In 1655 occurred an event which called forth all the Great Elector’s powers of statesmanship. The old hostility between Poland and Sweden, the two most dangerous neighbours of Brandenburg, suddenly flamed out again. John Casimir, king of Poland, refused to acknowledge Charles Gustavus, who had succeeded to the Swedish throne on the abdication of Christina. [Sidenote: =War between Sweden and Poland, 1655.=] Charles, who had been brought up in the school of the Thirty Years’ War and was no mean soldier, determined to avenge the insult, and demanded from the Great Elector the right of passage through eastern Pomerania into Poland, in order to avoid the difficult task of the siege of the sea fortress of Dantzig, which had cost Gustavus Adolphus many weary hours some twenty-five years before. Frederick William was not in a position to resist, and after making a few demonstrations to cover appearances, gave the required permission. The Swedes, using Pomerania as their basis of operations, poured across Brandenburg into Poland, defeated John Casimir, drove him back on Cracow and then returned leisurely into west Prussia to form the siege of Dantzig. The Great Elector now thought he saw his opportunity. [Sidenote: =Unsuccessful intrigue of Frederick William against Sweden.=]The Poles were beaten not conquered. Denmark was ever ready to strike a blow at her old enemy across the Baltic. Charles X. was fully occupied round Dantzig. A well-planned alliance and a well-timed stroke might bring Sweden to her knees and win his own independence of Poland. But Charles was too quick for him. [Sidenote: =Acknowledgment of Swedish suzerainty over East Prussia, 1656.=] Hearing of the negotiations in the middle of the winter of 1655–56 he at once broke up his camp and marched into east Prussia on Königsberg. Frederick William had to make his peace as best he could. By the treaty of Königsberg, developed by the treaty of Marienbad, concluded in June 1656, Brandenburg was forced to acknowledge the suzerainty of Sweden over east Prussia, instead of that of Poland, to grant to the Swedes free passage through the country, and to provide a contingent to serve under Swedish orders in the Polish war. [Sidenote: =War with Poland, 1656.=]