Shortly after France declared the war on Spain in May 1635, a French army under the Marshals of France Urbain de Maillé-Brézé and Gaspard III de Coligny, allied with the Dutch States Army, invaded the Spanish Netherlands from two sides and threatened Brussels before investing Leuven. The siege ended in a costly failure because of bad logistics and organization, and as the French army was decimated by the plague. The Cardinal-Infante Ferdinand, governor of the Spanish Netherlands, counterattacked and expelled the invaders, concentrating his resources against the Dutch over the following months. The recapture by the statholder Frederick Henry of Orange of the key fortress of Schenkenschans did not discouraged the Spanish, and the Count-Duke of Olivares continued determined to concentrate the war effort against the Dutch.
After suffer further defeats against the armies of the Duke Charles of Lorraine and the Imperial generalissimo Matthias Gallas in the Rhine, Alsace and Lorraine, the French armies remained focused in the defense and reconquest of strategic places in these territories. The conquest of the Franche-Comté, entrusted to Henri de Bourbon, Prince de Condé and Charles de La Porte de La Meilleraye, soon became an absolute priority to the Cardinal Richelieu. The Holy Roman Emperor, Ferdinand II, whose position in Germany had strengthened since the Peace of Prague, meanwhile, projected an invasion of the eastern France under Matthias Gallas, but as logistical and financial problems diminished his force, he proposed a joint invasion to the Cardinal-Infante. Philip IV of Spain and Olivares rapidly agreed.
A lightly equipped army ranging from 10,000–12,000 infantry and 13,000 cavalry soldiers to 18,000 soldiers of both types, including an imperialist contingent under Ottavio Piccolomini was gathered at Mons during June. On 4 July the Cardinal-Infante crossed the frontier via Avesnes and took the fortresses of Le Catelet and La Capelle. Though having large garrisons, La Capelle surrendered after only four days of siege and Le Catelet, one of the strongest fortresses of France, after three days thanks to the exploding shells used by the Spanish army, a recent innovation yet unfamiliar to the French. The alarming advance of the Cardinal-Infante forced Louis XIII to return to Paris from Fontainebleau. By then Ferdinand was in Cambrai and had left the command of his army to the Prince Thomas Francis of Carignano, the commander of the Army of Flanders.