The political tangle that produced the Battle of the Dunes was almost beyond satire. Cromwell's Commonwealth had allied with Catholic France against Spain. The exiled Charles II, scraping by in Bruges, had allied himself with Spain in hopes of one day invading England with Spanish gold and a Royalist army. Philip IV of Spain gave Charles enough money to raise five regiments - not the invasion force Charles wanted, but the nucleus of one. Meanwhile, French rebels of the Fronde, led by the Great Conde, a prince of the blood, fought on the Spanish side against their own king. So when Spain's relief army marched out to break Turenne's siege of Dunkirk, it carried English Royalists fighting against English Republicans, and French Frondeurs fighting against French royalists, under a Spanish commander. Almost nobody on either side had a simple national reason for being there.

Don John of Austria's relief column - 15,000 men, mostly Spanish, Walloon, and German foot with the English and Irish royalists under James, Duke of York - reached the dunes northeast of Dunkirk on 13 June. They had left their artillery a day's march behind. Both Conde and the Duke of York warned Don John that Turenne would attack immediately. Don John refused to believe it. Turenne, leaving 6,000 men to keep the city under siege, marched out before dawn on the 14th with about 14,000 - infantry, cavalry, and ten cannon - and arrayed his line with the left flank on the beach itself, where the receding tide would shortly expose the Spanish right to a cavalry charge along the wet sand.

The fighting began about ten in the morning. Turenne fired four or five artillery salvoes from his two unopposed batteries. English warships off the coast added harassing fire. Then the Cromwellian regiments on the French left went forward against a Spanish tercio under Don Caspar Boniface posted on a dune higher than the rest. Sir William Lockhart, Cromwell's ambassador to Paris and now infantry commander, watched his men climb. They crossed pikes at the summit with Spanish veterans and drove them down the far slope. The English formation became exposed. The Duke of York led two cavalry charges into its flank, riding personally at the head of his small troop of guards. Lockhart's regiment took the brunt - Lieutenant-Colonel Roger Fenwick and two captains were killed, nearly every other officer wounded - but the position was held.

While the redcoats fought their dune, Turenne played his real card. As the tide receded he funnelled most of his cavalry along the beach itself - forty squadrons of horsemen on hard wet sand, sweeping wide around the Spanish right where the dunes ended and the open shore began. The Spanish wing crumbled under that envelopment. The French infantry in the centre - the Guards, the Swiss, the regiments of Picardy and Turenne - advanced and met little resistance. The German and Walloon tercios in the centre fell back. The reserve Spanish cavalry got entangled with its own retreating foot. Only Conde, on the Spanish left, conducted a real defence: he counterattacked, was unhorsed, almost taken prisoner, and finally led his Frondeurs off the field in good order. By noon, two hours after it began, the battle was over.

Spanish losses were about 1,200 killed, 3,000 wounded, and 5,000 captured. The French lost about 400, perhaps half of them English. The Duke of York's troop of guards, which had charged repeatedly with the Duke at its head, was cut down but kept formation. After the battle the entire Royalist Army in Exile numbered perhaps 800 men - the dream of a Spanish-backed invasion of England died on those dunes. Dunkirk surrendered ten days later. The Treaty of the Pyrenees in 1659 confirmed France's annexation of Artois and other Flemish towns and ended a war that had lasted twenty-four years. Cromwell, who never saw the news from Dunkirk, died on 3 September. Twenty months later Charles II was on the English throne, and two years after that he sold Dunkirk back to France for £320,000 - selling, in effect, the prize his brother had failed to keep.