Labiau involved an essential modification of Charles's Baltic policy; but the alliance with the elector of Brandenburg had now become indispensable for him on almost any terms. The difficulties of Charles X in Poland are believed to have caused him to receive the tidings of the Danish-Norwegian declaration of war on 1 June 1657 with extreme satisfaction. He had learnt from Torstensson that Denmark was most vulnerable if attacked from the south, and he attacked Denmark with a velocity which paralysed resistance. At the end of June 1657, at the head of 8,000 seasoned veterans, he broke up from "Bromberg" (Bydgoszcz) south of Pomerania and reached the borders of Holstein on 18 July. The Danish army dispersed and the Swedes recovered the duchy of Bremen. In the early autumn Charles's troops swarmed over Jutland and firmly established themselves in the duchies. But the fortress of Fredriksodde (Fredericia) held Charles's smaller army at bay from mid-August to mid-October, while the fleet of Denmark - Norway, after two days’ battle, compelled the Swedish fleet to abandon its projected attack on the Danish islands. The position of the Swedish king had now become critical. In July Denmark-Norway and Poland-Lithuania concluded an offensive and defensive alliance. Still more ominously for the Swedes, the elector of Brandenburg, perceiving Sweden's difficulties, joined the league against Sweden and compelled Charles to accept the proffered mediation of Oliver Cromwell, Coenraad van Beuningen and Cardinal Mazarin. The negotiations foundered, however, upon the refusal of Sweden to refer the points in dispute to a general peace-congress, and Charles received encouragement from the capture of Fredriksodde, 23–24 October, whereupon he began to make preparations for conveying his troops over to Funen in transport vessels. But soon another and cheaper expedient presented itself. In the middle of December 1657 began the great frost, which would prove so fatal to Denmark-Norway. In a few weeks the cold had grown so intense that the freezing of an arm of the sea with so rapid a current as the Small Belt became a conceivable possibility; and henceforth meteorological observations formed an essential part of the strategy of the Swedes.