Official titles in Latin: Ioannes Casimirus, Dei Gratia rex Poloniae, magnus dux Lithuaniae, Russie, Prussiae, Masoviae, Samogitiae, Livoniae, Smolenscie, Severiae, Czernichoviaeque; nec non Suecorum, Gothorum, Vandalorumque haereditarius rex, etc.
His father Sigismund, grandson of Gustav I of Sweden, had in 1592 succeeded his own father to the Swedish throne, only to be deposed in 1599 by his uncle, Charles IX of Sweden. This led to a long-standing feud wherein the Polish kings of the House of Vasa claimed the Swedish throne, resulting in the Polish-Swedish War of 1600-1629. Poland and Sweden were also on opposite sides in the Thirty Years' War (1618-1648), although in that war Poland for the most part avoided taking part in any major military actions.
Jan Kazimierz for most of his life remained in the shadow of his brother, Władysław IV Waza. He had few friends among the Polish nobility (szlachta), as he openly sympathized with Austria and showed disregard and contempt for Polish culture. Unfriendly, secretive, dividing his time between lavish partying and religious contemplation, and disliking politics, he did not have a strong power base nor influence at the Polish court. He did display talent as a military commander, showing his abilities in the Smolensk War against Muscovy (1633).
Between 1632 and 1635, Władysław IV sought to enhance his brother's influence by negotiating a marriage for Jan Kazimierz to Christina of Sweden, then to an Italian princess, but to no avail. In 1635 Jan Kazimierz undertook a diplomatic mission to Vienna, which he abandoned to join the army of the Holy Roman Empire and fight against the French. After his regiment was defeated in battle, he spent a year living lavishly at the Viennese court.
In 1636 he returned to the Polish-Lithuanian Commonwealth and fell in love with Baroness Guldentern, but his desire to marry her was thwarted by King Władysław. In return, Władysław attempted to make him the sovereign of Courland, but this was vetoed by the Commonwealth parliament (Sejm). Taking offence at this, Jan Kazimierz in 1638 left for Spain to become Viceroy of Portugal, but was captured by French agents and imprisoned by order of Cardinal Richelieu until 1640. He was then freed by a diplomatic mission of the Voivod of Smolensk, Aleksander Gosiewski.
In 1641 Jan Kazimierz decided to become a Jesuit. In 1642 he again left the Polish-Lithuanian Commonwealth, accompanying his sister to Germany. In 1643 he joined the Jesuits, against vocal opposition from King Władysław, causing a diplomatic rift between the Commonwealth and the Pope. Jan Kazimierz was made a cardinal, but in December 1646, finding himself unsuited to ecclesiastical life, he returned to Poland. In October 1647 he resigned as cardinal to stand in elections for the Polish throne. He attempted to gain the support of the Habsburgs and marry an Austrian princess.
In 1648 Jan Kazimierz succeeded to the Polish throne. The reign of the last of the Vasas in the Polish-Lithuanian Commonwealth would be dominated by war with Sweden ("The Deluge"), the scene for which had been set by the Commonwealth's two previous Vasa kings. During the Deluge, nearly all of Poland was captured by the Swedes, who, though unable to retain most of their conquests and forced to retreat, had fairly devastated the entire country.
Jan Kazimierz (kneeling) takes an oath at Lwów in 1655, during "The Deluge," that he would drive the Swedes from Poland, improve the lot of the peasants, and honor the rights and privileges of the szlachta. Painting by Jan Matejko.
In 1660 Jan II Kazimierz was forced to renounce his claim to the Swedish throne and acknowledge Swedish sovereignty over Livonia and the city of Riga.
Jan Kazimierz had married his brother's widow, Mary Louise of Mantua (Polish: Maria Ludwika), who was a major support to the depression-prone King. Maria Ludwika died in 1667.
On September 16, 1668, Jan II Kazimierz abdicated the Polish-Lithuanian throne, and returned to France, where he joined the Jesuits and became abbot of St. Martin's monastery in Nevers. He died in 1672.
Jan Kazimierz left no surviving children. All his brothers and sisters having predeceased him without surviving issue, he was the last of the line of Bona Sforza. With him, all the legitimate issue of Alfonso II of Naples died out. His heir in Ferrante I of Naples and in the Brienne succession was his distant cousin, Henry de La Tremoille, Prince of Talmond and Taranto, the heir-general of Federigo di Aragona (second son of Ferrante I and Isabella of Taranto), who also was the heir-general of Federigo's first wife, Anne of Savoy.
