"The Northern War of 1655-60 was one of three wars that involved the majority of the Baltic powers of the time. In different historical traditions it is either the First Northern War (German, Russian, Scandinavia and British traditions) or the Second Northern War (Polish tradition). In the Polish tradition the Livonian War of 1558-83 was the First Northern War.
The Northern War of 1655-60 began as a Swedish attack on Poland-Lithuania, already at war with Russia and suffering from a Cossack revolt in the Ukraine. During the fighting Brandenburg and Denmark both entered the war and even Britain became involved. The war saw a dramatic collapse in Poland followed by an almost as dramatic revival.
Poland-Lithuania had been facing a Cossack revolt in the Ukraine since 1648. In 1654 the Cossacks signed a treaty with Muscovy that triggered the Thirteen Years War between Poland-Lithuania and Muscovy (1654-1667, with some gaps as we will see below). In 1654 the Russians had recaptured Smolensk, lost to them earlier in the century and in 1655 they advanced into Lithuania.
The Muscovite advance directly threatened Swedish interests around the Baltic. If it continued, Muscovy could soon control large parts of the Baltic coast. Sweden could have responded by supporting Poland-Lithuania, but there was a real danger that such a move would lead to a settlement between Poland-Lithuania and an alliance against Sweden (Just has would happen at the end of 1656). There was also a personal reason for attacking Poland. Charles X had only come to the Swedish throne in July 1654, after his cousin Christina abdicated. John II Casimir of Poland-Lithuania was the son of Sigismund III, who had briefly been king of Sweden before being deposed in 1599. Neither Sigismund nor John Casimir was willing to forget his claim to the Swedish throne
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