For several decades, the interests between the Radziwłł family and the state (Polish–Lithuanian Commonwealth) had begun to drift apart, as the Radziwiłłs increased their magnate status and wealth.

[1] In September 1628 he departed for a four-year-long voyage, primarily in Germany and the Netherlands, returning to the Commonwealth's capital of Warsaw by autumn 1632.

[12] In 1646 he became a member of the senate of Poland, as he an important office that granted him this privilege: in April that year, he became the Field Hetman of Lithuania.

[14] Later that year, the Commonwealth suffered two drastic events: death of king Władysław (succeeded by John II Casimir Vasa), and the beginning of the Khmelnytsky Uprising.

Some historians have speculated that Siciński might have acted on orders from Janusz Radziwiłł, through Wisner observed there is no evidence to support this theory.

The morale was further damaged by the order of king John II Casimir Vasa to royal troops (about 5,000 men) to retreat to Marienburg.

Most valued treasures, including the coffin of Saint Casimir, main books of Lithuanian Metrica, and valuables from Vilnius Cathedral, were transported outside the city.

[28] Janusz Radziwiłł took up defensive position on the northern shore of the Neris river near the present-day Green Bridge to cover the evacuations.

Together with his cousin Bogusław Radziwiłł, Janusz began talks with Swedish king Charles X Gustav of Sweden, discussing how to reverse the Union of Lublin which created the Commonwealth.

John II Casimir had few friends among the Polish szlachta, as he openly sympathized with Austria and showed disregard and contempt for the Commonwealth.

Although much of the Commonwealth, including Warsaw, Kraków, and the western portions of the Grand Duchy, were taken by the Swedes,[35] King John II Casimir and his allies were able to regain power after a few years starting with the Jasna Góra resistance and the Tyszowce Confederation.

The Swedish defeat and eventual retreat from the territories of the Commonwealth spelled an abrupt end for the plans of Janusz and Bogusław.

[34] Janusz died in Tykocin, besieged by loyal Commonwealth forces (desperate Swedish defenders later blew themselves up).

Sienkiewicz, in his Trilogy, wrote about Radziwiłł's death: "Earthly ruin, a fallen soul, darkness, nothingness-that is all he managed to attain as a reward for service to himself".