Battle of Prostken The Battle of Prostken was fought near Prostken , Duchy of Prussia ( today Prostki in Ełk County , Poland ) on October 8 , 1656 between forces of the Polish - Lithuanian Commonwealth and allied Crimean Tatars commanded by hetman Wincenty Gosiewski on one side , and on the other allied Swedish and Brandenburg forces commanded by Prince Georg Friedrich of Waldeck , reinforced by cavalry of Prince Bogusław Radziwiłł . The Commonwealth forces won the battle , annihilating enemy forces and taking Radziwiłł captive . Background . In the late summer of 1656 , Swedish and Brandenburgian armies left Warsaw , retreating towards northwest . Polish - Lithuanian commanders decided then to invade the Duchy of Prussia , which had been a vassal state to Kingdom of Poland , and which , as Brandenburg - Prussia , had been an ally of the Swedish Empire ( see Treaty of Konigsberg ( 1656 ) . The objective of the invasion was to make “ The Great Elector ” , Frederick Wilhelm , to end the alliance with Sweden . Polish - Lithuanian forces were commanded by Lithuanian Field Hetman Wincenty Korwin Gosiewski . They were supported by a unit of Crimean Tatars , under Subchan Ghazi Aga . Altogether , Gosiewski ’ s army had some 8 , 000 - 10 , 000 men ( including 2 , 000 Tatars ) . In early October 1656 , it crossed the Narew river , either near Lomza or Wizna , and headed northwards , to Lyck ( Elk ) . Prussian commandant Prince Georg Friedrich of Waldeck concentrated his forces near Wasosz . His army had probably 3 , 500 men , mostly reiter s and dragoon s . Waldeck reached Prostken ( Prostki ) on October 6 , and camped on the eastern bank of the Elk river , near a bridge , which he planned to defend . He also got in touch with Boguslaw Radziwill , whose cavalry unit of 800 stationed in Rajgrod . Furthermore , additional Prussian units were stationed at some distance from Lyck . Upon hearing of the invasion , they set off to help Waldeck , but only some 1 , 000 men reached Prostken before the battle . A unit of 2 , 000 men and 12 cannons left Lyck on October 8 , which was too late . The Battle . In the morning of October 8 , first Tatar and Lithuanian units reached Prostken , while main forces were some two hours behind them . Gosiewski , who enjoyed numerical superiority over the enemy , decided to make the Prussians abandon their positions behind the river , and to destroy them in open field . At the same time , he sent Tatars towards Lyck , to face the Prussian reinforcements . In the first stage of the battle , the Lithuanians pretended to retreat , after an initial clash . This worked , as the Prussians crossed the river , while Waldeck , upon receiving news of Tatars , sent a cavalry unit of 500 towards Lyck . This unit was reinforced by the cavalry of Boguslaw Radziwill , which soon afterwards clashed with Tatars . When main Lithuanian force attacked the Prussians , Waldeck ordered his troops to return behind the river . Gosiewski attacked the Prussians , forcing them to retreat . Meanwhile the Tatars crossed the Elk , and attacked the camp . Soon afterwards , they were joined by the Lithuanians . The unit of Boguslaw Radziwill was completely destroyed , while Radziwill himself was captured . Prince Waldeck , with 500 soldiers , managed to escape , as the Tatars and the Lithuanians were too busy looting the Prussian camp . Altogether , the battle lasted app . five hours . Aftermath . After the victory , Gosiewski sent a letter to Frederick Wilhelm , urging him to abandon the Swedes . The Great Elector rejected this , which resulted in widespread looting and destruction of southeastern corner of the Duchy of Prussia , toether with neighbouring Polish counties of Wizna and Rajgrod . Gosiewski then marched with his army to Lithuania , while the Tatars returned to the Crimea .