Belarus–Ukraine relations
Belarus–Ukraine relations (, "Byelaruska-ŭkrainskiya adnosiny", , "Ukrains'ko-Bilorus'ki vidnosyny", , "Belorussko-ukrainskiye otnosheniya") are foreign relations between Belarus and Ukraine. Both countries were successively ruled by the Polish-Lithuanian Commonwealth, the Second Polish Republic, and ultimately, the Russian Empire. Prior to the 1991 breakup, both countries were part of the Soviet Union as the Belarusian SSR and Ukrainian SSR when they were the founding members of the United Nations in 1945. Being Slavic nations, both Belarus and Ukraine share closely related cultures and are predominantly inhabited by the East Slavic ethnic groups of Belarusians and Ukrainians respectively, along with a few Russians. Today, the two countries share an 891–km border.
Russian language
Judging by the historical records, by approximately 1000 AD the predominant ethnic group over much of modern European Russia, Ukraine and Belarus was the Eastern branch of the Slavs, speaking a closely related group of dialects. The political unification of this region into Kievan Rus' in about 880, from which modern Russia, Ukraine and Belarus trace their origins, established Old East Slavic as a literary and commercial language. It was soon followed by the adoption of Christianity in 988 and the introduction of the South Slavic Old Church Slavonic as the liturgical and official language. Borrowings and calques from Byzantine Greek began to enter the Old East Slavic and spoken dialects at this time, which in their turn modified the Old Church Slavonic as well.
Dialectal differentiation accelerated after the breakup of Kievan Rus' in approximately 1100. On the territories of modern Belarus and Ukraine emerged Ruthenian and in modern Russia medieval Russian. They became distinct since the 13th century, i.e. following the division of that land between the Grand Duchy of Lithuania, Poland and Hungary in the west and independent Novgorod and Pskov feudal republics plus numerous small duchies (which came to be vassals of the Tatars) in the east.