 Maximian was born near Sirmium ( modern Sremska Mitrovica , Serbia ) in the province of Pannonia , around 250 into a family of shopkeepers . Beyond that , the ancient sources contain vague allusions to Illyricum as his homeland , to his Pannonian virtues , and to his harsh upbringing along the war @-@ torn Danube frontier . Maximian joined the army , serving with Diocletian under the emperors Aurelian ( r . 270 – 275 ) and Probus ( r . 276 – 282 ) . He probably participated in the Mesopotamian campaign of Carus in 283 and attended Diocletian 's election as emperor on November 20 , 284 at Nicomedia . Maximian 's swift appointment by Diocletian as Caesar is taken by the writer Stephen Williams and historian Timothy Barnes to mean that the two men were longterm allies , that their respective roles were pre @-@ agreed and that Maximian had probably supported Diocletian during his campaign against Carinus ( r . 283 – 285 ) but there is no direct evidence for this . 
