What was the major conflict of the thirty years war?

They fought over the religion of the Holy Roman Empire. The H.R.E. was Catholic and they fought the Protestants. Since Spain, Austria, parts of the Spanish Netherlands, and the H.R.E. were run by the Habsburgs(Hapsburgs), France joined the Protestants because they wanted to weaken the Habsburgs. This system was known as politique. The Catholics won the Bohemian and Danish phases with the help of Albrecht von Wallenstein in the latter, and the Protestants won the Swedish Phase with the help of Swedish King Gustavus Adolphus who died in battle (The French funded the Swedish). During the French phase in which the Swedish funded the French, it was a stalemate resulting in the Peace of Westphalia, which weakened the Habsburgs and the H.R.E., established France as a dominant power, gave independence to Dutch Netherlands and Switzerland who were neutral, and allowed Princes in the H.R.E. to choose Calvinism as well as Lutheranism and Catholicism as their state religion. The Balance of power in Europe shifted towards France because of this.