Although Poland-Lithuania escaped the ravages of the Thirty Years' War (see Glossary), which ended in 1648, the ensuing two decades subjected the country to one of its severest trials. This colorful but ruinous interval, the stuff of legend and the popular historical novels of Nobel laureate Henryk Sienkiewicz (1846-1916), became known as the potop, or deluge, for the magnitude of its hardships. The emergency began with an uprising of Ukrainian Cossacks that persisted in spite of Warsaw's efforts to subdue it by force. After the rebels won the intervention of Muscovy on their behalf, Tsar Aleksei conquered most of the eastern half of the country by 1655. Taking advantage of Poland's preoccupation, Charles X of Sweden rapidly overran much of the remaining territory of the commonwealth in 1655. Pushed to the brink of dissolution, Poland-Lithuania rallied to recover most of its losses to the Swedes. Swedish brutality raised widespread revolts against Charles, whom the Polish nobles had recognized as their ruler in the meantime. Under Stefan Czarniecki, the Poles and Lithuanians drove the Swedes from their territory by 1657. Further complicated by noble dissension and wars with the Ottoman Turks, the thirteen-year struggle over control of Ukraine ended in the Truce of Andrusovo in 1667. Although Russia had been defeated by a new Polish-Ukrainian alliance in 1662, Russia gained eastern Ukraine in the peace treaty.

Despite the improbable survival of the commonwealth in the face of the potop, one of the most dramatic instances of the Poles' knack for prevailing in adversity, the episode inflicted irremediable damage and contributed heavily to the ultimate demise of the state. When Jan II Kaziemierz abdicated in 1668, the population of the commonwealth had been nearly halved by war and disease. War had destroyed the economic base of the cities and raised a religious fervor that ended Poland's policy of religious tolerance. Henceforth, the commonwealth would be on the strategic defensive facing hostile neighbors. Never again would Poland compete with Russia as a military equal.

Data as of October 1992