Historians infer from his account that the Byzantines were able to manipulate the Rus ' of Oleg 's time for their own political ends .
In one of his letters , Patriarch Nicholas Mysticus threatened to unleash a Rus ' invasion of Bulgaria .
Historians infer from Nicholas Mysticus ' account that the Byzantines were able to manipulate the Rus   of Oleg 's time for their own political ends .
In one of his letters , Patriarch Nicholas Mysticus threatened to unleash a Rus ' invasion of Bulgaria .