30 years war wasn't just started as a religious conflict but it was a nessesity for Ferdinand II to ensure his power over Bohemia to secure his emperorship without the crown of bohemia Ferdinand's chance of becoming the Holy Roman Emperor would have gotten much smaler since it was common that the king of Bohemia would also become emperor come election.

Now the reformation struck hard in northern europe and the treaty of Augsburg gave the Protestants right to practise their religion with the famous quote "Cuius regio, eius religio" wich translates in to "Whose realm, his religion"

Back to Bohemia, the Protestants wanted more saying in the ruling of their country and wanted a Protestant king instead of a Catholic from the house of Habsburg. So Ferdinand sent some envoys on his behalf to Bohemia to strike a treaty that would secure him his crown. But the negortiations backfired terribly and the spanish diplomats was thrown out of the town halls tower window, they survived due to a pile of horse manure, and escaped to the spannish low countries. This was a stain on Ferdinands honor and the quickly gathers his banners and marched on Prague to show he was a man of action and wouldn't stand being humiliated like that again.

The Bohemians knew that Ferdinands armys was on it's way so they sought for help from Saxony their ally and one of the powerful nations in the Evangelical Union which was created so protestant princes attacked by a catholic nation would aid one and other. Saxony decided not to help Bohemia due to the immence strenght of the Imperial Army backed by Spain.

The bohemians was slaughterd and Prague was occupied by the imperial army and a trial was held and 27 citizens was convicted and executed. and over 30,000 protestants was forced to move. This forced the Evangelical Union in to action and thus the start of the 30 years war.