 Following Maximian 's failure to invade in 289 , an uneasy truce with Carausius began . Maximian tolerated Carausius ' rule in Britain and on the continent but refused to grant the secessionist state formal legitimacy . For his part , Carausius was content with his territories beyond the Continental coast of Gaul . Diocletian , however , would not tolerate this affront to his rule . Faced with Carausius ' secession and further challenges on the Egyptian , Syrian , and Danubian borders , he realized that two emperors were insufficient to manage the Empire . On March 1 , 293 at Milan , Maximian appointed Constantius to the office of Caesar . On either the same day or a month later , Diocletian did the same for Galerius , thus establishing the " Tetrarchy " , or " rule of four " . Constantius was made to understand that he must succeed where Maximian had failed and defeat Carausius . 
