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 all the dockyards work went on with relays of men night and day. In less than a month Tromp found himself at the head of 105 sail with twelve fire-ships. They were smaller ships than those of his adversary, but they were more than enough to ensure victory. On October 21, after detaching Vice-Admiral Witte de with 30 ships to watch Pennington's squadron, Tromp bore down straight upon the Spanish fleet though they were lying in English waters. Rarely has there been a naval triumph more complete. Under cover of a fog Oquendo himself with seven vessels escaped to Dunkirk; all the rest were sunk, burnt, or captured. It is said that 15,000 Spaniards perished. On the side of the Dutch only 100 men were killed and wounded. The Spanish power at sea had suffered a blow from which it never recovered.
Charles I was very angry on learning that English ships had been obliged to watch the fleet of a friendly power destroyed in English waters before their eyes. The king had inherited from his father a long series of grievances against the Dutch; and, had he not been involved in serious domestic difficulties, there would probably have been a declaration of war. But Charles' finances did not permit him to take a bold course, and he was also secretly irritated with the Spaniards for having sought the hospitality of English waters (as written evidence shows) without his knowledge and permission. Aerssens was sent to London to smooth over the matter. He had no easy task, but by skill and patience he contrived, in spite of many adverse influences at the court, so to allay the bitter feelings that had been aroused by "the scandal of the Downs" that Charles and his queen were willing, in the early months of 1640, to discuss seriously the project of a marriage between the stadholder's only son and one of the English princesses. In January a special envoy, Jan van der Kerkoven, lord of Heenvlict, joined Aerssens with a formal proposal for the hand of the princess royal; and after somewhat difficult negotiations the marriage was at length satisfactorily arranged. The ceremony took place in London, May 12, 1641. As William was but fifteen years of age and Mary, the princess royal, only nine, the bridegroom returned to Holland alone, leaving the child-bride for a time at Whitehall with her parents. The wedding took place at an ominous time. Ten days after it was celebrated Strafford was executed; and the dark shadow of the Great Rebellion was already hanging over the ill-fated Charles. In the tragic story of the House of Stewart that fills the next two decades there is perhaps no more pathetic figure than that of Mary, the mother of William III. At the time this alliance gave added lustre to the position of the Prince of Orange, both at home and abroad, by uniting his family in close bonds of relationship with the royal houses both of England and France.