Under the command of hetman Bohdan Khmelnytsky, the Zaporozhian Cossacks, allied with the Crimean Tatars and local Ukrainian peasantry, fought against Commonwealth's forces.

It ended the Polish Catholic szlachta′s domination over the Ukrainian Orthodox population; at the same time, it led to the eventual incorporation of eastern Ukraine into the Tsardom of Russia initiated by the 1654 Pereiaslav Agreement, whereby the Cossacks would swear allegiance to the tsar while retaining a wide degree of autonomy.

"[9] In 1569 the Union of Lublin granted the southern Lithuanian-controlled Ruthenian voivodeships of Volhynia, Podolia, Bracław and Kiev—to the Crown of Poland under the agreement forming the new Polish–Lithuanian Commonwealth (Rzeczpospolita).

[10] This assimilation of Polish culture on the part of the Ruthenian nobility alienated them from the lower classes, and most especially to the Cossacks, who proved stubbornly resistant to Catholicism and Polonization.

This szlachta, along with the actions of the upper-class Polish magnates, oppressed the lower-class Ruthenians, with the introduction of Counter-Reformation missionary practices and the use of Jewish arendators to manage their estates.

The pressure of Catholic expansionism culminated with the Union of Brest in 1596, which attempted to retain the autonomy of the Eastern Orthodox churches in present-day Ukraine, Poland and Belarus by aligning themselves with the Bishop of Rome.

He participated in campaigns for Grand Crown Hetman Stanisław Koniecpolski, led delegations to King Władysław IV Vasa in Warsaw and generally was well respected within the Cossack ranks.

In 1647 Chyhyryn deputy of starosta (head of the local royal administration) Daniel Czapliński openly started to harass Khmelnytsky on behalf of the younger Koniecpolski in an attempt to force him off the land.

On two occasions raids were made to Subotiv, during which considerable property damage was done and his son Yurii was badly beaten, until Khmelnytsky moved his family to a relative's house in Chyhyryn.

The Cossacks were already on the brink of a new rebellion as plans for the new war with the Ottoman Empire advanced by the Polish king Władysław IV Vasa were cancelled by the Sejm.

On January 25, 1648, Khmelnytsky brought a contingent of 400–500 Cossacks to the Zaporizhian Sich and quickly killed the guards assigned by the Commonwealth to protect the entrance.

Either because they underestimated the size of the uprising,[12] or because they wanted to act quickly to prevent it from spreading,[13] the Commonwealth's Grand Crown Hetman Mikołaj Potocki and Field Crown Hetman Marcin Kalinowski sent 3,000 soldiers under the command of Potocki's son, Stefan, towards Khmelnytsky, without waiting to gather additional forces from Prince Jeremi Wiśniowiecki.

In addition to the loss of significant forces and military leadership, the Polish state also lost King Władysław IV Vasa, who died in 1648, leaving the Crown of Poland leaderless and in disarray at a time of rebellion.

[14] News of the peasant uprisings now troubled a nobleman such as Khmelnytsky; however, after discussing information gathered across the country with his advisers, the Cossack leadership soon realized the potential for autonomy was there for the taking.

After obtaining the ransom, he moved to besiege Zamość, when he finally heard about the election of the new Polish King, John Casimir II, whom Khmelnytsky favored.

He made a triumphant entry into Kiev on Christmas Day in 1648, and he was hailed as "the Moses, savior, redeemer, and liberator of the people from Polish captivity... the illustrious ruler of Rus".

In February 1649, during negotiations with a Polish delegation headed by nobleman Adam Kysil in Pereiaslav, Khmelnytsky declared that he was "the sole autocrat of Rus" and that he had "enough power in Ukraine, Podolia, and Volhynia... in his land and principality stretching as far as Lviv, Chełm, and Halych".

When Poland–Lithuania and Russia signed the Truce of Vilna and agreed on an anti-Swedish alliance in 1657, Khmelnytsky's Cossacks supported the invasion of the Commonwealth by Sweden's Transylvanian allies instead.

Yeven Mezulah, the contemporary 17th-century chronicle by Nathan ben Moses Hannover, an eyewitness, states: Wherever they found the szlachta, royal officials or Jews, they [Cossacks] killed them all, sparing neither women nor children.

According to the book known as History of the Rus, Khmelnytsky's rationale was largely mercantile and the Jews of Brody, which was a major trading centre, were judged to be useful "for turnovers and profits" and thus they were only required to pay "moderate indemnities" in kind.

But even if we accept the lower figure, the number of victims still remains colossal, even exceeding the catastrophes of the Crusades and the Black Death in Western Europe.

In the localities on the right shore of the Dnieper or in the Polish part of Ukraine as well as those of Volhynia and Podolia, wherever Cossacks had made their appearance, only about one tenth of the Jewish population survived.

"[36] Martin Gilbert in his Jewish History Atlas published in 1976 states, "Over 100,000 Jews were killed; many more were tortured or ill-treated, others fled ...."[38] Many other sources of the time give similar figures.

"[44]A 2003 study by Israeli demographer Shaul Stampfer of Hebrew University dedicated solely to the issue of Jewish casualties in the uprising concludes that 18,000–20,000 Jews died out of a total population of 40,000.

[46] Orest Subtelny concludes: Between 1648 and 1656, tens of thousands of Jews—given the lack of reliable data, it is impossible to establish more accurate figures—were killed by the rebels, and to this day the Khmelnytsky uprising is considered by Jews to be one of the most traumatic events in their history.

At the initial stages of the uprising, armies of the magnate Jeremi Wiśniowiecki, on their retreat westward inflicted terrible retribution on the civilian population, leaving behind them a trail of burned towns and villages.

After the Cossacks' alliance with Tsardom of Russia was enacted, the Tatar raids became unrestrained; coupled with the onset of famine, they led to a virtual depopulation of whole areas of the country.

The extent of the tragedy can be exemplified by a report of a Polish officer of the time, describing the devastation: I estimate that the number of infants alone who were found dead along the roads and in the castles reached 10,000.