The great Battle of Beresteczko (Berestechko in Ukrainian) in Volhynia (Wołyń in Polish, Volyn in Ukrainian), Ukraine, lasted from June 28 to June 30, 1651, between the Polish army under King Jan II Casimir and Ukrainian Cossack and peasant forces, led by Hetman Bohdan Khmelnytsky, and their Crimean Tatar allies. The Polish army had around 57,000 troops, the Ukrainians around 100,000 plus 25,000 Tatar cavalry. The battle was won by the Poles after the Tatars deserted the battlefield. As a result, Khmelnytsky was forced to sign the Treaty of Biała Cerkiew (Bila Tserkva in Ukrainian) with the Poles.
The battle was probably the biggest in the 17th century.