Final answer:
France supported Protestant forces during the Thirty Years' War to balance power in Europe and prevent the Habsburgs from gaining dominance, prioritizing political strategy over religious concerns.
Step-by-step explanation:
Catholic France supported the Protestant cause during the Thirty Years' War to undermine Habsburg power and thereby increase their own influence in Europe. The overarching goal was to prevent the Habsburgs, who controlled the Holy Roman Empire and Spain, from becoming too powerful. While France itself was largely Catholic and had signed the Concordat of Bologna in 1516 making Catholicism the official religion of France, the political concerns regarding the balance of power in Europe took precedence over religious uniformity. Additionally, the aftermath of the Saint Bartholomew's Day massacre and subsequent political events, such as the Edict of Nantes, had created a background where political considerations were often as important, if not more so, than the religious affiliations within France itself.