Many historians agree that Sigismund considered Poland as a tool to eventually regain the throne of Sweden.
To this end he tried to strengthen his royal power and allied himself with Habsburgs and Counter-Reformation forces.
His policies were opposed by many within the circles of the wealthy Polish nobility (the szlachta), most notably the chancellor Jan Zamoyski.
This led to a semi-legal rebellion against the king (rokosz), known as Zebrzydowski Rebellion (1606–08), which was a response to Sigismund's attempt to introduce majority voting in place of unanimity in the Sejm.
Eventually Sigismund's forces were victorious, but the rebels went unpunished.
Partially in order to pacify the restless szlachta, Sigismund supported war with Muscovy (the Dimitriads, 1605–18).
Although Commonwealth forces were almost constantly shuffled between wars in the East (with Muscovy), north (with Sweden) and South (with Ottomans in the Polish–Ottoman wars), Sigismund took advantage of the civil war in Russia (the Time of Troubles) and secured temporary territorial gains for the Commonwealth.

While Sigismund never managed to regain the Swedish throne, his personal ambition to do so did succeed in provoking a long series of conflicts between the Commonwealth and Sweden, which was temporarily allied with Muscovy.
While the Sejm managed to thwart many of the plans of Sigismund (and later of his son, Władysław), the Vasa dynasty nonetheless succeeded in partially drawing the Commonwealth into the Thirty Years' War.
The conflict with Sweden, combined with wars against Ottomans and Muscovy, culminated well after Sigismund's death in the series of events known as the Swedish Deluge, which ended the Golden Age of the Polish–Lithuanian Commonwealth that spanned almost a century.

During his reign Sigismund permitted the Brandenburg Hohenzollerns to inherit Ducal Prussia.
This decision later greatly strengthened the Duchy.
The Commonwealth suffered a major military defeat in the Second Northern War, during the reign of Sigismund's younger son, John II Casimir.
Under the terms and conditions of the Treaty of Oliva, Prussia became a sovereign state and would eventually Partition Poland, together with Austria and the Russian Empire, in the late 18th century.

Sigismund's relationship with the jesuit Piotr Skarga, the royal chaplain from his accession, and other religious leaders, served as a basis for his depiction in the famous painting by Jan Matejko, illustrating the preaching of Piotr Skarga in the presbyterium of Wawel Cathedral.
Many historians agree that Sigismund considered Poland as a tool to eventually regain the throne of Sweden. To this end he tried to strengthen his royal power and allied himself with Habsburgs and Counter-Reformation forces. His policies were opposed by many within the circles of the wealthy Polish nobility (the szlachta), most notably the chancellor Jan Zamoyski. This led to a semi-legal rebellion against the king (rokosz), known as Zebrzydowski Rebellion (1606–08), which was a response to Sigismund's attempt to introduce majority voting in place of unanimity in the Sejm. Eventually Sigismund's forces were victorious, but the rebels went unpunished. Partially in order to pacify the restless szlachta, Sigismund supported war with Muscovy (the Dimitriads, 1605–18). Although Commonwealth forces were almost constantly shuffled between wars in the East (with Muscovy), north (with Sweden) and South (with Ottomans in the Polish–Ottoman wars), Sigismund took advantage of the civil war in Russia (the Time of Troubles) and secured temporary territorial gains for the Commonwealth.
While Sigismund never managed to regain the Swedish throne, his personal ambition to do so did succeed in provoking a long series of conflicts between the Commonwealth and Sweden, which was temporarily allied with Muscovy. While the Sejm managed to thwart many of the plans of Sigismund (and later of his son, Władysław), the Vasa dynasty nonetheless succeeded in partially drawing the Commonwealth into the Thirty Years' War. The conflict with Sweden, combined with wars against Ottomans and Muscovy, culminated well after Sigismund's death in the series of events known as the Swedish Deluge, which ended the Golden Age of the Polish–Lithuanian Commonwealth that spanned almost a century.
During his reign Sigismund permitted the Brandenburg Hohenzollerns to inherit Ducal Prussia. This decision later greatly strengthened the Duchy. The Commonwealth suffered a major military defeat in the Second Northern War, during the reign of Sigismund's younger son, John II Casimir. Under the terms and conditions of the Treaty of Oliva, Prussia became a sovereign state and would eventually Partition Poland, together with Austria and the Russian Empire, in the late 18th century.
Sigismund's relationship with the jesuit Piotr Skarga, the royal chaplain from his accession, and other religious leaders, served as a basis for his depiction in the famous painting by Jan Matejko, illustrating the preaching of Piotr Skarga in the presbyterium of Wawel Cathedral.
Sigismund III Vasa