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.
Ukrainians
Differentiation between separate East Slavic groups began to emerge in the later medieval period, and an East Slavic dialect continuum developed within the Polish–Lithuanian Commonwealth, with the Ruthenian language emerging as a written standard. The active development of a concept of a Ukrainian nation and a Ukrainian language began with the Ukrainian National Revival in the early 19th century. In the Soviet era (1917–1991), official historiography emphasized "the cultural unity of 'proto-Ukrainians' and 'proto-Russians' in the fifth and sixth centuries".