John II Casimir was King of Poland and Grand Duke of Lithuania from 1648 until his abdication in 1668 as well as titular King of Sweden from 1648 until 1660. He was the second son of Sigismund III Vasa and Constance of Austria. His older brother and predecessor on the throne was Władysław IV Vasa.
As a prince, John Casimir embarked at Genoa for Spain in 1638 to negotiate a league with Philip IV against France, but was captured by Cardinal Richelieu and imprisoned at Vincennes where he remained for two years. He was released when his brother, Władysław IV, promised never to wage war against France. John Casimir then travelled extensively throughout western Europe and entered the order of Jesuits in Rome in 1643. He was made cardinal by Innocent X, however, after returning to Poland, he became a layman and succeeded his brother in 1648. His reign commenced amid the confusion and disasters caused by the great revolt of the Cossacks under Bohdan Khmelnytsky in Ukraine, 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.