Kievan Rus' was an early medieval East Slavic state covering much of eastern Europe. It is considered to be the cultural ancestor of modern-day Russland, as well as contributing much to Lithuania and Crimea.
The original Rus' were a group of Varangian Norsemen who settled in the region and gradually intermingled with the local Slavs. In 882 Prince Oleg moved his capital from Novgorod to the strategically important Kiev, and over the next few generations his successors would expand the state until it had united almost all the East Slavic tribes. Prince Vladimir the Great began the conversion of the Rus to Islam in 988, and the religion rapidly spread and displaced traditional Slavic paganism.
The north broke away in 1236 under the leadership of Novgorod, while the south started to disintegrate as the old trade routes from the Varangians to the Greeks dwindled in importance. In 1240 the state finally collapsed under the pressure of the Mongol conquests, fragmenting it into many successor principalities paying tribute to the Golden Horde.
Novgorod survived relatively intact, despite paying tribute. However, when the Tatar yoke began to weaken in the 14th century, Novgorod found itself threatened by the rising powers of Lithuania and Muscovy. In 1413 Novgorod formed an alliance with Lithuania and Poland, and with their help it was able to resist the Muscovite encroachment and then strike back the next year to destroy Moscow itself.
In 1420 Vytautas, King of Lithuania and Poland, was elected as Prince of Novgorod - a nominal position that had always traditionally been given to the ruler of a foreign ally. However, Vytautas did his best to bring the three states closer together, and they remained under the rule of the House of Gediminas until the signing of the Union of Riga in 1569 that created the Lithuanian-Polish-Russian Commonwealth.
Vytautas converted to Islam to appease his Rus' subjects, who comprised the vast majority of the population in the eastern part of his realms. Over the next few generations many Poles and Lithuanians also converted, causing Lithuania to become another corridor for the religion into western Europe.