It was not an occupation at all. Cuz Ruthenians (Ukr/Bel) and Lithuanians had fought together against Mongols and Moscovia. Ruthenian language was official one. Grand Duchy of Lithuania is just continuation of Rus' Kingdom, but with Ruthenian/Lithuanian nobility together.
Moscovian and Soviet propaganda named it "an occupation".
Btw, its BelaRus (Not BelaRussia).