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.

From January 1569, the Polish and Lithuanian parliaments (sejms) held their debates in parallel, bargaining for a union agreement. The deliberations were acrimonious, but eventually led to the signing, in Lublin, of an act of real union between the Crown and the Grand Duchy of Lithuania. This happened on July 1, 1569.

The Union of Lublin provided for the merger of the two sovereign countries, Poland and Lithuania, into a single unit that became called ‘the Commonwealth [Rzeczpospolita] of the Two Nations’, to be ruled by a single monarch elected jointly by the two nations in free elections. The Rzeczpospolita was henceforth to have a joint Sejm (parliament) and a shared foreign policy. The central offices specific to each of the countries were preserved, as were their respective official languages, armies, treasuries, and judicial systems. A single currency was also introduced – however, it was minted separately in each of the two countries.