Why was the Thirty Years War fought?

The Thirty Years War is a series of wars fought between 1618 to1648 for reasons that range from religious to territorial. It devastated most of Europe especially Germany and ended with the Treaty of Westphalia in 1648. It began with the resistance of Protestant nobles in Bohemia against the Hapsburgs, the family which headed the Holy Roman Empire. Since its beginning, political affiliations among states played a role in the causes and the outcomes of the series of wars. When the war started, the German Protestants allied with Austria.
They were defeated by another alliance, that of Brussel and Spain, which was sent by King Ferdinand II. In 1625, Denmark renewed the war but was defeated by General Wallenstein’s army, also sent by Ferdinand. Wallenstein was a Bohemian, but who sided with the Roman Empire in return for the spoils of war. The final phases of the war were initiated by Denmark, Sweden and France, all of whom were led to the fray in concern with the increasing Hapsburg power in their territories.
The Peace of Westphalia, two treaties which ended the war, returned the German states which were seized by the Holy Roman Empire in the duration of the war to their former rulers. It also allowed them to practice and choose their religions. Countries like Switzerland and Netherland were granted independence. The treaty introduced religious tolerance and ended religious warfare. Matters could have been resolved earlier during the conflict, avoiding the death and destruction that it inflicted upon the affected countries has the Holy Roman Empire been not driven by ambition and greed for more land.

The reason why states waged the wars against the empire was their fear of the increasing power of the Hapsburg over Europe threatening their very borders. King Ferdinand could have also appeased the Protestants within Germany; that he would leave them alone, way before the war expanded outside Germany.
Reference
The Columbia Encyclopia, (2001) Sixth Edition, 2001-2007. Columbia University Press. Retrieved on May 21, 2008. http://www. bartleby. com/65/th/ThirtyYe. html.