In the previous year, Oliver Cromwell's government had passed the 1651 Navigation Act with the intent of harming Dutch commerce. In addition, he demanded that all ships must salute the English flag when in English waters.
On May 29 (May 19 in England), a Dutch fleet of 40 ships under Admiral Maarten Tromp encountered a British fleet of 25 under "General-at-Sea" Robert Blake. Nevertheless, Blake bravely demanded that the Dutch dip their ensigns first. When they refused to do so, he added in a few cannonshots, to which Tromp replied with a broadside from his flagship Brederode, and the battle was on.
After five hours, the English had managed to sink two Dutch ships, and as darkness fell, the Dutch fleet withdrew.
War was declared in July, and the Dutch began to gather their merchantmen into convoys to protect against attack by the English fleets.