sentences:  On 28–30 July the combined Swedes and Brandenburgers, 18,000 strong, after a three days’ battle, defeated John Casimir's army of 40,000 at Warsaw, however the Polish-Lithuanian forces promptly withdrew with no large losses and apparent strong will to fight another day, while Swedish host reoccupied the Polish capital again, causing much destruction to the city and its inhabitants.However, this feat of arms did not have the desired result for Charles, and when Frederick William compelled the Swedish king to open negotiations with the Poles, they refused the terms offered, the war resumed, and Charles concluded an offensive and defensive alliance with the elector of Brandenburg (Treaty of Labiau, 20 November 1656) which stipulated that Frederick William and his heirs should henceforth possess the full sovereignty of East Prussia.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.