From The Gates of Europe: A History of Ukraine, by Serhii Plokhy (Basic Books, 2017), Kindle pp. 161-164:
WHATEVER THE LEGAL and ideological underpinnings of the Pereiaslav agreement, the tsar honored Buturlin’s promise and gave the Cossacks what the Polish king had never agreed to: recognition of Cossack statehood, a Cossack register of 60,000, and privileged status for the Cossack estate. He also recognized the liberties enjoyed by other social strata under the Polish kings.
First and foremost, however, the agreement laid the foundations for a military alliance. It established no western boundary for the Cossacks’ territory—they could go as far as their sabers would take them. The Muscovite and Cossack armies entered the war against the commonwealth on their separate fronts: the Cossacks, assisted by a Muscovite corps, led the offensive in Ukraine, within the boundaries of the Kingdom of Poland; the Muscovite troops launched an offensive near Smolensk and moved west through Belarus and then into Lithuania, north of the Lublin border between the grand duchy and the kingdom. The joint offensive of Muscovite and Cossack troops brought unexpected results. Whereas in 1654 the Polish and Lithuanian troops, assisted by the Crimean khan, had managed to resist the offensive from the east, in the summer and fall of 1655 the Polish-Lithuanian counteroffensive collapsed: the Cossacks once again besieged Lviv, and Muscovite troops entered Vilnius, the capital of the grand duchy.
This was the beginning of the era known in Polish history as the Deluge. Not only did the Muscovite and Cossack armies move deep into the commonwealth, but in July 1655 the Swedes launched an offensive of their own across the Baltic Sea. By October, both Warsaw and the ancient Polish capital of Cracow were in Swedish hands. Alarmed by the prospect of a complete Polish collapse and a dramatic expansion of Sweden, which now claimed the parts of the Grand Duchy of Lithuania conquered by Muscovite troops, in the fall of 1656 Muscovite diplomats concluded an agreement with the commonwealth in Vilnius that put an end to Polish-Muscovite hostilities. Khmelnytsky and the Cossack officials were enraged at being denied access to the negotiations. The separate peace with Poland was leaving the Cossacks one on one with their traditional enemy. As far as they were concerned, the tsar was reneging on his main obligation under the Pereiaslav agreement—the military protection of his subjects.
Bohdan Khmelnytsky ignored the Muscovite-Polish deal and sent his army to help an ally of Sweden, the Protestant ruler of Transylvania, fight the Poles. Now, even the military alliance between the tsar and the Cossacks came into question. Khmelnytsky had been looking for new allies since Sweden’s entry into the war with Poland. The Swedes seemed determined to destroy the commonwealth, which Khmelnytsky also wanted. Negotiations to conclude a Ukrainian-Swedish agreement that would put an end to the commonwealth and guarantee the inclusion not only of Ukraine but also parts of what is now Belarus in the Cossack state gained new impetus from what the hetman regarded as the tsar’s betrayal of Ukraine.
Khmelnytsky, however, did not live to see the conclusion of this new international alliance. He died in August 1657, leaving the state he had created and the Cossacks he had led at a crossroads. Although Khmelnytsky believed that his alliance with the tsar had already run its course, he formally abided by the deal he had made in Pereiaslav. Events there became an important part of the old hetman’s large and contradictory legacy. Cossack chroniclers of the eighteenth century celebrated him very much in the same vein as the professors and students of the Kyivan College had done on his entrance into Kyiv in December 1648. They extolled him as the father of the nation, the liberator of his people from the Polish yoke, and the hetman who had negotiated the best possible arrangement with the tsar: they considered the Articles of Bohdan Khmelnytsky, approved by the tsar after Pereiaslav, a Magna Carta of Ukrainian liberties in the Russian Empire.
Far Outliers 