Gustavus Adolphus had been drawn into the war in 1630 principally out of a need to defend Sweden’s possessions on the eastern and southern shores of the Baltic, and had in the process occupied virtually the whole shoreline. Under his successor Charles X Gustavus, Sweden went on to champion the Protestant cause and to challenge Habsburg influence in Germany, thereby engaging the sympathy and support of France. It was largely because Zygmunt III had always been perceived in Sweden as an agent of Habsburg designs on the Baltic region that the Swedes had invaded Poland.
Another threat had arisen in the shape of Brandenburg. In 1600, this sparsely populated state planted on poor, sandy soil, with no mineral deposits, no coastline and an unmanageable river system, had been diplomatically irrelevant, and its ruling family, the Hohenzollern, carried little weight with the older courts of Europe. Their only other resources consisted in a collection of rich but small dynastic lands in the west, along the border with the Dutch Republic, and a fragile connection with the former lands of the Teutonic Order in the east.
The last Grand Master of the order had been a Hohenzollern cousin of the Elector of Brandenburg, and after its secularisation in 1520 he had continued to rule the province, now known as Ducal Prussia, as a vassal of the Polish crown. He was succeeded by his son and grandson, who both paid homage to the King of Poland on their succession. The Elector of Brandenburg obtained from the Polish crown the right of succession if the Prussian line of Hohenzollerns were to die out. When it did, in 1618, Zygmunt III was at war with Sweden, and a handsome subsidy from the Elector of Brandenburg, John Sigismund, eased the passage of the duchy into his line of the family. In 1641 John Sigismund’s grandson Frederick William (who would be known as the Great Elector) knelt in homage before Władysław IV at Warsaw Castle on succeeding as ruler of Ducal Prussia. Sixteen years later, in return for military support against Sweden, he extorted from a desperate Jan Kazimierz the right to rule there as a sovereign prince. There was much resistance to this in the duchy itself, where the noble estates feared the loss of their rights and appealed to Warsaw for protection, but with Habsburg support Frederick William managed to have his right to rule as a sovereign in the duchy confirmed by the Treaty of Oliwa in 1660. The duchy was thereby detached from the Polish crown and the Commonwealth. And it was obvious that the rulers of Brandenburg-Prussia would never rest until they had joined up their dominions, and that would necessitate acquiring a swathe of Polish territory and cutting the Commonwealth off from the sea. They had exploited the possibilities offered by the Thirty Years’ War and the Swedish invasion of Poland to acquire both territory and prestige, while the army they had built up meant that they were now sought after as allies.
The Polish Commonwealth’s pool of potential allies was limited by the fact that its policy was fundamentally pacific and it had little to offer in the way of an army. The Habsburgs had expended much energy in the first half of the century to bring Poland into their orbit and use it as an ally against Sweden. From the 1640s France had taken an interest in Poland as a potential ally against the Habsburgs, but the Francophile court party had failed to bring this about. Three decades later, as Louis XIV engaged in his struggle with the Habsburgs, he again looked to Poland. And Jan III saw in this an opportunity, not just to reaffirm the power of the Commonwealth and gain the prestige necessary to carry out some fiscal reforms, but also to further his own dynastic aspirations.
In 1675 he signed the Treaty of Jaworów with France, which offered to finance an invasion of Ducal Prussia (which Jan III hoped would be given to his son as a hereditary vassal duchy) while her other ally, Sweden, invaded Brandenburg. France undertook to neutralise the Habsburgs and persuade the Ottoman Porte to give back Kamieniec and other lands ceded by the treaty of Buczacz. Sweden duly went into action against Brandenburg, but the Polish forces were not able to invade Prussia because Turkey not only refused to give back Kamieniec, but launched a new offensive into Poland. The large army which had been assembled was used not against Prussia but against the Turks, who were defeated at żurawno in 1676. By the time this operation was complete, Sweden had made peace with Brandenburg, and the opportunity had passed.