The Polish-Swedish union (1592–1599) was a brief but pivotal episode in Swedish history, characterized by the personal union of Sigismund III Vasa, who was both the elected King of Poland and the King of Sweden. The union began when Sigismund, a Catholic, inherited the Swedish throne from his father John III in 1592, uniting the Polish-Lithuanian Commonwealth and Sweden under his rule. However, it was fraught with religious and political tensions, as most of Sweden had converted to Lutheranism, and Sigismund's Catholic faith raised fears of a Catholic resurgence.
Sigismund's authority in Sweden was contested by his Protestant uncle, Duke Charles of Södermanland, who gained increasing support from the Swedish nobility and clergy. Sigismund’s promises to uphold Lutheranism in Sweden were quickly broken as he began favoring Catholics, which deepened opposition to his rule. Charles capitalized on this unrest, eventually leading to a civil conflict.
By 1598, Sigismund attempted to reassert control over Sweden but was decisively defeated at the Battle of Stångebro. In 1599, he was officially deposed by the Swedish Riksdag, and Charles took the throne as Charles IX. The union collapsed after only seven years, marking a permanent split between Sweden and Poland. Sigismund's ambition to reclaim the Swedish crown led to a series of Polish-Swedish wars, culminating in the Deluge during the mid-17th century, a devastating invasion of Poland by Sweden. This period reinforced Sweden's break from Catholic influence and set the stage for its rise as a Protestant power in Northern Europe.