On 17 July he again landed on Zealand and besieged Copenhagen with its king Frederick III of Denmark and Norway, but Copenhagen repelled a major assault and managed to hold out long enough for the Dutch fleet under Lieutenant-Admiral Jacob van Wassenaer Obdam to relieve the city, defeating the Swedish fleet in the Battle of the Sound on 29 October 1658. Jutland had been reconquered by Denmark–Norway's allies, Poland and Brandenburg, and in 1659 Danish and forces liberated the Island of Funen in the Battle of Nyborg. As Baltic trade was vital to the Dutch economy they made clear to Charles they would not allow Sweden to control the Sound.