France, although overwhelmingly 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 had signed the Treaty of Bärwalde with Gustavus Adolphus, by which France had 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 appeared doubtful, Richelieu had made the decision to enter into direct war against the Habsburgs.