Union of Kėdainiai

Union of Kėdainiai (or Agreement of Kėdainiai, Polish: Umowa Kiejdańska) was an agreement between several magnates of the Grand Duchy of Lithuania and the king of the Swedish Empire, Charles X Gustav. It was signed on 20 October 1655 during the "Swedish Deluge", part of the Second Northern War. In contrast to the preceding Treaty of Kėdainiai of 17 August, which put Lithuania under Swedish protection, the purpose of the Swedish-Lithuanian union was to end Lithuania's union with Poland, and set up two separate principalities in the Grand Duchy of Lithuania. One of these was to be ruled by the Radziwiłł (Radvila) family, while the rest of the duchy was to remain a Swedish protectorate.

The agreement did not last for long and never came into effect, as the Swedish defeat in the Battles of Warka and Prostki as well as a popular uprising in both Poland and Lithuania put an end both to Swedish power and the influence of the Radziwiłłs.

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 (except Samogitia and parts of Suvalkija and Aukštaitija).

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