A short summary:

This war, fought mainly in German countries 1618-1648, had several causes but it is often described as a religious war between Catholics and Protestants.

It started when Protestant German nobles in the Hapsburg-dominated principality of Bohemia revolted against the Catholic king Ferdinand and the Holy Roman Empire. The first act of violence was supposed to be when Protestant nobles hurled two royal officers through a window in Prague, an event often referred to as "the defenestration of Prague". A full scale revolt ensued and Ferdinand was deposed of the throne of Bohemia. At this time the revolt spread to other Hapsburg dominions.

The Holy Roman Empire and the Catholic League under general Tilly reacted strongly, defeated the Bohemians in November 1620 and began a period of severe repression in Bohemia.

Meanwhile in the Palatiniate (German: Pfalz), the revolt continued under von Mansfeld and Christian of Brunswick only to be quenched by Tilly in 1622. The conflict may have ended there if it wasn't for the fact that foreign powers had noticed the success of the Hapsburgs and feared that the power balance was being tipped too far in the Catholic direction.

The first to react was the Danish king Christian IV. With the support of England he sent troops into Germany, only to be defeated by Tilly and Wallenstein, who retaliated by attacking Danish provinces.

At this point the Swedish king Gustavus Adolphus (Swedish: Gustav II Adolf), who had designs on Poland, began to feel threatened by the Catholic forces as well. A devout Protestant, he also claimed religious reasons for sending troops into Germany. With the support of John George of Saxony, the Swedes defeated Tilly at Breitenfeld securing northern Germany for the Protestant union.

Wallenstein, who had been removed from his command in 1630, managed to get back into the action in 1632 and met the Swedish forces at Lützen. The battle ended with no significant victory for either side, and Gustavus Adolphus was killed leading his cavalry. The Protestant union was left disorganized, even though they were ahead on the battlefields and the Catholic army was all but defeated. The people of the German countries, where the battles were fought, also grew wary of foreign soldiers marching across their homelands and support was low.

In 1635 France entered the war opposing the Hapsburgs and, religious matters aside, the Swedes decided to support Cardinal Richelieu to save what they could of their hold in Germany. Most of Europe from the Iberian peninsula to Scandinavia was involved in the war at this point.

After a few successful attacks on France, the Empire began losing the war. Bernard of Saxe-Weimar and the Swedish generals Banér, Torstensson and Wrangel won important battles across Germany.

The war ended in 1648 with The Peace of Westphalia, where France and Sweden together with their German allies stood as victors.

I apologize for the many spelling errors in the first version of this write-up. I have corrected those I found so far and also corrected the fact about the outcome of the battle of Lützen.

The Thirty Years' War (1618-1648) was—well—obviously a war which lasted thiry years! It was fought in the Holy Roman Empire and was the most destructive war of the Wars of Religion.

During the second half of the 16th century, Germany was a land of 360 autonomous political entities. With the Peace of Augsburg (1555), each had received a significant degree of sovereignty within its own borders, such as choosing the areas religion. Germany had become a politically decentralized and fragmented confederation.

The population in the Holy Roman Empire was about equally divided between Catholics and Protestants, leaving Calvinism unrecognized as a legal religion by the Peace of Augsburg. When Frederick III (r. 1559-1576), a convert to Calvinism, became the elector Palatinate and made Calvinism the official religion of his domain, the Lutherans felt threatened, and disapproved. Religious tensions began to mount, when, in 1609, Maximilian, the Duke of Bavaria, had organized a Catholic League to counter a new Protestant alliance which was forced under the leadership of the Calvinist elector Palatinate, Frederick IV (r. 1583-1610). The war shortly followed. It was mainly divided into four periods, or phases: The Bohemian Period (1618-1625), the Danish Period (1625-1629), the Swedish Period (1630-1635), and the Swedish-French Period (1635-1648).

The Bohemian Period (1618-1625)

The Habsburg Ferdinand, also in line of succession for the imperial throne, ascended to the Bohemian throne in 1618. He revoked the religious freedoms of the Bohemian Protestants, and in response to this, in May of 1618, the Protestant nobility in Prague threw his regents out the window. This was also referred to as the “defenestration of Prague.”

Next year, by the unanimous vote of the seven electors, Ferdinand became the Holy Roman Emperor as Ferdinand II. The Bohemians deposed him in Prague and declared the Calvinist Elector Palatinate, Frederick V (r. 1616-1623) as their overlord. Spain sent troops to help Ferdinand II, and he found more allies in Maximilian of Bavaria, and the opportunistic Lutheran elector, John George I of Saxony (r. 1611-1656). Ferdinand II’s army, under Cardinal Tilly, routed Frederick V’s troops at the Battle of White Mountain (1620). By 1622, Ferdinand II had not only managed to subdue and re-Catholicize Bohmeia, but also to conquer the Palatinate. Fighting had also continued into Northwest Germany.

The Danish Period (1625-1629)

The Lutheran King Christian IV (r. 1588-1648) of Denmark sent troops to Germany to support the Protestant cause. He was also eager to extend Danish influence over the coastal towns of the North Sea.

Military success had made Maximilian stronger and more difficult to control. Ferdinand II needed a more pliant tool for his policies, so he hired a powerful complex mercenary named Albrecht of Wallenstein (1583-1634). Wallenstein penetrated Denmark with an occupying army, and had broken Protestant resistance very successfully.

Ferdinand II issued his notorious Edict of Restitution in 1629 which basically countered the Peace of Augsburg, reaffirmed the illegality of Calvinism, and ordered the return of all church lands acquired by the Lutherans since 1552. Ferdinand was clearly attempting to recreate a Catholic Europe.

The Swedish Period (1630-1635)

Gustavus Adolphus of Sweden (r. 1611-1632), a deeply pious king of a unified Lutheran nation, became the new leader of the Protestant forces within the empire. Adolphus was bankrolled by the French minister, Cardinal Richelieu. Richelieu’s foreign policy was to protect French interests by keeping Hapsburg armies preoccupied in Germany. Adolphus found ready allies in the electors of Brandenburg and Saxony, and soon won a victory at Breitenfeld.

During the Battle of Lützen in November of 1632, Adolphus was killed at the hands of Wallenstein. Then in 1634, Ferdinand had Wallenstein assassinated.

In 1635, the Peace of Prague was made—an agreement which the Swedes refused to join. The German Protestant states, led by Saxony, reached a compromise with Ferdinand. The Swedes received continued support from France and the Netherlands.

The Swedish-French Period (1635-1648)

In 1635, the French had openly entered the conflict, and were fighting the war just for war itself. The French (who were on the side with the Protestants) and the Spanish (on the side of the Catholics) took advantage of the war and fought for territory in the German lands. The two countries, being evenly matched, did not bring out a clear victor—neither had the resources to win a quick victory. By 1644, about one third of the German population died directly from the war.

The Treaty of Westphalia

The Treaty of Westphalia, or Peace of Westphalia, finally brought all hostilities in the Holy Roman Empire to an end. It rescinded Ferdinand II’s Edict of Restitution, and reasserted the Peace of Augsburg, but recognized the Calvinists. The Swiss Confederacy and the United Provinces of Holland gained independence, and Bavaria was elevated to the rank of an elector state. The German princes remained supreme over their principalities.

France and Spain remained at war outside the empire until 1659, when, amazingly, the French won! French victories forced on the Spanish a humiliating Treaty of the Pyrenees.

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