Consequently, the Cossacks regularly rebelled for the purpose of expanding the register and obtaining a guarantee of their rights, which would have protected them from having the peasant status imposed on them. The greatest of the rebellions took place in 1648, led by Bohdan Khmelnytsky. The weakened Commonwealth found itself unable to oppose it. A few compromising military defeats resulted in the rebels’ strengthening on the south-eastern lands, of which Warsaw lost control. The war, which was dragging on and which neither side was able to settle in its favor, ruined the Ukrainian lands and also put a heavy economic burden on the Commonwealth’s shoulders. The Cossacks also dragged Moscow into the conflict, subjecting Ukraine to the tsar.

When Ivan Vyhovsky became the Hetman of the rebellion, supporters of an agreement with the Poles prevailed among the Cossacks. The willingness to end the destructive war was dominant also on the Polish side. In consequence, in 1658, the Treaty of Hadiach was signed. It transformed the Polish-Lithuanian Commonwealth into a Commonwealth of Three Nations, granting a separate status to the territories inhabited by Cossacks. It guaranteed the Orthodox population’s rights and also the Cossacks’ gradual ennoblement. A separate ladder was created for offices on the Ukrainian lands, which were to be held only by individuals originating from those territories. The nobility could return to its landed estates.

The controversy whether the Treaty of Hadiach could put an end to the destructive civil war continues. Shortly after its signing, rank-and-file Cossacks rebelled against Vyhovsky under Moscow’s influence. Vyhovsky was then deprived of the mantle of Hetman and the war against the Commonwealth resumed.

The Swedish invasion of 1655 proved a truly catastrophic event. Faced with the capture of most of the Grand Duchy of Lithuania and Ukraine by Russia and its Cossack allies, a significant number of the magnates and nobles in conflict with Jan Kazimierz surrendered to the Swedes almost without a fight. Jan Kazimierz had to flee the country. Yet the severe pillaging visited upon the land by Swedish soldiers and the German mercenaries in their ranks, coupled with their religious otherness (they were Lutherans), quickly sparked resistance from the plebeian masses and the nobility alike. The successful defence of the Jasna Góra monastery, home to a venerated painting of the Madonna (the painting was in Habsburg Silesia during the siege), at the end of 1655 marked a turning point in the war.

The royal court used this event for propaganda purposes when Jan Kazimierz returned to Poland in 1655/1656. Lwów (Leopolis), where the Swedes had not penetrated, became the command centre of resistance against the invaders. It was there, in the city cathedral, on 1 April 1656, that Jan Kazimierz made a solemn vow before the altar of Our Lady of Graces, establishing the Holy Mother as Queen of the Polish Crown and promising remissions for the peasants. By doing so, the ruler sanctioned the religious nature of the conflict with Sweden and made concessions to the commoners, exploiting their zeal in the fight against the enemy. The ceremonial mass at the Lwów Cathedral was celebrated by the papal nuncio, Pietro Vidoni, with numerous dignitaries waiting on the king in attendance. Although the social programme enounced in the Lwów vows was never put into action, they have come down in Polish history as an important symbol of national awakening and unity in the face of external danger.

Defeated by the Swedes in combat, Jan Kasimir fled to Silesia, to the terrains under the Habsburg rule. The nobility and the royal clerks recognized the king of Sweden as the king of Poland in return for a guarantee of rights. At the same time Great Hetman of Lithuania Janusz Radziwiłł surrendered the Great Duchy of Lithuania to Sweden. With time, however, the Swedes began to treat the captured territory in an increasingly harsh way. They started to transport valuables and works of art out of the Commonwealth. They also accepted the fact that their soldiers were robbing peasants. To make the renegade king’s contact with the country more difficult the Swedes began to capture towns and fortresses on the border with Silesia. That was when they came to the foothill of the fortified cloister called Luminous Mount (Jasna Góra), the most important Marian sanctuary in the Crown. Their attempt to capture it, combined with other Swedish misdeeds, outraged the nobility, which turned back to Jan Kasimir. Under those circumstances, in December 1655 hetmans of the Crown with the army, representatives of the disappointed nobility, and the Lithuanian troops which had turned against Radziwiłł assembled in Tyszowce. They decided to establish a confederation, that is, an armed union of citizens formed to defend their violated rights. The Confederates declared their subordination to Jan Kasimir, promised to fight against the Swedes, and announced universal conscription. The Confederation of Tyszowce is regarded as the symbolic turning point in the Polish-Swedish War, which transformed from the invaders’ triumphant march into a bloody and difficult war, which came to an end in 1660 with the signing of the Treaty of Oliva and the Swedes’ retreat.