The Radziwiłł family owned vast areas of land in Lithuania and Poland and some of its members were dissatisfied with the role of the magnates, who in the Polish–Lithuanian political system theoretically had the same rights as the Polish nobility. Eventually, the interests of the wealthy clan, known as the "Family", and the Crown began to drift apart. In 1654, during the Swedish and Russian invasion of Poland, known as The Deluge, two notable princes of the Radziwiłł clan, Janusz and Bogusław, began negotiations with the Swedish king Charles X Gustav, aimed at dissolving the Commonwealth and the Polish–Lithuanian Union. At the time Lithuania was in turmoil: it was being attacked on two separate fronts by Russia and Sweden, while the Ukrainian peasant revolt known as the Khmelnytsky Uprising was spilling into the Grand Duchy's southern regions from Ukraine. Eventually, most of the Polish-controlled Lithuanian army capitulated to the Swedes and the state collapsed. Most of the Crown of Poland along with western parts of Lithuania were occupied by Swedish forces, while the Russians seized most of Lithuania. On 17 August, Janusz Radziwiłł signed the Treaty of Kėdainiai, placing the Grand Duchy under Swedish protection. On 10 October 1655, Janusz and Bogusław Radziwiłł signed an agreement with the Swedes at their castle at Kėdainiai. According to the treaty, signed by the two cousins in the name of all Lithuanian nobility, the Polish–Lithuanian Union was declared null and void. In exchange for military assistance against Russia, the Grand Duchy of Lithuania was to become a protectorate of Sweden, with a personal union joining two states. In addition, The Family was to be given two sovereign principalities carved from their lands within the Grand Duchy and the Lithuanian nobility was to retain its liberties and privileges. The agreement never came into force. Its main proponent, Janusz Radziwiłł, died only 2 months after it was signed, on 31 December at Tykocin Castle, which was then besieged by forces loyal to the King of Poland and the Grand Duke of LithuaniaJohn II Casimir. The castle was taken soon after by Paweł Jan Sapieha, who immediately succeeded Janusz Radziwiłł to the office of Grand Hetman of Lithuania. The tide of the war soon turned and a popular uprising in Poland broke the power of the Swedish army. The Swedish occupation of Lithuania sparked a similar uprising in Lithuania. The Swedish defeat and eventual retreat from the territories of the Commonwealth abruptly ended the plans of Janusz's cousin Bogusław, who lost his army in the Battle of Prostki and died in exile in Königsberg on 31 December 1669. With the passing of the two cousins, the Radziwiłł family fortunes waned. Bogusław became commonly known as Gnida by his fellow nobles while Janusz was termed Zdrajca. Their treason against the Commonwealth largely overshadowed the deeds of numerous other family members of the next generation, including Michał Kazimierz Radziwiłł, who served faithfully against the Swedes.
Assessment
Although seen as an act of treason by contemporaries, modern views on the Swedish-Lithuanian accord differ. Some argue that the arrangement with the Swedes was made by Janusz Radziwiłł not out of greed and the political ambition, but rather out of Realpolitik. According to another theory, Janusz Radziwiłł was merely attempting to secure a strong ally against Russia. The Grand Duchy of Lithuania lacked the resources to fight a war on two fronts, and the Polish Crown, which by this time had its own serious problems, was able to supply only trifling amounts of money and military forces. Ultimately, Janusz Radziwiłł's choice was perhaps wrong – the Swedes proved to be not much better than the Russians.
