From Lemberg Khmelnitzki proceeded with his troops in the direction of Warsaw, where at that time the election of a new king was taking place. The choice fell upon John Casimir, a brother of Vladislav IV., who had been Primate of Gnesen and a Cardinal (1648-1668). The new King entered into peace negotiations with the leader of the rebels, the hetman Khmelnitzki. But owing to the excessive demands of the Cossacks the negotiations were broken off, and as a result, in the spring of 1649, the flame of civil war flared up anew, accompanied by the destruction of many more Jewish communities. After a succession of battles in which the Poles were defeated, a treaty of peace was concluded between John Casimir and Khmelnitzki, in the town of Zborov. In this treaty, which was favorable to the Cossacks, a clause was included forbidding the residence of Jews in the portion of the Ukraina inhabited by the Cossacks, the regions of Chernigov, Poltava, Kiev, and partly Podolia (August, 1649). At last the Jews, after a year and a half of suffering and tortures, could heave a sigh of relief. Those of them who, at the point of death, had embraced the Greek Orthodox faith, were permitted by King John Casimir to return to their old creed. The Jewish women who had been forcibly baptized fled in large numbers from their Cossack husbands, and returned to their families. The Council of the Four Lands, which met in Lublin in the winter of 1650, framed a set of regulations looking to the restoration of normal conditions in the domestic and communal life of the Jews. The day of the Niemirov massacre (Sivan 20), which coincided with an old fast day in memory of the martyrs of the Crusades, was appointed a day of mourning, to commemorate the victims of the Cossack rebellion. Leading rabbis of the time composed a number of soul-stirring dirges and prayers, which were recited in the synagogues on the fateful anniversary of the twentieth of Sivan. But the respite granted to the Jews after these terrible events did not last long. The Treaty of Zborov, which was unsatisfactory to the Polish Government, was not adhered to by it. Mutual resentment gave rise to new collisions, and civil war broke out again, in 1651. The Polish Government called together the national militia, which included a Jewish detachment of one thousand men. This time the people's army got the upper hand against the troops of Khmelnitzki, with the result that a treaty of peace was concluded which was advantageous to the Poles. In the Treaty of Byelaya Tzerkov, concluded in September, 1651, many claims of the Cossacks were rejected, and the right of the Jews to live in the Greek Orthodox portion of the Ukraina was restored.[129]