The Thirty Years War was a time of great religious conflict. The Calvinists opposed the Lutherans and the Catholics opposed the Protestants. Each side had no tolerance for the other and felt a deep hatred for them.
The war consisted of four stages, or periods. The first of these stages was the Bohemian period. When Ferdinand came to power, in Bohemia, he immediately abolished religious freedom. The Protestant nobility, who had previously gained their rights, responded with a revolt against the king. As the conflict continued, the fighting spread to other lands, leading us into the Danish period.
King Christian V of Denmark was a Lutheran king eager to expand his empire and influence. Unfortunately, he was unsuccessful and was forced to retreat to Denmark. Wallenstein was a mercenary hired by Ferdinand II. Wallenstein crushed the Protestants and it became obvious that a Catholic Europe is what Ferdinand sought. The Protestants needed help and called upon the Swedish king, Gustavus Adolphus.
The Swedish period began with the Protestant leadership of the Lutheran Adolphus. During this period is when the French began to show a great deal of interest in the war. The French minister, Cardinal Richelieu helped to fund the Swedish forces. At first, this act of the French seems quite unusual because France is a Catholic power. It would make sense that the French would aid the Catholic forces rather than those of the Protestants, yet they did not. The French had a strong interest in keeping the Habsburg armies occupied and tied down in Germany. This factor was more important than having the territories under Catholic influence.
In the last period, the French openly entered the war. The French came in on the side of the Protestants. The French period resulted in mass destruction of Germany and a significant loss in their population.
Of the four periods of war, the two most surprising were the ones of the Swedish and the French. I would have never imagined that the French would support the Protestants over their fellow Catholics. These wars were based on religious differences, yet the French chose to help a religion other than their own, in order to prevent Habsburg domination.