By 26 May 1656 he received the position of the "chorąży koronny" (Standard-bearer of the Crown). During the three-day-long battle of Warsaw of 1656, Sobieski commanded a 2,000-man strong regiment of Tatar cavalry. He took part in a number of engagements over the next two years, including the Siege of Toruń in 1658. In 1659 he was elected a deputy to the Sejm (Polish parliament), and was one of the Polish negotiators of the Treaty of Hadiach with the Cossacks. In 1660 he took part in the last offensive against the Swedes in Prussia, and was rewarded with the office of "starost" of Stryj. Soon afterward he took part in the war against the Russians, participating in the Battle of Slobodyshche and Battle of Lyubar, and later that year he again was one of the negotiators of a new treaty with the Cossacks (the Treaty of Cudnów).