Pereiaslav agreement and war with Russia

A new enemy

In 1654 an agreement at Pereiaslav brought the Cossacks under the tsar’s authority. The Polish Army Museum describes it as an event that effectively began Russia’s war against the Commonwealth. It did not end the Cossack uprising; it widened the conflict¹.

Why it matters here

Commonwealth armies were already fighting in the east before Sweden crossed the border. A Russian offensive worsened Lithuania’s position in 1655, while the threat on several fronts made defence against Sweden harder²³.

See the article on the invasion’s causes for wider context.