of his army was instructed to follow Spanish orders instead of Ferdinand's orders if necessary. In 1635, the French attacked Namur, planning to merge with the Dutch near Maastricht but were held off by well prepared defenses. At the siege of Leuven, the invading Franco-Dutch forces, suffering badly from supply problems and greatly depleted by disease and desertion, were forced to withdraw when a Spanish relief force arrived. This allowed Spanish forces under Ferdinand to go on the offensive. The Dutch were driven back and the French retreated. Ferdinand subsequently was able to capture Diest, Goch, Gennep, Limbourg, and Schenkenschanz. In 1636, Ferdinand disempowered the last Protestant priests in the Spanish Netherlands, and continued his military counter-offensive by capturing Hirson, Le Catelet, and La Capelle, and securing Luxembourg using the usual mixed nationalities typical of the early modern age that included Croatian troops, and reaching as far a stronghold in France as Corbie, threatening Paris. In 1637, with Spanish forces concentrated in the fight with the French, a relatively lightly defended Breda, that had been under Spanish control for twelve years, was recaptured by the Dutch after a 10-month siege by the Prince of Orange. The loss of Breda was a propaganda blow to Spanish prestige but of minor strategic importance compared to the gains made by Ferdinand that year in taking the Dutch towns of Venlo and Roermond in the Meuse, effectively cutting Maastricht from the Dutch Republic and, thus, preventing further Dutch attacks on the Spanish Netherlands from the east. In the southern front Ferdinand lost the towns of La Capelle, Landrecies, and Damvillers to the French, but then he forced them to retreat south of Maubeuge. In 1638, Ferdinand's army successfully defended Antwerp, Saint-Omer and Geldern from the Dutch and French armies. Ferdinand achieved a great victory over the Dutch at the Battle of Kallo. In a letter to his brother the King of Spain shortly afterwards, Ferdinand described the battle as "the greatest victory which your Majesty's arms have achieved since the war in the Low Countries began". In 1639, Ferdinand managed again to thwart Franco-Dutch plans. The Dutch navy defeated an important Spanish fleet in the Battle of the Downs, off the English coast, but it failed to prevent most of the army it was carrying, some 7,000 to 10,500 infantry, from landing at Dunkirk. While Ferdinand frustrated the Prince of Orange's move against Hulst,