After George Frederick's death in 1603, the Polish king Sigismund III Vasa appointed Joachim Frederick as regent in 1605, and permitted his son, John Sigismund, to succeed him in 1611.
He paid his feudal homage in person to the King of Poland, Sigismund III Vasa, in September 1621 in Warsaw (the Duchy of Prussia was a fief of the Kingdom of Poland at the time).
In 1611, John Sigismund traveled from Königsberg to Warsaw, where on 16 November 1611 he gave feudal homage to Sigismund III Vasa, King of Poland (the Duchy of Prussia was a Polish fief at the time).
For many centuries Kraków was the royal capital of Poland, until Sigismund III Vasa relocated the court to Warsaw in 1596.
The ruler from Transylvania was followed by Sigismund III Vasa of Sweden, whose election marked gradual decline of the province.
His determined Prussian wife showed a strong dislike for this Swedish suitor, because Prussia was a Polish fief and the Polish King Sigismund III Vasa still resented his loss of Sweden to Gustavus Adolphus' father Charles IX. Maria Eleonora had additional suitors in the young William of Orange, Wladislaw Vasa of Poland, Adolf Friedrich of Mecklenburg and even the future Charles I of England.
Gregory XV interfered little in European politics, beyond assisting Ferdinand II, Holy Roman Emperor, and the Catholic League against the Protestants—to the tune of a million gold ducats—as well as Sigismund III Vasa, King of the Polish–Lithuanian Commonwealth, against the Ottoman Empire.
King Sigismund III Vasa, a talented but somewhat despotic ruler, involved the country in many wars, which subsequently resulted in the successful capture of Moscow and loss of Livonia to Sweden.
Five days later, the assembly elected Sigismund III Vasa king.
Maximilian II's son, Archduke Maximilian III, was elected king but was contested by the Swedish Sigismund III Vasa, who defeated Maximilian at the Byczyna and succeeded as ruler of the Commonwealth.
Due to its central location between the Commonwealth's two major cities of Kraków and Vilnius, Warsaw became the capital of the Commonwealth and the Polish Crown when Sigismund III Vasa transferred his royal court in 1596.
However, the rule of an ultra-Catholic King Sigismund III Vasa was marked by numerous attempts to spread the Catholicism, mostly through his support for counterreformation and the Jesuits.
He soon placed himself under the suzerainty of Rudolf II, the Holy Roman Emperor, and, in 1599–1600, intervened in Transylvania against Poland's king Sigismund III Vasa, placing the region under his authority;
Sigismund III Vasa (Polish: Zygmunt III Waza, Swedish: Sigismund, Lithuanian: Žygimantas Vaza;
Equestrian portrait of king Sigismund III Vasa by Peter Paul Rubens.
A 40-ducat coin depicting King Sigismund III Vasa, 1621.
Renaissance amber inkwell of king Sigismund III Vasa, 1590s.
Also, in the last years of the 16th century and the first part of the 17th century, a number of Italian musicians were guests at the royal courts of Sigismund III Vasa and Władysław IV. These included Luca Marenzio, Giovanni Francesco Anerio, and Marco Scacchi.
The heir to the throne was John's eldest son, Sigismund III Vasa, already king of Poland and a devoted Catholic.
In 1621, on the occasion of the Polish victory over the Ottoman Empire at Chocim, one of the most valuable and largest coins in the history of Europe was minted in Bydgoszcz – 100 ducats of Sigismund III Vasa.