Ferdinand II was the king of Germany. There were a few princes of Germany that decided whether their little part of Germany was Catholic or Protestant. But Ferdinand was a devote Catholic and began making laws to get rid of the Protestants.
A little part of Germany that was Protestant called Bohemia was REALLY angry at Ferdinand, so they demanded to see him. But Ferdinand was visiting the sister-city (Pawnee, Indiana, and was having a pretty hard time with the Parks and Recreation department) so he sent two officials into Bohemia to deal with the Protestants.
Because there was a huge angry Protestant mob, and only two Catholic officials, the officials ended up being thrown out the window.
The Protestants were very angry that Ferdinand even TRIED to send officials to reason with them so they declared themselves free, not thinking dumb-little-Ferdinand would try to fight back.
Ferdinand wanted to become Holy Roman Emperor. In order to do that, he needed two of the German princes to elect him, they were called electors. The problem was, those two German princes were Protestant. So Ferdinand convinced Spain to help him take over the rebellious Bohemia. He took away all the land from the Protestants and gave it to the loyal Catholics, and finally forced the electors to elect him.
The other Protestant countries were all like "WHAT THE HEEELLLLL". They weren't involved in the rebellion, but they were worried that they might be next. So the king of Denmark, Christian IV, gathered his army and marched to Germany.
Ferdinand was worried about being attacked by Christian IV. So he hired Albert of Wallenstien to help train his army. Wallenstien LOVED war, so Ferdinand's army was trained amazingly well. Ferdinand's army won the war, with Wallenstein's help.
The Protestant King of Sweden, Gustavus, was all like "WHAT THE HEEELLLLL" because he was worried that Ferdinand might want to attack him. So he trained and even BETTER army and attacked Ferdinand. Gustavus kicked Ferdinand's army's BUTTOCKS, so he marched into Germany and convinced the other Protestants to fight with him. They created a huginourmous army called The Protestant Union. They ran through the streets slitting thoughts and almost won, until Gustavus himself died, so things fell apart.
The Protestant union was willing to make peace, so they sighed a treaty that basically said "none of this ever happened okay?!" And all was well
THEN ALL WAS NOT WELL
The king of France, Cardinal Reichelieu, attacked Germany in hopes of taking it over. But then both Cardinal and Ferdinand died, so everyone decided:
"You know what? This wasn't worth it. Let's just stop fighting."