A reply to /u/MissedAirstrike .

Have you looked at some of the FAQ entries related to the 30YW?

Can you define what you mean by "religious" and "religious", please? These concepts were understood differently in that time frame than it is today.

Can I ask if this is for an assignment?

I want to know: was it even religious from the start?

Perhaps a previous post may be of help. In particular, you may want to read up on the Swedish and French phases of the war.

Swedish period

France and the Swedish king Gustavus Adolphus was alarmed by HRE success, came up with an agreement whereby the Swedish army was to join in the 30YW. Sweden's entry and participation in the 30YW was very dependent on French financial support. It can be said that the French proposal came at the perfect time. The Swedish army was just handed a heavy defeat at the hands of the Polish-Imperial army in 1629, from which the Swedish King Gustaf Adolf II barely escaped with his life. The French had lobbied for Bavaria to break against the Imperial army, but failed. Their former ally Denmark refused to continue its war. Gustaf Adolf II was eager to extricate himself from the war against Poland, so he signed a six-year truce where he conceded territorial control but earned the very-important toll revenue of all ships calling at ports of Poland and Prussia. This ended up being a third of Sweden's revenues. So as she entered the 30YW, Sweden had an experienced body of troops, a well-organized state.

With his new style of warfare, using smaller and more agile infantry formations, he turned the table on the Catholics and won a great battle in Breitenfeld, in the end costing Tilly his life. However, Sweden's advance was dependent on many factors. It is telling that after the major victory that was Breitenfeld, the road to Bavaria and Austria were open, but Gustav II Adolf had to instead turn right westward onto the fertile Rhinelands, lest his army would starve on its march south. This gave the Imperials and Catholic League the needed breathing space to conduct massive recruiting, re-armament and re-evaluation of tactics at all levels.

French Enters the War

Seeing the danger, Wallenstein was recalled to lead Imperial troops. Wallenstein decided to threaten Sweden's lines of communication, thus setting the stage for the Battle of Lutzen where Sweden won but their king was killed. At this point, Spain increased her investment in the war, and in the battle of Nordlingen they won against the Protestants. The stalemate worried France, who saw a Spanish-Imperial victory at Nördlingen neutralize of Sweden as a force in German politics. Thereafter, France joined as a direct participant in the conflict.

France's idea of fighting against Habsburg encirclement was promulgated in the 30YW, so Richelieu arranged for an attack in the Pyrenees against France even as he was being threatened by a joint Spanish-Imperial army from Flanders. Initial gains by the Cardinal-Infante Ferdinand, opening the road from Flanders to Paris, was checked by his untimely death and the subsequent loss in Rocroi. At the same time, the Dutch re-captured Breda, cutting the lines of communication between Spain's army of Flanders and the rest of Germany. In turn, this isolated the conflict in the Pyrenees and that in Germany.

Over time, signs of exhaustion was showing on all sides, but it took years to reach the agreement that was Westphalia.

Interesting, I've always seen it the other way round - it started as a politico-religious conflict with the Catholic Imperial forces putting down a Bohemian rebellion which would put an important electorship in Protestant, non-Habsburg hands. When the Bohemian/Palatine rebels were defeated, first the Danes and then the Swedes stepped in as Defenders of Protestant Liberty but really in search of more influence within the Empire.

All pretence of it being a religious conflict then died off when Catholic France intervened on the side of Protestant Sweden for the sole purpose of putting one over on the Habsburgs.

But then that's on the basis of a dimly-remembered university course...