The union was initially established because the Vasa king secured election in Poland-Lithuania, immediately promising to propel both realms to new heights and grant them unparalleled, unquestionable dominance over Northern Europe, since they essentially encircled the Baltic Sea. However, the union was beset by major troubles from its inception, primarily due to religious friction. While Sweden maintained a wholly Lutheran makeup, the king began to encroach upon its Protestant populations. Given that Poland-Lithuania was both Catholic and significantly more populous than Sweden, it quickly assumed primacy over religious affairs, positioning the combined realm advantageously into an alliance with the Catholic Habsburgs, who were rivals of the Eastern Romans.

This religious imbalance caused deep resentment among the Lutheran Swedes, who spent years stewing over the matter. By the summer of 1594, this discontent escalated as numerous Swedish agitators, including peasants and nobles, formally expressed their ire over the oppression of their faith. Initially, the court in Warsaw and the king chose to ignore these tensions, believing the Swedes would eventually "learn their place". To ensure perpetual stability and quell these major complaints, an intervention was needed, leading to the enactment of the Edict of Toleration on April 1, 1598. This pivotal charter formally ensured that Lutheranism would remain the state religion of Sweden and that the ancient rights and privileges of the Swedish Riksdag would be respected. While these compromises led to a general stabilization of the Polish-Swedish Union by the early 17th century, the foundational structure of the state was inherently volatile. The sources emphasize that despite the outward appearance of stability, the core makeup of the realm remained a fragile facade, guaranteed to breed future conflict and division.