La Capelle
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.