The naval Battle of Portland, or Three Days' Battle, took place during 18–20 February 1653 (28 February – 2 March 1653 (Gregorian calendar)),[a] during the First Anglo-Dutch War, when the fleet of the Commonwealth of England under General at Sea Robert Blake was attacked by a fleet of the Dutch Republic under Lieutenant-Admiral Maarten Tromp escorting merchant shipping through the English Channel.

The First Anglo-Dutch War was caused by friction between the two naval powers of the century, competing for strategic supremacy over the world's merchant routes.

This changed when Parliamentary armies finally routed the Royalists at the Battle of Worcester, effectively ending the English Civil War.

The translation of Parliament's words into action came when Sir George Ayscue folded Barbados into the Commonwealth and seized 27 Dutch merchantmen.

This fleet was put under the command of Admiral Maarten Tromp, who had defeated the sixth and final Spanish Armada at the Battle of the Downs, 31 October 1639.

He set to return to his home port, but first anchored off La Rochelle to repair and resupply his ships and waited for expected merchantmen coming from the Atlantic.

He attempted to set sail on 20 February with 152 merchantmen, but was held back for three days by high winds and rough seas.

On 24 February Tromp finally set sail, entering the area off Portland four days later where he spotted Blake's fleet attempting to cut them off.

The battle ended for the day when Blake drew off, after forcing the Dutch to fight to the point where they only had around half an hour worth of shot left.

They concluded that the adoption of line tactics by the English would make it impossible for the Dutch to compensate inferior firepower with better seamanship and they urged the States-General to finally start building real heavy warships instead of replacing losses by recruiting armed merchants.