King Sigismund III Vasa was a monarch who ruled over both Poland and Sweden during the late 16th and early 17th centuries. Born in 1566, he was the son of King John III of Sweden and Catherine Jagellon. His reign in Poland began in 1587, and he became the king of Sweden in 1592, although his rule there was contested. Sigismund's reign was marked by political struggles and conflicts, particularly with the Swedish nobility. He aimed to strengthen the Polish-Lithuanian Commonwealth and promote Catholicism, which led to tensions with Protestant factions. His rule ended in 1632, and he is remembered for his efforts to unify the two crowns.