The 15,000 French supported by 6,000 troops from the English Commonwealth besieged Dunkirk. Dunkirk was Spain's greatest base for privateers, and these privateers had wreaked havoc on English merchant shipping. It was defended by a garrison of about 3,000 in May 1658, while an English fleet of 18 ships, under Edward Mountagu, blockaded the port and prevented any reinforcement or supply by sea. The Spanish and their allies were caught by surprise as they were convinced that Turenne would attack Cambrai, while they thought Dunkirk was merely a diversion, and they responded belatedly and hastily. The approach to Dunkirk was made difficult as the inhabitants had opened the sluices and flooded the area, but Turenne persisted and opened the trenches on the night of 4/5 June.