John II Casimir, king of Poland, born March 21, 1609, died in Nevers, France, Dec. 16, 1672. He was a younger son of Sigismund III., of the house of Vasa, by an Austrian princess, who was baffled in her schemes to procure him the throne by his loyal adherence to his elder half brother Ladislas, who after the death of Sigismund was elected king (1632). In 1638 he embarked at Genoa for Spain to negotiate a league with Philip III. against France; but suffering shipwreck on the coast of Provence, he was seized and by order of Richelieu imprisoned at Vincennes, where he remained two years, and was only released on promise of his brother the king of Poland never to wage war against France. He then travelled through various countries of western Europe, entered the order of Jesuits in Rome, was made cardinal by Innocent X., but after his return to Poland again became a layman, and, having succeeded his brother in 1648, married his widow Maria Luisa Gonzaga. His reign commenced amid the confusion and disasters caused by the great revolt of the Cossacks under Chmielnicki, who had advanced into the very heart of Poland. The power of the king had been stripped of almost all its prerogatives by the growing influence of the nobles.

Russia and Sweden, which had long been active enemies of Poland, availed themselves of its distracted condition, and renewed their attacks. George Rakoczy of Transylvania, too, invaded the Polish territory, while diet after diet was dissolved by abuses of the liberum veto. Charles Gustavus of Sweden triumphantly marched through the country, and occupied Cracow (1655), John Casimir having fled to Silesia. Before Czentochowa, however, the Swedes met with an unexpected check, and a confederation of the nobles against all enemies of the country having been formed, Czar-niecki won a series of victories over the Swedes, Transylvanians, Cossacks, and Russians. The wars with the Swedes and Russians were terminated by treaties involving considerable cessions of provinces on the Baltic and the Dnieper on the part of Poland, which also lost its sway over the Cossacks, who put themselves under the protection of the czar. During these long disturbances John Casimir, though feeble and of a peaceful disposition, frequently proved his patriotism and bravery.

The intrigues of his wife in favor of the duke of Enghien, son of the prince of Con-de, as successor to the throne, having brought about a rebellion under George Lubomirski, and a bloody though short civil war, the king finally resolved upon abdication, and resigned his crown at the diet of Warsaw, Sept. 16, 1668. In the following year he retired to France, where he was hospitably treated by Louis XIV. His wife had died without issue before his abdication. His body was removed to the cathedral of Cracow in 1676, his heart only being interred in St. Germain des Pres, of which Louis XIV. had made him abbot. John Casimir's reign was one of the most disastrous in the history of Poland, whose dismemberment by the houses of Moscow, Brandenburg, and Hapsburg, as it took place 100 years after his death, he predicted in a memorable speech to the diet of 1661.