The surrender of Ujście of 25 July 1655 was a key moment in the collapse of Polish resistance in the first months of the Northern War of 1655-60 against Sweden. When Sweden attacked, the main Polish army was absent in the Ukraine, facing a combined Cossack-Muscovite threat, while much of Lithuania was threatened or occupied by the Muscovites. The Swedes invaded Poland from east and west in the summer of 1655. Over 30,000 Swedish troops were involved. Against them the Poles could only field a force of just over 14,000 levies, while attempting to gather together 8,735 regular troops.
When faced with the first Swedish army to enter Poland from the west, 13,650 men commanded by Arvid Wittenberg, the levy surrendered. The palatinates of Poznań and Kalisz accepted Charles’s promises of protection, and to honour and obey him as if he was king of Poland.
The surrender of Ujście left central Poland vulnerable to Swedish occupation. Despite the best efforts of John Casimir, Cracow fell into Swedish hands on 26 October. A similar change of sides would follow in Lithuania with the treaty of Kiejdany of 17 August 1655.