Ruthenian

The Ruthenians are the East Slavic heirs of Kievan Rus' in the Dnieper basin and western Rus' lands, whom Latin documents called Rutheni to distinguish them from the Muscovites. After the Mongol sack of Kyiv in 1240, principalities turned to the Grand Duchy of Lithuania, whose 1362 Blue Waters victory brought Kyiv and the steppe border under Vilnius; Lithuanian chanceries ran in their written tongue, Ruthenian.

The Union of Lublin of 1569 transferred the Ruthenian lands of Ukraine to the Polish Crown, bringing Polish nobility and Catholic pressure producing the Union of Brest in 1596, when part of the Orthodox hierarchy entered communion with Rome as the Greek Catholic Church. Orthodox resistance centred on the Mohyla Academy, founded in 1632, while runaways swelled the Dnieper Cossack host.

The Cossack uprising of 1648 under Bohdan Khmelnytsky and the Pereyaslav agreement of 1654 drew Muscovy into Ruthenian affairs, and through later partitions the vernaculars matured into the Ukrainian and Belarusian languages.