Polish–Lithuanian–Ruthenian Commonwealth

The Polish–Lithuanian–Ruthenian Commonwealth (Polish: Rzeczpospolita Trojga Narodów, lit. 'Republic of Three Nations', Lithuanian: Trijų Tautų Respublika, Ukrainian: Річ Посполита Трьох Народів) was a proposed European state in the 17th century that would have replaced the existing Polish–Lithuanian Commonwealth, but it was never actually formed.

A way to establishment a Grand Duchy of Ruthenia was considered by the Ukrainian hetman Bohdan Khmelnytsky at various times, particularly during the 1648 Cossack insurrection, against Polish rule in the primarily-ethnically Ukrainian territories (see Khmelnytsky Uprising). Such a Ruthenian duchy, as proposed in the 1658 Treaty of Hadiach, would have been a full-fledged member of the Polish–Lithuanian Commonwealth, which would thereby have become a tripartite Polish–Lithuanian–Ruthenian Commonwealth. In May 1659, the Polish Diet (Sejm) ratified the treaty with an amended text. The plan, as envisioned by Yuri Nemyrych, would have ennobled a portion of the Cossacks, which would then run the Grand Principality of Ruthenia (1658). Hetman Ivan Vyhovsky could not get enough Cossacks agree to keeping the Uniate Church, which the Catholic church refused to liquidate, but many Cossacks were strongly against the idea.

The plan meant an annulment of the Pereiaslav Agreement's arrangements and as such renewed hostilities between the Commonwealth and the Tsardom of Russia.

The idea of a Ruthenian Duchy within the Commonwealth was completely abandoned in 1659 after the second Agreement. The Canadian historian Paul Robert Magosci believes that happened because of the divisions among the Cossacks and because of the Russian invasion. However, those events were much earlier than the signing of the Treaty of Hadiach. The Russian historian Tairova-Yakovleva regards the resistance of Polish society and papal pressure as the reasons for the failure in ratification.

The idea of a Polish–Lithuanian–Ruthenian Commonwealth revived during the January Uprising when a patriotic demonstration took place at Horodło in 1861. The so-called Second Union of Horodło was announced there by the szlachta of Congress Poland of the former Grand Duchy of Lithuania of Volhynia and of Podolia. The New Commonwealth, based on the Second Union of Horodło, was to be based on the three nations, and its proposed coat of arms included the Polish eagle, the Lithuanian Pahonia, and the patron saint of Ruthenia, the Archangel Michael.