The Belorussians, whose lands had been incorporated into the expanding Grand Duchy of Lithuania after the disintegration of Kievan Rus' in the thirteenth century, at first exerted a strong religious and cultural influence on the pagan Lithuanians; but the position of the Orthodox Belorussians began to decline after the dynastic union of Lithuania and Poland in 1386 and the Lithuanians’ consequent conversion to Latin Christianity.