The Thirty Years' War was fought from 1618 until 1648. Though it was primarily centered in Germany, several other countries became involved in the conflict, including France, Spain, and Sweden. In fact, almost all of the powerful countries in Europe were involved in the war. It began as a fight about religion — the Protestants and Catholics were the two groups that disagreed. As the war continued, the Habsburg dynasty (a Catholic family) and other organizations used the war to try and get more power. One of the examples of this is that Catholic France fought for the Protestants. This made the France-Habsburg  even worse.

The Thirty Years' War caused things like famine and disease in almost every country involved. The war lasted for 30 years, but the problems that caused the war were not fixed for a long time after the war was over. The war ended with the Treaty of Westphalia.

Origins of the War 
There were several reasons that the Thirty Years' War started.

First, the Peace of Augsburg (1555), which was signed quickly by Charles V, stopped the fighting between the Lutherans and the Catholics in Germany.

The Peace of Augsburg said that:
 German Princes (there were 225 princes) could choose the religion (whether they were Lutheran or Catholic) in their states (this was called cuius regio eius religio).
 Lutherans that lived in a state under the control of a bishop, called an ecclesiastical state, could stay Lutherans.
 Lutherans could keep the land that they had taken from the Catholic Church after the Peace of Passau (1552).
 The bishops of the Catholic Church that switched to Lutheranism had to give their land back (the principle called reservatum ecclesiasticum).
 People that lived in a state that had chosen Lutheranism or Catholicism were not allowed to change their religion.
The Peace made the violence end for a bit. But it did not fix the real reason that the Lutherans and Catholics were fighting. Both of them said it meant different things. The Lutherans said it was only an agreement that would last for a short time. Calvinism came quickly into Germany. Calvinism was a third Christian group in Germany, but it was not part of the Peace of Augsburg. This meant it argued with both Lutherans and Catholics. 

Second, the powerful countries in Europe in the 17th century often disagreed about matters of Politics or Economics. Spain wanted land in some of the German states, because the Germans owned some of the Spanish Netherlands. The Dutch fought the Spanish to get independence. They got it in some wars that ended in 1609.
 France was afraid of the two Habsburg states on both of France's sides (Spain and the Holy Roman Empire). France wanted to show its power to the weak German states.
 Sweden and Denmark wanted to control the German states in the north next to the Baltic Sea.

Third, the Holy Roman Empire was a broken group of nations inside a bigger empire. The empire had nations like the Austrian House of Habsburg, Bavaria, the Electorate of Saxony, the Margravate of Brandenburg, the Electorate of the Palatinate, Hesse, the Archbishopric of Trier and Württemberg, and other small nations and towns. Only Austria was capable of operating on its own. Countries often made alliances with other places ruled by relatives. Because there were so many nations inside the empire, the nations disagreed with each other a lot and the government couldn't control the whole empire very well. This meant that the government couldn't fix the problems in the country.

Fourth, religious groups were not agreeing during the second half of the 16th century. The Peace of Augsburg was not working because some bishops had not given up their bishoprics, and Catholic rulers in Spain and Eastern Europe wanted to make Catholicism strong in the region. This caused fighting between the groups. The Catholics made many Protestants leave their home lands.
Some places gave Protestants permission to worship. These disagreements caused violence.

Fifth, the Holy Roman Emperor Matthias died without any children to take his place in 1619. He was Catholic. His lands were given to his cousin Ferdinand of Styria. Ferdinand was Matthias's closest male relative. He became Ferdinand II, Holy Roman Emperor. Ferdinand had been educated by the Jesuits, and was a Catholic. He wanted to make Catholicism the only religion again. This made him unpopular in the German state of Bohemia (the people in Bohemia were mostly a non-Catholic religion called Hussites). They rejected Ferdinand and launched the Thirty Years' War. The War can be divided into four major phases: the Bohemian Revolt, the Danish intervention, the Swedish intervention, and the French intervention.

The Bohemian Revolt 
Time: 1618–1625

Emperor Matthias, who had no children, wanted to give the throne of the Holy Roman Empire to Ferdinand II when he died. So, to ensure that the transition would work, he wanted to make Ferdinand the Crown Prince of Bohemia (a country that was part of the Holy Roman Empire) in the meantime. Ferdinand was extremely Catholic, so some of the Protestant leaders of Bohemia thought Ferdinand would take away their religious rights. They liked the Protestant Frederick V, Elector of the Palatinate better. However, some of the other Protestants supported Ferdinand. Enough people preferred Ferdinand that, in 1617, he was elected to become the Crown Prince of Bohemia. 

Ferdinand sent two Catholic governors to Prague, the capital of Bohemia, in May 1618. Ferdinand wanted them to run the government while he was gone. Suddenly, a lot of angry Protestants took them and threw them out of the high palace window (this is called the Defenestration of Prague). This made the Catholics angry at the Protestants.

The Protestants who threw the Catholics out the window created a new Protestant government in Bohemia. A lot of Protestants in Bohemia and the nearby countries started hating the rest of the Holy Roman Empire. The Holy Roman Empire wanted to stop them from doing that, so it wanted to fight Bohemia and borrow Spain's money to do that. However, they thought that if that happened, Bohemia's Protestant friends might come in and start fighting them. So they tried to peacefully make an agreement with Bohemia to stop the fighting. But when Emperor Matthias died in 1619, Bohemia decided not to make the agreement because they thought the Holy Roman government was weak now.

Bohemia was so mad about the Catholics that they decided to not make Ferdinand II the king of Bohemia anymore and made Frederick V the king instead. But now because Matthias was dead and Ferdinand was the next person waiting to be the emperor, Ferdinand was in charge of the whole Holy Roman Empire now. Some people also thought Frederick shouldn't be allowed to be king of Bohemia because they had already decided Ferdinand was king and they weren't allowed to undo that. Because of all this, the Holy Roman Emperor decided to fight Bohemia.

A big army that got money from a German politician called Maximilian I and was led by a man called Count Tilly invaded Bohemia. At the Battle of White Mountain, the Holy Roman Empire beat the Bohemian rebels. Frederick ran away and the revolt collapsed.

Frederick was also in charge of an German nation called the Palatinate. Maximilian I, who was in charge of a nearby nation called Bavaria, wanted more power and decided to take over some of the Palatinate. Spain (a Catholic country) joined the war and tried to take over the rest of the Palatinate, but the Netherlands said Frederick should be in charge of the whole Palatinate instead. This was the first time other countries were involved in this war. After some fighting, Maximilian and Spain won, and Emperor Ferdinand decided that all of the Palatinate should go to Maximilian. This made some of the other Protestant nations very scared because this meant that some Protestant areas were being taken over by Catholics.

Related pages
Battle of White Mountain

17th century in Europe
 
Wars involving England
Wars involving France