The entry of France into the Thirty Years War had blocked off the overland "Spanish Road" to Flanders. To support the Spanish army of Flanders of Cardinal-Infante Ferdinand, the Spanish navy had to ferry supplies by sea via Dunkirk, the last Spanish-controlled port on the North Sea coast.

In the spring of 1639, the Count-Duke of Olivares ordered the construction and assembly of a new fleet at A Coruña for a new relief jaunt to Dunkirk. 29 warships were assembled in four squadrons, soon joined by an additional 22 warships (also in four squadrons) from the Spanish Mediterranean fleet. Twelve English transport ships also arrived, contracted to carry the Spanish army under the flag of English neutrality.

From intelligence networks, the Dutch learned that the Spanish fleet might attempt to make for the anchorage known as The Downs, off the English coast, between Dover and Deal. The naval Battle of the Downs was a decisive defeat of the Spanish, by the United Provinces of the Netherlands, commanded by Lieutenant-Admiral Maarten Tromp.