In the summer of 1655 the second-largest state in Europe collapsed. The Commonwealth of Poland-Lithuania had struggled for seven years to contain a massive Cossack revolt led by Bohdan Khmelnytskyi which had broken out in the Ukraine in the summer of 1648. Khmelnytskyi, unable after six years of inconclusive fighting to achieve an acceptable settlement, had looked to Alexis, tsar of Muscovy (1645-76), to break the deadlock. By the treaty of Pereiaslav (18 January 1654), the Cossacks put themselves under the tsar's protection; shortly afterwards Muscovite armies swept into the Commonwealth. The military reputation of Poland-Lithuania had been shaken by the Swedish victories of the 1620s and the failure to defeat Khmelnytskyi, but nobody expected the collapse which followed: by the summer of 1655 Alexis had occupied most of Lithuania, while joint Cossack and Muscovite forces in the Ukraine had driven the Poles back to Lwów. On 9 August 1655 the tsar's triumphal entrance to Wilno, capital of Lithuania, crowned a remarkable campaign which had upset the whole balance of power in eastern Europe.

The coup de grace was administered by Sweden, whose vulnerable Baltic empire was directly threatened by the Muscovite drive into Lithuania.