Union with Sweden
Janusz Radziwiłł trust in the Poles was damaged during the Battle of Vilnius in 1655 when he, being the Great Hetman of the Grand Duchy of Lithuania, had to defend the Grand Duchy's capital Vilnius from the approaching Tsardom of Russia forces. When a large Russian army approached Vilnius (at least 41,000 men), Janusz Radziwiłł could muster just 5,000 to 7,000 men. 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. City residents began hasty evacuations. Most valued treasures, including the coffin of Saint Casimir, main books of Lithuanian Metrica, and valuables from Vilnius Cathedral, were transported outside the city.
Janusz Radziwiłł took up defensive position on the northern shore of the Neris river near the present-day Green Bridge to cover the evacuations. The battle started around 6 a.m. on 8 August 1655 and lasted the whole day. The Lithuanians managed to capture three Russian flags. At night, the Lithuanian army split into two groups that retreated to Vilkaviškis and to Kėdainiai. The garrison of the Vilnius Castle Complex surrendered two days later. The invading forces plundered the city and murdered its inhabitants for several days. A fire consumed part of the city. Tsar Alexis of Russia arrived to the city on 14 August. The city was so badly devastated that he could not find suitable accommodations in the city and instead built a large tent in Lukiškės.
With the war against Russia still ongoing, the Commonwealth suddenly had to face a new enemy. In June 1655 Swedish forces started to advance across the northern territories of the Commonwealth. The Swedish invasion of Polish–Lithuanian Commonwealth, part of the Second Northern War, is known in Lithuanian history as the Deluge. 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. Janusz first declared the Grand Duchy a Swedish protectorate in the Treaty of Kėdainiai on 17 August 1655, then the brothers signed another treaty on 20 October according to which the Swedish–Lithuanian union was founded, and in which Radziwiłł's were to rule a part of the Grand Duchy.
Janusz was not alone in abandoning the Polish side and acting in favour of the Lithuanian raison d'état; many Polish nobles, such as Deputy Chancellor of the Crown Hieronim Radziejowski and Grand Treasurer of the Crown Bogusław Leszczyński, believing that John II Casimir was a weak king or a Jesuit-king, encouraged Charles Gustav to claim the Polish crown. John II Casimir had few friends among the Polish szlachta, as he openly sympathized with Austria and showed disregard and contempt for the Commonwealth. Poznań Voivode Krzysztof Opaliński surrendered Greater Poland to Charles Gustav, and soon other voivodes followed.
Although much of the Commonwealth, including Warsaw, Kraków, and the western portions of the Grand Duchy, were taken by the Swedes, 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. Janusz died in Tykocin, besieged by loyal Commonwealth forces (desperate Swedish defenders later blew themselves up).
