, who was catholic but considered the Habsburgs too powerful. The French declared war on Spain and the Holy Roman Empire. They were initially successful, then driven back beyond their own borders At Rocroi in 1643 CE they scored a great victory that halted the counterattacks. Menwhile Spain faced a rebellion in Catalonia, Portugal regained its independence, the Dutch took Limburg and the Swedes under Lennart Torstenson re-asserted themselves. But the anti-Habsburg party could not sustain their successes. In 1643 CE all sides were worn out, though it still took them five years to conclude the Peace of Westphalia. During this time the fighting went on as before, all sides trying to make gains to be able to negotiate from a position of power.
After the Swedish intervention in 1630 CE the total number of men under arms first rose further to around 250,000. The war became more destructive and some areas were severely depopulated. Where humans died, the number of rodents and wolves rose. Recruitment got more difficult and in the later years of the war the size of the armies decreased again. When the French got involved, the fighting switched to other areas of Germany and those that had suffered earlier could recover somewhat.
Several factors made the war bigger and last longer than earlier wars. Financing through (forced) taxation instead of plunder and more tight financial control from the royalty made more money available, so armies grew larger. However the average skill level of the soldiers was low, as the weapons used did not require much skill. This meant that if an army was defeated, a new one that almost as strong could be recruited quickly. Most of the extra money went to increase numbers, not the quality of the soldiers or their life. Many were not paid promptly; instead their employers built up large debts to them. They were hoping that many would die before the war was over, in which case payment could be forfeited, which happened in many cases. The overdue soldiers' wages were an incentive for the royalty to continue the war instead of ending it, even after all parties were exhausted.
Though most victories in the war were offset by subsequent defeats, the fighting did re-arrange the power balance in Europe. The Holy Roman Empire got more decentralized, but it was no longer the Medieval patchwork of old; a proto nation-state appeared. The feeling of nationality was also strengthened in other countries and the role of religion in politics declined. Spain lost power while Sweden and the Dutch republic gained it. England, Brandenburg-Prussia and especially France emerged even stronger, positioning themselves for dominance in the 18th century CE.
The Westphalian Congress was groundbreaking in establishing a platform where states, not just rulers, negotiated on equal footing. It was still pre-modern; reputation and honor were considered just as important as political business. Though the Peace of Westphalia ended the Thirty Years' War and the Eighty Years' War was concluded in the same year, Europe did not remain free of conflict. Powerful absolutist monarchs fought each other on and off for about 1½ century more, until the French revolution and Napoléon finished the ancién regimes.