In 1648, the Zaporozhian Cossacks under Hetman Bohdan Khemelnytsky revolted against the Polish-Lithuanian Commonwealth under King Wladyslaw Vasa. The Cossacks were descendants of the original multiethnic settlers that settled the Ruthenian (Ukrainian) steppe after the fracturing of the Golden Horde in the 14th and 15th Centuries. They formed the border marches against the Tartars in the Crimea and the Ottoman Empire in the Balkans. The Orthodox Cossacks were normally very loyal to the Catholic Commonwealth, but the Counter-Reformation after the destructive Thirty Years War and rise of the magnates, the real power in the Commonwealth, changed that.
The Commonwealth was supposedly ruled by a king elected by a large gentry and nobility that made up the upper and middle classes. But in reality, it was ruled by about thirty ultra-rich landowners called magnates. The King had no real authority and ruled through the Parliament. However, most legislation was defeated due to a parliamentary device known as the “Liberum Veto”, where any noble could single handedly veto a bill. With perpetual gridlock, the magnates ruled vast estates virtually autonomously. In 1648, one magnate, Daniel Czaplinski, attempted to confiscate the technically free land of the Cossacks, including Khemelnytsky’s. Khemelnytsky appealed to King Wladyslaw Vasa, but the powerless king could do nothing. So Khemelnytsky revolted, and brought the entire Zaporozhian Sech with him. Additionally, Khemelnytsky acquired the help of the Tartar Horde under the ruthless Khan Tugar-Bey, and together they drove the Poles and Lithuanians from the Steppe.
After three years of brutal fighting which ravaged the eastern provinces (It was said that a good Polish or Ruthenian slave could be bought for just five piastres in the markets of Istanbul or Crimea), the new king, Jan II Casimir, managed to convince the Parliament to call a levee en masse of the Szalchta, or gentry, and with the personal armies of several magnates met the Zaporozhian Host and the Tartar Horde outside of the town of Berestechko in the Volhynia on 28 June 1651.
For three days, 300,000 soldiers and warriors of the two armies clashed in the largest land battle of the 17th century. The winged Polish husaria, mailed panzerini, and saber wielding szalchta of the Polish and Lithuanian Hetmans (warlords) met the Tartar arrows and scimitars, and the muskets and lances of the Cossacks in a freewheeling frenzy that more resembled a massive brawl than any organized battle. Both sides would win the field at times only to be stopped at the massive circled wagon-forts of the camps which allowed each side to regroup and counterattack.
On the third day, Togar-Bey was killed and the Tartars deserted away from the camp. Seeing the Tartars fleeing, many Cossacks followed suit. The Cossack wagon-fort fell when the only professional infantry that the King had, German mercenary musketeers, stormed the weakly defended camp. Khemelnytsky was captured but was pardoned on his word to bring the Cossacks back into the fold after some reforms by the Poles. But Khemelnytsky knew the king couldn’t keep his word due to the liberum veto.
Casualties were high on both sides due to the battle, but Cossacks’ were much higher, so much so that Khemelnytsky sought assistance from the Russian Tsar when he revolted again in 1654. The Russians would eventually subsume the Cossacks and invade the Commonwealth proper. While occupied with Russia, Sweden would also invade in 1655, followed by Brandenburg Prussia in 1656, and Transylvania, Wallachia, and Moldova (read: Ottomans) in 1657.
Poland-Lithuania would eventually prevail in what is now known as “The Deluge”, but the country was devastated, and the eastward expansion into Siberia would not be made by the Poles and Lithuanians, but the Russians.