Complex sentence: After the battle, the Tatars, with blessings from the Cossacks, slaughtered the Polish captives to avenge Chmielnicki's defeat at Berest (Brest) in June 1651.
Split into simpler sentences: After the battle, the Tatars, with blessings from the Cossacks, slaughtered the Polish captives. This avenged Chmielnicki's defeat at Berest (Brest) in June 1651.