of the Spanish economy, which depended upon silver and gold from the Americas and thereby damaged Spain's capacity for waging war. The English lost 1,500 to 2,000 merchant ships to Spanish privateers and instead of using captured English ships to replace their destroyed convoys, the Spanish government placed the care of Spanish trade in the hands of neutral Dutch merchantmen. Flanders An Anglo-French alliance against Spain was established when the Treaty of Paris was signed in March 1657. Based on the terms of the treaty, the English would join with France in her continuing war against Spain in Flanders. France would contribute an army of 20,000 men, England would contribute both 6,000 troops and the English fleet in a campaign against the Flemish coastal fortresses of Gravelines, Dunkirk and Mardyck. It was agreed that Gravelines would be ceded to France, Dunkirk and Mardyck to England. Dunkirk, in particular, was on the Commonwealth's mind mainly because of the privateers that were causing damage to the mercantile fleet. For Cromwell and the Commonwealth, the question of possession of Dunkirk thus passed from regional diplomatic possibility to urgent political necessity. The combined Anglo-French army for the invasion of Flanders was commanded by the great French Marshal Turenne. The Spanish Army of Flanders was commanded by Don Juan-José, an illegitimate son of the Spanish King Philip. The Spanish army of 15,000 troops was augmented by a force of 3,000 English Royalists—formed as the nucleus of potential army for the invasion of England by Charles II, with Charles's brother James, Duke of York, among its commanders. The Commonwealth fleet blockaded Flemish ports but, to Cromwell's annoyance, the military campaign started late in the year and was subject to many delays. Marshal Turenne spent the summer of 1657 campaigning against the Spanish in Luxembourg and made no move to attack Flanders until September. Mardyck was captured on 22 September and garrisoned by Commonwealth troops. Dunkirk was besieged in May 1658. A Spanish relief force attempted to lift the siege but was defeated on 4 June at the Battle of the Dunes. The Commonwealth contingent in Turenne's army fought with distinction and impressed their French allies with a successful assault up a strongly defended sandhill 150 feet high during the battle. When Dunkirk surrendered to Turenne on 14 June, Cardinal Mazarin honoured the terms of the treaty with Cromwell and handed the port over to the Commonwealth,