Charles X Gustavus had just ascended the throne. His country was bankrupt after the Thirty Years’ War, its only asset a huge and now redundant army. In spite of the twenty years of peace with the Commonwealth, the Swedes still dreamed of extending their possessions on the Baltic seaboard. The discontent of many Polish magnates, the confusion attendant on the Cossack and Muscovite invasions, and finally the appeal from the Radziwiłł, all paved the way for an invasion, which took place at the beginning of 1655.
The Swedes rapidly made themselves masters of Pomerania and advanced into Wielkopolska. A Polish army barred their way, but was made to capitulate by Krzysztof Opaliński, Palatine of Poznań, and his colleague the Palatine of Kalisz, who officially transferred their provinces to Swedish overlordship. The enemies of Jan Kazimierz all over the country announced his dethronement in favour of Charles X. With three enemy armies operating in the country confusion reigned. Disoriented by a rumour that Jan Kazimierz had abdicated, Hetman Stanisław Rewera Potocki capitulated. Isolated groups of szlachta and small bodies of troops on their way to join the army either surrendered or dispersed. Jan Kazimierz advanced to face the enemy with a small army commanded by Stefan Czarniecki, Castellan of Kiev. In September 1655 they were defeated at żarnowiec and fell back on Kraków. The King then took refuge in Silesia, while Czarniecki tried unsuccessfully to hold Kraków.
On 22 October Janusz Radziwiłł signed an agreement at Kiejdany detaching Lithuania from Poland and placing it under the protection of Sweden. Swedish troops appeared in every province, often accompanied by magnates or szlachta who supported Charles. Since there was little to choose between one Vasa king and another, many accepted what appeared to be a
Charles was only interested in provinces such as Pomerania and Livonia, which would give Sweden control of the Baltic, and treated the rest of Poland as occupied territory. He and his generals removed everything they could lay hands on—pictures, sculpture, furniture, entire libraries. His troops burnt down churches, having first emptied them of everything portable, and this sacrilege incensed the peasants, who were hardly concerned as to who sat on the throne. They began to massacre lone soldiers, then lone detachments. A guerrilla war developed, with bands of szlachta and peasants making life unpleasant for the Swedes.
A handful of fortresses such as Gdańsk, Lwów, Kamieniec and Zamość continued to hold out for Jan Kazimierz, while the fortified monastery of Częstochowa fought off a siege in a manner which would pass into legend. All around the country groups of szlachta only needed a signal to take up arms for the King.
The Tatars were prescient enough to see that if the Commonwealth were defeated it would only be a matter of time before Muscovy and the Cossacks would devastate the Crimea, and Mehmet Girey therefore signed an alliance with Jan Kazimierz and despatched several thousand warriors to assist the Polish army.
In January 1656, Jan Kazimierz took the offensive at the head of the army he had rallied. Although the Swedes, assisted by a large contingent of troops from Brandenburg, managed to win the threeday Battle of Warsaw in July, fortune began to turn against them. Denmark and Holland joined the Polish alliance, and in June a Dutch fleet broke the Swedish blockade and sailed in to relieve Gdańsk. The Swedes, who had already made an alliance with the Elector of Brandenburg, now enlisted the support of the prince of Transylvania and briefly managed to reoccupy Warsaw in 1657, but they and their allies were decisively beaten in the following year, when Brandenburg switched sides and joined Poland, along with the Habsburgs. In 1660 peace was signed at Oliwa on the basis of a return to the
The death of Chmielnicki in 1657 had ended the Cossack threat. He was succeeded by Jan Wyhowski, a moderate who quickly brought negotiations to a head. On 16 September 1658 the Union of Hadziacz turned the Commonwealth of Two Nations into a Commonwealth of Three Nations. Ukraine was to have its own chancellor, treasurer, marshal and hetman, chosen by the king from candidates proposed by the Cossacks. It was to have its own courts, its own mint, and its own army. Several hundred Cossacks were ennobled, and the Metropolitan of Kiev and the Orthodox bishops of Lwów, Przemyśł, Chełm, Łuck and Mścisław were to have seats in the Senate. Polish and Lithuanian troops were barred from entering the three Palatinates, in which only Orthodox Ukrainians were to hold office. Ukraine was to have two universities and a number of schools, paid for by the Commonwealth.