The siege began as a bookkeeping problem. Privateers sailing out of Dunkirk had carried somewhere between 1,500 and 2,000 English merchant ships into Flemish ports in a single year - a wound that bled the Commonwealth's treasury faster than any tax could heal. So in 1657 Oliver Cromwell, Lord Protector and reluctant European, signed a treaty with the Catholic French he had spent a lifetime distrusting. The deal was blunt: 6,000 New Model Army infantry and an English fleet would join Marshal Turenne's campaign in the Spanish Netherlands, on one condition. When Dunkirk fell, it became English. Mazarin agreed. The strangest alliance of the century was set.

On 25 May 1658, Turenne's army of 7,000 reached Dunkirk through ground the defenders had deliberately flooded by opening sluices. Soldiers waded chest-deep through standing water, rifles and powder held overhead, while engineers behind them laid corduroy roads for the artillery train. The advice had been clear: bring no guns under these conditions. Turenne brought them anyway. Within days, reinforcements swelled his force past 20,000. Admiral Edward Montagu's 18 English warships sealed the seaward approach. The Spanish garrison, around 3,000 strong under the Marquis of Lede, found itself encircled before its commanders quite realised what was happening - they had been certain Turenne was marching on Cambrai.

Sieges in seventeenth-century Flanders were not patient affairs of starvation but constant, exhausting violence. The garrison sortied on the very first night the trenches opened. They came again the next morning, cavalry and twenty guns. On the fourth day, a strong onshore wind drove sand into French faces, blinding them, and the defenders rushed out under that cover, filling in the lead trench and killing or wounding about a hundred men of the regiments of Picardy and Plessis. The tide knocked down the wooden stockade the French had built along the dunes. Bomb-chests had to be carried on and off the beach with each cycle of the sea. Through it all the trenches crept forward, foot by foot, toward the counterscarp.

Don Juan of Austria's relief army arrived on 13 June - too quickly, having outpaced its own artillery by more than a day's march. Both Condé and the exiled Duke of York warned him that Turenne would attack before the guns caught up. Don Juan dismissed the warning. The next morning at ten o'clock, on a line of sand-hills above the beach, the Battle of the Dunes began. By noon the Spanish army was broken: about 1,200 killed, 3,000 wounded, 5,000 captured. The Marquis of Lede was mortally wounded in a final summons to surrender, and on 25 June 1658 - twenty-two days after the trenches opened - the 1,800 surviving defenders marched out under terms.

Louis XIV himself placed the keys of Dunkirk into the hand of Sir William Lockhart on 26 June. England now held a Continental city for the first time since Calais was lost a hundred years earlier. Cromwell did not enjoy it long - he died two months later. His son Richard lasted nine months as Lord Protector before the Commonwealth collapsed. Charles II returned to the throne in May 1660, inherited Dunkirk, and in 1662 sold it back to Louis XIV for £320,000 to settle his own debts. The town that had taken thousands of lives and reshaped a treaty became, in the end, a line in the royal household accounts.