Why did Sweden invade?

Sweden did not attack a country at peace in the summer of 1655. The Commonwealth had faced the Cossack uprising led by Bohdan Khmelnytsky since 1648 and had been at war with Muscovy since 1654. This crisis preceded the Deluge. It did not make a Swedish invasion inevitable: the decision to go to war was made in Stockholm¹².

The Vasa claim was only part of the story

The Polish branch of the Vasa dynasty retained its claim to the Swedish crown. John II Casimir, king of the Commonwealth from 1648, continued to assert that claim. It complicated relations between the two courts, but it cannot by itself explain why Sweden attacked at this moment. Historian Stellan Dahlgren, in his biography of Charles X Gustav for Svenskt biografiskt lexikon, also describes strategic concerns and the Swedish king's expansionist aims³².

The Baltic and Muscovy's advance

Swedish leaders feared that Muscovy's advance into Commonwealth territory threatened Baltic lands important to Sweden. According to Dahlgren, Charles X Gustav considered attacking Muscovy or the Commonwealth and chose the latter. He saw a weakened opponent and attached importance to Prussia and Courland. The struggle was therefore about Baltic influence as well as dynastic claims².

A state already fighting on several fronts

The Grand Duchy of Lithuania bore the weight of the war with Muscovy while the Crown faced fighting in the southeast. The Museum of the Polish Army describes Lithuania's military situation in 1655 as disastrous. That situation did not justify Swedish aggression; it helps explain why the invasion was so dangerous¹⁴.

The next chapter follows the rapid change in the balance of power. For an overview of the whole conflict, see The Swedish Deluge.