Thread by @akoz33

#OTD in 1651 began a huge (one of the largest in the 17th century) battle of Berestechko between the army of the Polish-Lithuanian Commonwealth, commanded personally by King John II Casimir Vasa and a combined Cossack-Tatar army

commanded by Bohdan Khmelnitsky & Khan Islam III Giray. The 10 day battle (which actually consisted of a 3 day field battle followed by a 7 day siege of a fortified Cossack camp, after the Tatars ran away talking Khmelnitsky with them) ended with a total Commonwealth victory

appeared to put an end to the Khmelnytski Uprising. However the hostility between the king and the nobles allowed the Cossack forces to recover and the war continued after a brief ceasefire at Bila Tsyrkva.
Both armies were enormous although their exact sizes are uncertain. The

Commonwealth Army (which also included loyal Cossacks and Polish & Lithuanian Tatars) is estimated at about 34,000 professional royal troops, 30,000 nobles and 15,000 private troops of Lithuanian magnates. There were about 100,000 Cossacks, of whom 40,000 were so called

“registered Cossacks” and 60,000 of so called “chern” (armed peasants - they were mostly responsible for the notorious massacres of Jews) and about 30,000 Crimean Tatars personally lead by the Khan.
The Kmelnytsky Uprising began with disastrous Polish defeats, which lead to the

treaty of Zborow, which was favorable to the Cossacks - it not only increased the so called “register” (the number of Cossacks who were in the pay of the Commonwealth), granted Khmelnytsky the title of Hetman & full autonomy to Ukraine but it also forbade Catholics and Unites

to hold any offices in Ukraine and forbade Jews to reside there.Polish nobles never intended to accept this and both sides immediately started to prepare to resume hostilities, which is why the two armies were unusually large & well equipped.

Khmelnytsky told the Crimean Tatar Khan that the Commonwealth was very weak and it’s army would collapse without fighting, and the Tatars would able to pillage at will without serious resistance. This proved the Cossacks undoing.
The other reason for the Polish victory was

John Casimir’s decision to adopt the so called “Dutch” or “Swedish” formation (used, for example, by Gustav Adolf at Breitenfeld). The basic idea was arranging the infantry and cavalry in a “chessboard” like mutually supporting squares. This formation was completely

unfamiliar to the Tatars, who while charging Polish cavalry suffered huge losses from musket fire. These losses angered the Khan so much that on the third day he ordered his army to leave. Khmelnytsky and a few Cossacks chased them to persuade them to return themselves taken

hostage by the Tatars.
The remaining Cossacks formed a fortified camp, which was besieged by the Commonwealth troops. Negotiations began but the conditions offered were so harsh, that they didn’t get anywhere. About 2,000 elitę Cossack cavalry lead by the famous colonel Bohun,

managed to escape, before the camp was stormed and most of its defenders slaughtered. It is estimated that between 10,000 to 30,000 Cossacks were killed.
On the field of battle (in Ukraine) there is now a monument to the Cossack dead