Prior to 1300s the territories of Belarus was settled by different peoples not necessarily related to each other.
Slavs : Dregoviches (south), Radimiches (south-east), Kriviches (centre, east, north-east)
Balts : Yotvingians (west), Lithuanians (north-west). Latgalians (north)

After ancient Rus state was collapsed under Mongol invasion in 1241, nobility of present day territories of Belarus leaned towards Lithuanians, who had a powerful army and could defend the territories. Balts and Slavs struck several dynasty marriages and Lithuanian grand dukes inherited much of the land on territories of Belarus.

By 1320 the grand duke of Lithuania Gedyminas consolidated the borders of what is present day Lithuania (without Samogitia) and Belarus. Since that time the ethnogenesis of Belarusians began. By 1795 half of Belarusian ancestors were identifying themselves as Litvins. Many nobility was polonised though. If not for the Russian intervention in 1795 likely all Belarusians would be identifying themselves Litvins today. Instead, we got Belarusian ethnonym. Most Belarusians are happy with it. Some say our ancestors didn’t identify by this ethnonym, so it’s not ours.