With the signing of the treaty Lockhart's difficulties had only begun. The French laid siege in September to Fort Mardyke, which was taken and handed over to the English before the end of the month. Lockhart urged on Turenne the necessity of proceeding immediately to the siege of Dunkirk, but this was delayed till June 1658, by which time the Spaniards had strongly entrenched their position. On the death of Sir John Reynolds, the English general, Lockhart undertook the command of the English forces, and in the pitched battle before Dunkirk he charged the Spanish foot. The town was surrendered on 15 June, and on the 24th handed over to Lockhart, who was made governor by Cromwell, and proceeded to put it in a state of defence. He received no assistance from the French.