The most devastating religious conflict in European history happened in the middle of the Holy Roman Empire; it ultimately dragged on for decades and saw the reduction of the population in the German Lands of between 20 – 40%. That conflict, the Thirty Years’ War, saw the most horrific acts of violence, the greatest loss of life, and the greatest suffering among both soldiers and civilians of any of the religious wars of the period.
Leading up to the outbreak of war, there was an uneasy truce in the Holy Roman Empire between the Catholic emperor, who had limited power outside of his own ancestral (Habsburg) lands, and the numerous Protestant princes in their respective, mostly northern, territories. As of 1618, that compromise had held since the middle of the sixteenth century and seemed relatively stable, despite the religiously-fueled wars across the borders in France and the Netherlands.
The compromise fell apart because of a specific incident, the attempted murder of two Catholic imperial officials by Protestant nobles in Prague, when the emperor Ferdinand II attempted to crack down on Protestants in Bohemia (corresponding to the present-day Czech Republic). Ferdinand sent officials to Prague to demand that Bohemia as a whole renounce Protestantism and convert to Catholicism. The Bohemian Diet, the local parliament of nobles, refused and threw the two officials out of the window of the building in which they were meeting; that event came to be known as the Defenestration of Prague - "defenestration" literally means "un-windowing."
The Diet renounced its allegiance to the emperor and pledged to support a Protestant prince instead. A flurry of attacks and counter-attacks ensued, ultimately pitting the Catholic Habsburgs against the German Protestant princes and, soon, their allied Danish king. The Habsburgs led a Catholic League, supported by powerful Catholic princes, while Frederick of the Palatinate, a German Calvinist prince, led the Protestant League against the forces of the emperor.
From 1620 – 1629, Catholic forces won a series of major victories against the Protestants. Bohemia itself was conquered by Catholic forces and over 100,000 Protestants fled; during the course of the war Bohemia lost 50% of its population. Catholic armies were particularly savage in the conflict, living off the land and slaughtering those who opposed them. The Danish king, Christian IV, entered the war in 1625 to bolster the Protestant cause, but his armies were crushed and Denmark was briefly occupied by the Catholic forces. This period of Catholic triumph saw the Emperor Ferdinand II issue an Edict of Restitution in 1629 that demanded the return of all Church lands seized since the Reformation – this was hugely disruptive, as those lands had been in the hands of different states for over 80 years at that point!
In 1630, the Swedish king, Gustavus Adolphus, received financial backing from the French to oppose the Habsburgs and their forces. Under the leadership of its savvy royal minister, Cardinal Richelieu, France worked to hold its Habsburg rivals in check despite the shared Catholicism of the French and Habsburg states. Adolphus invaded northern Germany in 1630, then won a major victory against the Catholic forces in 1631. He went on to lead a huge Protestant army through the Empire, reversing Catholic gains everywhere and exacting the same kind of brutal treatment against Catholics as had been inflicted on Protestants. In 1632, Adolphus died in battle and the military leader of the Catholics, a nobleman named Wallenstein, was assassinated, leaving the war in an ongoing, bloody stalemate.
In 1635 the French entered the war on the Protestant side. At this point, the war shifted in focus from a religious conflict to a dynastic struggle between the two greatest royal houses of Europe: the Bourbons of France and the Habsburgs of Austria. It also extended well beyond Germany; follow-up wars were fought between France and Spain even after the 30 Years’ War itself ended in 1648, and Spain provided both troops and financial support to the Habsburg forces in Germany as well.
For the next thirteen years, from the French intervention in 1635 until the war finally ended in 1648, armies battled their way across the Empire, funded by the various elite states and families of Europe but exacting a terrible toll on the German lands and people. From 1618 – 1648, the population of the Empire dropped by 8,000,000. Whole regions were depopulated and massive tracts of farmland were rendered barren; it took until close to 1700 for the Empire to begin to recover economically. In 1648, exhausted and deeply in debt, both sides finally met to negotiate a peace. The result was the Treaty of Westphalia, which was negotiated by a series of messages sent back and forth between the two sides, since the delegations refused to be in the same town.
The end result was that the already-weak centralized power of the Holy Roman Empire was further reduced, with the constituent states now enjoying almost total autonomy. In terms of the religious map of the Empire, there was one major change, however: despite the fact that the Catholic side had not “won” the war per se, Catholicism itself did benefit from the early success of the Habsburgs. Whereas roughly half of Western and Central Europe was Protestant in 1590, only one-fifth of it was in 1690; that was in large part because few people remained Protestants in Habsburg lands after the war.
The “winners” of the war were really the relatively centralized kingdoms of France and Sweden, with Austria’s status as the most powerful individual German state also confirmed. The big loser was Spain: having paid for many of the Catholic armies for thirty years, it was essentially bankrupt, and its monarchy could not reorganize in a more efficient manner as did its French rivals. Likewise, Spain missed out on the subsequent economic expansion of Western Europe; the war had undermined the economy of Central Europe, and the center of economic dynamism thus shifted to the Atlantic seaboard, especially France, England, and the Netherlands. There, a mercantile middle class became more important than ever, while Spain remained tied to its older agricultural and bullion-based economic system.
If the war had a positive effect, it was that it spelled the end of large-scale religious conflict in Europe. There would be harsh, and official, intolerance well into the nineteenth century, but even pious monarchs were now very hesitant to initiate or participate in full-scale war in the name of religious belief. Instead, there was a kind of reluctant, pragmatic tolerance that took root across all of Europe - the same kind of tolerance that had emerged in France half a century earlier at the conclusion of the French Wars of Religion.
Perhaps the most important change that took place in the aftermath of the wars was that European elites came to focus as much on the way wars were fought as the reasons for war. The conduct of rapacious soldiers had been so atrocious in the wars, especially in the Holy Roman Empire, that many states went about the long, difficult process of creating professional standing armies that reported to noble officers, rather than simply hiring mercenaries and letting them run amok.