Wielkopolska was the region most vulnerable to attack as it lay in the immediate vicinity of Swedish-ruled West Pomerania. In the summer of 1655, two Swedish groups from Pomerania and one from Livonia headed for Poland. The army of Field Marshall Arvid Wittenberg camped in the vicinity of Ujście in July. Although the conditions favoured the defenders, the levy en masse under the command of Poznań Voivode Krzysztof Opaliński surrendered after only putting up token resistance – a decision that proved disastrous for the entire national defence plan.
The invaders were unfeeling and ruthlessly set about plundering the territories they occupied. This behaviour led to the formation of guerrilla units that started fighting the Swedes on their own initiative. It is now generally accepted that Podlachia Voivode Piotr Opaliński organised the first large-scale guerrilla unit in Wielkopolska and that the Starosta of Babimost, Krzysztof Żegocki, soon followed.
The intensive fighting between Polish troops and the Swedes and their Brandenburg allies raged on around Gniezno, Kłecko and Babimost with constantly fluctuating fortunes for ages (Poznań changed hands several times as well). The famous field hetman Stefan Czarniecki also operated in Wielkopolska. The situation became patently obvious to everyone after two years. A council of war was convened in the capital of Wielkopolska on 26 November 1657 to thrash out the details for a protracted war against the Swedes. King Jan Kazimierz (John II Casimir Vasa), Stefan Czarniecki and Piotr Opaliński all took part.
The Swedish “deluge” devastated the entire country and was the first step in its gradual decline. Wholesale destruction, coupled with the rampant pillaging of the invaders and behaviour from our own armies that was not much better, arrested the period of steady development.