The truce of Zboriv did not last long, and in the spring of 1651 Khmelnytsky's Cossacks begun advancing west again. On 1 June 1651 Wiśniowiecki brought his private army to face the Cossacks in Sokal. He commanded the left wing of the Polish–Lithuanian army in the victorious Battle of Berestechko on 28–30 June . The Polish–Lithuanian army advanced after the retreating Cossacks, but on 17 July the King "left the whole army to Potocki ... and having given the order that the army march into Ukraine, the King himself parted ... to Warsaw to celebrate his victories over the Cossacks." Later that year, on 14 August, Wiśniowiecki suddenly fell ill while in a camp near the village of Pawołocz, and died on 20 August 1651, at the age of only 39. His cause of death was never known, while some (even contemporaries) speculated he was poisoned, but no conclusive evidence to support such a claim have ever been found. Based on sparse descriptions of his illness and subsequent investigations, some medical historians suggest the cause of death might have been a disease related to cholera. However, one account states, "following a cheerful conversation with other officers who had congregated for a military council in his tent on Sunday 13 August N.S. he had eaten some cucumbers with zest and washed them down with mead, and from that contracted dysentery. After lying ill for a week, he died there, at Pavoloch". He was given a "ceremonial funeral with the entire army present. On 22 August Wiśniowiecki's body was seen off with the utmost pomp on its journey to his residence".