The War against Sigismund (1598–1599) was a pivotal conflict in Sweden's history, marking the final break between Sweden and the Polish-Lithuanian Commonwealth. The war erupted after Sigismund III Vasa, who was both King of Sweden and King of Poland, clashed with his uncle, Duke Charles of Södermanland (later Charles IX of Sweden). The conflict centered on Charles's resistance to Sigismund’s rule, driven by concerns over Sigismund’s Catholic faith in a largely Lutheran Sweden and his failure to abide by agreements guaranteeing religious freedom for Protestants.
After Sigismund's coronation in 1594, tensions escalated when Duke Charles, backed by the Swedish nobility and Lutheran clergy, defied the king's orders and took control of Sweden in Sigismund's absence. Despite Sigismund's attempts to reclaim the Swedish throne, including landing with a force of Polish and mercenary troops in 1598, his efforts were thwarted. The pivotal moment came with Sigismund’s defeat at the Battle of Stångebro in September 1598, after which he was forced to relinquish control of Sweden.
In 1599, Sigismund was officially deposed by the Swedish Riksdag (Parliament), and Charles consolidated his power, eventually becoming King Charles IX. This ended the brief Polish-Swedish union and initiated a series of wars between Sweden and Poland, with Sigismund's attempts to regain the Swedish crown fueling conflicts that would last for decades. The war solidified Sweden’s break from Catholicism and its political alignment with Protestantism under the Vasa dynasty, shaping its future as a major European power.