Sigismund III Vasa was King of Poland and Grand Duke of Lithuania from 1587 to 1632 and, as Sigismund, King of Sweden and Grand Duke of Finland from 1592 to 1599. He was the first Polish sovereign from the House of Vasa. A religious zealot, he imposed Roman Catholic doctrine across the vast realm, and his crusades against neighbouring states marked Poland's largest territorial expansion. As an enlightened despot, he presided over an era of prosperity and achievement, further distinguished by the transfer of the country's capital from Kraków to Warsaw.
Sigismund was the son of John III of Sweden and his first wife, Catherine Jagiellon, daughter of King Sigismund I of Poland. Elected monarch of the Polish–Lithuanian Commonwealth in 1587, he sought to unify Poland and Sweden under one Catholic kingdom, and when he succeeded his deceased father in 1592 the Polish–Swedish union was created. Opposition in Protestant Sweden caused a war against Sigismund headed by Sigismund's uncle Charles IX, who deposed him in 1599.
Sigismund attempted to hold absolute power in all his dominions and frequently undermined parliament.