Sigismund II or Sigismund Augustus, 1520–72, king of Poland (1548–72). Crowned in 1530 to assure his succession, he assumed the royal functions at the death of his father, Sigismund I. By the Union of Lublin (1569) he transferred his hereditary grand duchy of Lithuania to the Polish crown, creating the unified Polish-Lithuanian state. His great diplomatic skill enabled him to conciliate the dissident elements both in Poland and among the Lithuanian magnates who opposed the fusion. Upon the dissolution (1561) of the Livonian Brothers of the Sword, Sigismund gained control over Courland, Latgale, and other parts of Livonia. Opposed in this claim by Holy Roman Emperor Ferdinand I, Sigismund granted (1562) the elector of Brandenburg hereditary succession in the duchy of Prussia in exchange for diplomatic support. The widened frontiers brought Sigismund into conflict with Ivan IV of Russia, who took (1563) Polotsk. The Polish Reformation reached its height during Sigismund's reign; in 1570 most of the Protestant sects formed a union to strengthen their cause. An open-minded, tolerant monarch and a loyal Roman Catholic, Sigismund sought peacefully to counteract the Reformation; he abolished (1562) ecclesiastic courts but introduced (1565) the Society of Jesus (see Jesus, Society of), which successfully preached the Counter Reformation. The Renaissance flowered at this time (see also Polish literature), and Sigismund was an accomplished humanist and theologian. The last of the Jagiello dynasty to rule Poland, Sigismund died childless. After an interregnum and the brief rule of Henry of Valois (later Henry III of France), Stephen Báthory was elected (1575) king.
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