The plan was chiefly the work of Yuri Nemyrych, a nobleman and thinker who served as its principal designer, alongside Hetman Ivan Vyhovsky and the negotiator Pavlo Teteria. Their proposal was strikingly systematic. A Grand Principality of Ruthenia would be carved from the palatinates of Kyiv, Bratslav, and Chernihiv, joining Poland and Lithuania as an equal third partner in one indivisible commonwealth. The principality would be led by a hetman elected for life, chosen jointly by the Cossacks, the nobility, and the clergy, and confirmed by the king. It would have its own chancellor, marshal, treasury, and supreme court, and command its own army. In its ambition it echoed the confederal logic that had bound Poland and Lithuania together at Lublin in 1569, now extended to a third people.

The vision ran aground on the divisions it could not bridge. To many rank-and-file Cossacks, the treaty felt like too little, too late. They had risen against Polish rule in the Khmelnytsky Uprising of 1648, and they distrusted an alliance with the overwhelmingly Catholic Commonwealth; the Orthodox Tsardom of Russia looked to them like a more natural ally. Worse, the deal promised to return estates to the Polish szlachta and to ennoble only the Cossack elite, the starshyna, deepening the rift between ordinary fighters and their leaders. When Vyhovsky could not persuade enough Cossacks even to accept the compromise over the Uniate Church, the fault lines widened. A treaty meant to unite instead helped split Ukrainian society into pro-Russian and pro-Polish camps.

Moscow read the agreement as an act of war. The tsar, alarmed that the treaty loosened his grip on the Cossacks, sent an army into Ukraine even before it was ratified. Vyhovsky won an early and celebrated victory at the Battle of Konotop in June 1659, but it proved indecisive, and defeats followed. His Crimean Tatar allies were pulled home by a Zaporozhian raid on the Khanate, unrest spread through the Poltava region, and pro-Russian colonels rose against him, accusing the hetman of selling Ukraine to the Poles. Unable to continue, Vyhovsky resigned in October 1659 and retired to Poland. The Commonwealth's own fortunes sagged in the grinding Russo-Polish War, and the grand idea of a Ruthenian principality was abandoned outright.

In the end Russia prevailed. The Truce of Andrusovo in 1667 and the Treaty of Perpetual Peace in 1686 drew the Cossacks firmly into the Russian sphere, with far fewer privileges than Hadiach would have granted, and by the late eighteenth century the Russian Empire had all but extinguished Cossack political power. Yet the dream did not die entirely. After the November Uprising of 1831, patriots tried to revive the idea of a three-nation commonwealth, even designing a coat of arms for it, though Russian authorities banned the planned gathering at Hadiach and ringed the town with soldiers. That so many later generations reached back to this treaty is its truest legacy: a small town's name kept alive as shorthand for what Eastern Europe might have been.