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.
"At Brussels," says M.Guizot in his _Histoire de la Republique d'Angleterre et de Cromwell,_ "neither Don Juan nor the Marquis of Carracena would believe that Dunkerque was in danger; being at the same time indolent and proud, they disdained the counsel, at one time of vigilant activity and at another of prudent reserve, which was constantly given them by Conde; they would not have anybody come and rouse them during their siesta if any unforeseen incident occurred, nor allow any doubt of their success when once they were up and on horseback.