A Spanish fleet, commanded by Admiral Antonio de Oquendo, was decisively defeated by a Dutch force under Lieutenant-Admiral Maarten Tromp.

Victory ended Spanish efforts to re-assert naval control over the English Channel and confirmed Dutch dominance of the sea lanes, while it is also alleged to be the first major action to feature line of battle tactics.

The battle was initiated when Spanish chief minister Olivares sent a large convoy of troops and supplies for the Army of Flanders, escorted by some 50 warships.

Since 1621, Spanish naval activity in the Channel had focused on avoiding direct conflict with the superior Dutch fleet, while attacking their merchant ships from bases in Dunkirk and Ostend.

In a change from this policy, Oquendo was ordered to deliver the reinforcements but also bring the Dutch to battle; Olivares hoped victory would restore Spanish prestige and force the States General to negotiate peace terms.

Losses on both sides were minimal, but Oquendo took refuge in The Downs, an anchorage between the ports of Dover and Deal, where he was protected by English neutrality.

Although the Spanish Empire had far greater resources than any of its opponents, fighting on multiple fronts forced them to rely on long and vulnerable lines of communication.

[5] Stripping 22 galleys from the Mediterranean, combined with a hurried construction program, meant that by August 1639 Olivares had assembled a fleet of around 50 warships along with a number of smaller vessels, manned by 6,500 sailors and 8,000 marines.

[1] In addition, 9,000 reinforcements and three million escudos for the Army of Flanders were carried in 30 transports, which included ships chartered from Germany and England, contracted under an agreement with Charles I.

[8] Since 1621, the Spanish had avoided major conflict with the Dutch fleet in favour of raids conducted by the Dunkirk-based Armada de Flandre and privateers on their commercial shipping.

[9] Now confident enough to seek a full-scale battle, Olivares offered him command of the expedition, but de Hoces was sceptical of his own ability to defeat the Dutch, especially given that the galleys which formed a significant part of his force were far less effective in the Channel.

While this was being done, a squadron led by Maarten Tromp was ordered to sea to monitor and harass the Spanish if needed, although he was forbidden from engaging them in battle until joined by the rest of the fleet, some fifty vessels under Johan Evertsen.

Tromp divided his force into three; 12 ships under Joost Banckert [d] were positioned north of the Downs, in the unlikely event the Spanish took the long route around the British Isles.

[13] Attempts by the French admiral Henri de Sourdis to disrupt Spanish preparations failed and the fleet sailed on 27 August, entering the Channel on 11 September.

[8] On 15 September, they learned from a passing English ship that a Dutch squadron was anchored near Calais and the next day made contact with Tromp.

He failed to issue adequate instructions to his subordinates and negated his superior numbers by constantly seeking to engage Tromp's flagship.

The half-moon formation quickly disintegrated, and only the Dunkirk squadron and the galleon San Juan kept up with the Spanish flagship's pursuit of Tromp.

For Tromp, this was building up into a dangerous situation, as the Spanish units upwind would cut off his exit, and force the Dutch squadron to turn into the shoals of the bay of Boulogne and almost certainly run aground.

Ultimately, the Spanish calculated 6,000 troops had been landed, with 1,500 captured by the Dutch and another 1,500 either killed or interned in England, which allowed Ocquendo to later claim he had largely achieved the objectives set.

Meanwhile, the Spanish, who earlier had managed to sneak 13 or 14 Dunkirker frigates through the blockade, began to transport their troops and money to Flanders on British ships under an English flag.

Some of the large, unmanoeuverable Spanish ships panicked on approach of the Dutch fleet and grounded themselves deliberately; they were immediately plundered by the English populace, present in great numbers to watch the uncommon spectacle.

[17] According to Spanish naval historian Cesáreo Fernández Duro, of the 38 ships that attempted to break the Dutch blockade, twelve beached themselves on the English shore, nine of which were later refloated and managed to reach Dunkirk.

[23] The larger part of the Spanish infantry managed to reach Flanders along with nearly all the money, but the celebrated Dutch victory marked a significant moment in the shifting balance of naval power.