From 1653 to 1657, Turenne, seconded by Marshal La Ferte and sometimes by Cardinal Mazarin in person, constantly kept the Spaniards and the Prince of Conde in check, recovering the places but lately taken from France and relieving the besieged towns; without ever engaging in pitched battles, he almost always had the advantage. Mazarin resolved to strike a decisive blow. It was now three years since, after long negotiations, the cardinal had concluded with Cromwell, Protector of the Commonwealth of England, a treaty of peace and commerce, the prelude and first fruits of a closer alliance which the able minister of Anne of Austria had not ceased to wish for and pave the way for. On the 23d of March, 1657, the parleys ended at last in a treaty of alliance offensive and defensive; it was concluded at Paris between France and England. Cromwell promised that a body of six thousand English, supported by a fleet prepared to victual and aid them along the coasts, should go and join the French army, twenty thousand strong, to make war on the Spanish Low Countries, and especially to besiege the three forts of Gravelines, Mardyk, and Dunkerque, the last of which was to be placed in the hands of the English and remain in their possession. Six weeks after the conclusion of the treaty, the English troops disembarked at Boulogne; they were regiments formed and trained in the long struggles of the civil war, drilled to the most perfect discipline, of austere manners, and of resolute and stern courage; the king came in person to receive them on their arrival; Mardyk was soon taken and placed as pledge in the hands of the English. Cromwell sent two fresh regiments for the siege of Dunkerque. In the spring of 1658, Turenne invested the place. Louis XIV. and Mazarin went to Calais to be present at this great enterprise.