France, although Roman Catholic, is a rival of the Holy Roman Empire and Spain.

Cardinal Richelieu, the Chief Minister of King Louis XIII of France, feels that the Habsburgs are too powerful, since they hold a number of territories on France's eastern border, including portions of the Netherlands.

Richelieu had already begun intervening indirectly in the war in January 1631, when the French diplomat Hercules de Charnace signed the Treaty of Bärwalde with Gustavus Adolphus, by which France agreed to support the Swedes with one million livres each year in return for a Swedish promise to maintain an army in Germany against the Habsburgs.

The treaty also stipulated that Sweden would not conclude a peace with the Holy Roman Emperor without first receiving France's approval.

After the Swedish rout at Nördlingen in September 1634 and the Peace of Prague in 1635, as Sweden's ability to continue the war alone appears doubtful, Richelieu makes the decision to enter into direct war against the Habsburgs.

With Imperial forces threatening dominance in Germany, Spanish troops firmly settle on the western bank of the Rhine, and thus Habsburg armies surround France.

Thus, in alliance with the Protestant Dutch and Swedes and some of the German Protestant princes, Catholic France declares war on Spain in May 1635, thereby opening a second front in the Catholic Low Countries.