 In July 285 AD , Diocletian declared Maximian , another colleague from Illyricum , his co @-@ emperor . Each emperor would have his own court , his own military and administrative faculties , and each would rule with a separate praetorian prefect as chief lieutenant . Maximian ruled in the West , from his capitals at Mediolanum ( Milan , Italy ) or Augusta Treverorum ( Trier , Germany ) , while Diocletian ruled in the East , from Nicomedia ( İzmit , Turkey ) . The division was merely pragmatic : the Empire was called " indivisible " in official panegyric , and both emperors could move freely throughout the Empire . In 288 , Maximian appointed Constantius to serve as his praetorian prefect in Gaul . Constantius left Helena to marry Maximian 's stepdaughter Theodora in 288 or 289 . 
