1621 was also the year in which the Twelve Years Truce between the Netherlands and Spain expired. Consequently, it became necessary to prepare the fleet for war again. In 1624 Maarten received his first commission as a captain. He commanded the flagship of the new Lieutenant admiral Piet Heyn in 1629. Heyn sailed in 1629 with his fleet to form a blockade at the Spanish privateer haven Duinkerken. Piet Heyn died during a battle with 10 pirate vessels from Oostende.
He went on with the mission and was able to block the harbour of Duinkerken
very effectively for several years. His relations with Lieutenant Philips
van Dorp were strained, and this, combined with family circumstances led him
to resign in 1634.
In 1638 he commanded the Dutch fleet that managed to prevent the fleet
commanded by Michiel Dorne to leave from Duinkerken.
In February 1639 he defeated a fleet of privateers sailing from Duinkerken
under the command of Michiel Dorne.
In the second half of 1639 he encountered a large Spanish fleet that was
commanded by Admiral Antonio Oquendo and others. The Spanish fleet
consisted of 45 men-of-war and 30 merchant vessels. It was transporting
13.000 Spanish soldiers to Flanders. When tromp spotted the fleet near Beachy
Head on September 15, 1639, he did not immediately attack. At the time he had
only 13 vessels under his direct command. Other parts of the original fleet
that had been under the command of Piet Heyn were sailing near Dover and
Duinkerken. Witte Corneliszoon de With joined tromp with 5 ships on September
16. They decided to attack and after 6 hours of fighting the Spanish fleet
withdrew. They had suffered heavy losses and the number of casualties was
high, in part due to the overcrowded transport ships, filled with
soldiers.
No action could be taken on September 17 due to a lack of wind. On September
18, however, they attacked again in the strait of Dover after having been
reinforced by a squadron from Zeeland. In the afternoon Oquendo withdrew
again, this time to the roadstead of the Downs (= DUINS). Tromp sailed to Calais for
supplies of gunpowder after which he set out for the Downs to attack again.
An English squadron, under the command of Sir John Pennington, had joined
the Spanish fleet in the meanwhile creating a gambit for several weeks.
A squadron under the command of Michiel Dorne had
managed to escape the blockade at Duinkerken and had also joined the Spanish
fleet. After further reinforcements of the Dutch fleet Tromp attacked Oquendo
and Pennington on October 21. Despite the attempts at protection by Pennington
and Dorne the Spanish fleet was defeated at the Battle of the Downs. Only the
squadron commanded by Dorne managed to escape to Duinkerken with many sick
and wounded men.
Tromp received a knighthood from Louis XIII in 1640 and in 1642 he received
one from Charles I when he visited Dover. For several years after the Blockade
of Duinkerken and the Battle of the Downs he was able to do much damage to
the Spanish privateers. In 1640 he attacked a squadron of privateers that was after the Dutch herring-fleet. He managed to chase away two privateers and captured the third vessel. Tromp received a commission to attack fishing-boats sailing from Belgium in July 1641. The commission was issued by the Staten Generaal due to increasing pressure of the complaining Dutch fishing-communities. Up until that time the fishing-boats of Belgium had been left alone.
In 1644 the Dutch Republic and France made a treaty that was acted upon in 1646. A fleet under the command of Tromp blocked the harbour of Duinkerken while French troops attacked the city on land. For his actions he received the Order of St. Michael.
Maarten Tromp was killed during the First English War in the battle of Terheijde (near Scheveningen) on August 10 1653 after having lost two earlier battles at sea in March and June of that same year.