An alliance between Sweden and Hetman Bohdan Khmelnytsky against Poland was negotiated several times between 1651 and 1657.
After the 1656 peace in Vilnius between Muscovy and Poland Hetman Bohdan Khmelnytsky was infuriated and wrote to the tsar Alexis I of Russia: The Swedes are the honest people; when they pledge friendship and alliance, they honor their word.
However, the Tsar, in establishing an armistice with the Poles and in wishing to return us into their hands, has behaved most heartlessly with us.
Even after the Treaty of Pereyaslav in 1654 Hetman Bohdan Khmelnytsky continued to negotiate with the Swedes and in late 1655 his ambassador made proposals to the Swedish king Charles X Gustav to accept Ukraine as a Swedish vassal state and promised him the faithful service of the entire Zaporozhian Host.

Finally an agreement was signed between Sweden and three Ukrainian commanders (Ivan Bohun, the leader of the Ukrainian Protestants Yuri Nemyrych and Ivan Kovalivsky) on 6 October 1657 in Korsun where Sweden acknowledged the Ukrainian borders all the way to Wisła in the west and Prussia in the north.
But by then Bohdan Khmelnytsky had died one month earlier and after the Swedes had left Poland for military campaigns in Denmark the proposed alliance died.
An alliance between Sweden and Hetman Bohdan Khmelnytsky against Poland was negotiated several times between 1651 and 1657. After the 1656 peace in Vilnius between Muscovy and Poland Hetman Bohdan Khmelnytsky was infuriated and wrote to the tsar Alexis I of Russia: The Swedes are the honest people; when they pledge friendship and alliance, they honor their word. However, the Tsar, in establishing an armistice with the Poles and in wishing to return us into their hands, has behaved most heartlessly with us. Even after the Treaty of Pereyaslav in 1654 Hetman Bohdan Khmelnytsky continued to negotiate with the Swedes and in late 1655 his ambassador made proposals to the Swedish king Charles X Gustav to accept Ukraine as a Swedish vassal state and promised him the faithful service of the entire Zaporozhian Host.
Finally an agreement was signed between Sweden and three Ukrainian commanders (Ivan Bohun, the leader of the Ukrainian Protestants Yuri Nemyrych and Ivan Kovalivsky) on 6 October 1657 in Korsun where Sweden acknowledged the Ukrainian borders all the way to Wisła in the west and Prussia in the north. But by then Bohdan Khmelnytsky had died one month earlier and after the Swedes had left Poland for military campaigns in Denmark the proposed alliance died.
Sweden–Ukraine relations