 Ferdinand was also ordered, when the offensive operations had finished, to quarter his army near the Dutch frontier in order to protect Antwerp, which had become more vulnerable since the loss of Breda, and even to reinforce the garrisons of many secondary fortresses.In the end, however, the Spanish were pinned to the defensive by a coordinated Franco-Dutch attack in May 1638.Marshal Châtillon laid siege to Saint-Omer covered by Marshal La Force in Picardy while Frederick Henry marched on Antwerp commanding an army of 22,000 soldiers, determined to besiege the city.