The immense and growing prosperity of Amsterdam at this time was indeed mainly due to the fall of Antwerp from its high estate.  To reconquer Antwerp had indeed long been a favourite project of Frederick Henry.  In 1638 he made careful and ample preparations for its realisation.  But it was not to be.  Misfortune this year was to dog his steps.  The advance was made in two bodies.  The larger under the prince was to march straight to Antwerp.  The second, of 6000 men, commanded by Count William of Nassau, was instructed to seize some outlying defences on the Scheldt before joining the main force before the town.  Count William began well, but, hearing a false rumour that a fleet was sailing up the Scheldt to intercept his communications, he hastily retreated.  While his ranks were in disorder he was surprised by a Spanish attack, and practically his entire force was cut to pieces.  On hearing of this disaster the stadholder had no alternative but to abandon the siege.

Constant campaigning and exposure to the hardships of camp life year after year began at this time seriously to affect the health of the stadholder.  He was much troubled by attacks of gout, which frequently prevented him from taking his place in the field.  In 1639 there were no military events of importance; nevertheless this year was a memorable one in the annals of the Dutch republic.

It was the year of the battle of the Downs.  A great effort was made by Spain to re-establish her naval supremacy in the narrow seas, and the finest fleet that had left the harbours of the peninsula since 1588 arrived in the Channel in September, 1639.  It consisted of seventy-seven vessels carrying 24,000 men, sailors and soldiers, and was under the command of an experienced and capable seaman, Admiral Oquendo.  His orders were to drive the Dutch fleet from the Channel and to land 10,000 men at Dunkirk as a reinforcement for the Cardinal Infante.  Admiral Tromp had been cruising up and down the Channel for some weeks on the look-out for the Spaniards, and on September 16 he sighted the armada.  He had only thirteen vessels with him, the larger part of his fleet having been detached to keep watch and ward over Dunkirk.  With a boldness, however, that might have been accounted temerity, Tromp at once attacked the enemy and with such fury that the Spanish fleet sought refuge under the lee of the Downs and anchored at the side of an English squadron under Vice-Admiral Pennington.  Rejoined by seventeen ships from before Dunkirk, the Dutch admiral now contented himself with a vigilant blockade, until further reinforcements could reach him.  Such was the respect with which he had inspired the Spaniards, that no attempt was made to break the blockade; and in the meantime Tromp had sent urgent messages to Holland asking the Prince of Orange and the admiralties to strain every nerve to give him as many additional ships as possible.  The request met with a ready and enthusiastic response.  In