There are four causes of the Thirty Years War, which will be outlined in a clear manner below.
Religion In the past, everyone thought their religious beliefs where superior and would get very offended if someone said otherwise. Many believed that if they were to win a war against followers of the opposing belief, god preferred their religion and beliefs. Therefor, religion was one of the main causes of the Thirty Years War.
Politics and the Balance of Power Politics was one of the causes for the French getting involved. They feared the power and authority that the Habsburg had. France was also surrounded by countries and regions owned by the Habsburg's. This is why the chief minister of France, Cardinal Richelieu, had to think strategically of who to support to not only have an advantage for France but also so that the powers would be balanced out. The catholic France then joined forces with the protestants of Sweden to try an equal out the power through out Europe.
Alliances Throughout the Thirty Years there were two sorts of Alliances. Countries belonging to the same religion have a sort of alliance to help each other and their religion. This is one of the reasons why the Danish and the Swedish, which are both Lutherans, got involved in the war as the Protestants of the Bohemian Kingdom were losing against the Catholics.
Imperialism Imperialism is a term that is used to illustrate the domination of a region or province higher than others. A lot of the regions and provinces wanted to be imperial and also take over other countries. Scandinavian Protestants wanted to expand their territory to northern Germany and the Spanish wanted to reclaim Dutch Provinces.