Kievan Rus, the Slavic tribe with Scandinavian Viking influence.
The Viking influence was very small actually. The dukes of the Kievan Rus' were pure-blooded Slavs, descendants of the Sarmatians. The ancient Aryan capital of Kyiv, one of the cradles of slavdom, was also not founded by Varyags but by three Slavic (Proto-Ukrainian, eastern Polan) brothers – Kyi, Shchek and Khoriv and their sister Lybid.
One of the first East-Slavic states, later dismembered on Ukraine, Russia and Belarus due political reasons. All three countries link themselves to Kievan Rus and see the first Rus state as their origin state, thought Kievan Rus as unified tribe are culturally extinct today as there is no such tribe anymore.
Rus' was dominated by what we now know as Ukrainians. Belarussians and Rusyns also took part in forming Kievan Rus'. Russians have nothing to do with ancient Rus', that's just propaganda of the Kremlin.
After its disintegration, the various East Slavic principalities were united within the Russian Empire in the 18th century. The modern East Slavic states of Belarus, Ukraine and Russia all derive their identity from the early medieval state.
God, this just smells of Muscovite propaganda. Self-respecting Belarussians and Ukrainians never accepted Moscow as authority and rebelled against the oppressors any time they could.