Amsterdam and De Graeff were at the high point of their power and in 1656 mounted an expedition under Michiel de Ruyter to the Mediterranean Sea and - in Charles X Gustav of Sweden's war against Poland - another under Jacob van Wassenaer Obdam to the Baltic Sea. Fortunately for the Netherlands, this war ended in Danzig being declared neutral. The four strong-headed mayors decided to send Coenraad van Beuningen to Copenhagen to incite Denmark into a war against Sweden. To everyone's amazement, in the middle of the winter the Swedish king crossed over from Jutland, across the Great Belt, to Copenhagen. In a second expedition to relieve Copenhagen, Witte de With participated in the Battle of the Sound. Cornelis de Graeff was prepared to begin the fight against Sweden, against the advice of the Grand Pensionarys. When Charles X unexpectedly died in 1660, Sweden made peace. Amsterdam sent also admiral Michiel de Ruyter against the English, against the orders of the parliament of the Netherlands. Despite these differences in approach with De Witt, he and De Graeff stayed on good terms. De Witt in 1660 observed of Cornelis: "with the gentleman of Zuidpolsbroek, in nothing was to do something".