Sigismund III Vasa (also known as Sigismund III of Poland, Polish: Zygmunt III Waza, Swedish: Sigismund, Lithuanian: Žygimantas Vaza, English exonym: Sigmund; 20 June 1566 – 30 April 1632 N.S.) was King of Poland and Grand Duke of Lithuania, monarch of the united Polish–Lithuanian Commonwealth from 1587 to 1632, and King of Sweden (where he is known simply as Sigismund) from 1592 as a composite monarchy until he was deposed in 1599. He was the son of King John III of Sweden and his first wife, Catherine Jagellonica of Poland.

Elected to the throne of the Polish–Lithuanian Commonwealth, Sigismund sought to create a personal union between the Commonwealth and Sweden (Polish–Swedish union), and succeeded for a time in 1592. After he had been deposed in 1599 from the Swedish throne by his uncle, Charles IX of Sweden, and a meeting of the Riksens ständer (Swedish Riksdag), he spent much of the rest of his life attempting to reclaim it.

Shortly after his victory over his internal enemies, Sigismund took advantage of a period of civil unrest in Muscovy (known as the Time of Troubles) and invaded Russia, holding Moscow for two years (1610–12) and Smolensk thereafter. In 1617 the Polish–Swedish conflict, which had been interrupted by an armistice in 1611, broke out again. While Sigismund's army was also fighting Ottoman forces in Moldavia (1617–21), King Gustavus II Adolphus of Sweden (Charles IX's son) invaded Sigismund's lands, capturing Riga (1621) and seizing almost all of Polish Livonia. Sigismund, who concluded the Truce of Altmark with Sweden in 1629, never regained the Swedish crown. His Swedish wars resulted, moreover, in Poland's loss of Livonia and in a diminution of the kingdom's international prestige.

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