 Finally , Diocletian knew that single rule was dangerous and that precedent existed for dual rulership . Despite their military prowess , both sole @-@ emperors Aurelian and Probus had been easily removed from power . In contrast , just a few years earlier , the emperor Carus and his sons had ruled jointly , albeit not for long . Even the first emperor , Augustus , ( r . 27 BC – AD 19 ) , had shared power with his colleagues and more formal offices of co @-@ emperor had existed from Marcus Aurelius ( r . 161 – 180 ) on . 
