The brothers had planned to go to Turkey, but after hearing of the Khmelnytsky uprising they decided to return to Poland. Marek and John Sobieski left Brussels on 24 July 1648. Adulthood After returning to Poland, Sobieski and his brother came to Zamość, which at the time was being besieged by Cossacks. In 1648, 1649 and 1650 Sobieski was elected a member of the parliament (sejm walny) of the Polish–Lithuanian Commonwealth. In 1649 he was among the electors who voted for John II Casimir as King of Poland. Then, as a head of a chorągiew husarska (military unit formed by Hussars) of 100 horse, the young starosta joined the army of Jeremi Wiśniowiecki. Sobieski was a member of the Polish defense crew besieged by Cossacks at Zbarazh from 10 July to 22 August 1649. In 1650 he fought against the Cossacks at Kamieniec Podilskiy. Marek Sobieski also took part in the Battle of Beresteczko on 28–30 June 1651. After the Polish victory, due his bravery, he received a saber of Tugay Bey, who had been killed during the battle. In September 1651 he fought at the Battle of Biała Cerkiew. When the Poles, Cossacks and Tatars began negotiations, Marek Sobieski was sent to the Cossack camp as a guarantor of safety for the Cossack leader, Bohdan Khmelnytsky, who was in the Polish camp. In 1652 Sobieski fought against the Cossacks near Bracław and during Bila Tserkva campaign. Marching with Jan Odrzywolski, but without his own hussar banner, Sobieski and his retinue came to the field hetman Kalinowski's camp at Batoh on May 31. Marek Sobieski and Odrzywolski were assigned to command cavalry banners during the subsequent battle, and in that capacity they fought on June 1 and 2. On June 2, during the second day of the Battle of Batoh after the defeat of Polish cavalry in the field before the Polish camp, Sobieski commanded a cavalry group, perhaps containing wounded field hetman Kalinowski, that withdrew to the eastern redoubt in Polish camp, where Cossacks destroyed them after bringing artillery. Following Polish soldier Wespazjan Kochowski's writings Sobieski might have been the last Polish commander defending the Polish camp against the Cossack-Tatar army. Young commander was taken prisoner or surrendered, to be ransomed in a future as it was practice of the day, by the Tatars or Cossacks. After the battle, the Cossacks paid the Tatars for possession of the prisoners,