Roman Emperor, was adopted by Diocletian and named his co-regent in 285
Maximianus, MARCUS AURELIUS VALERIUS, surnamed HERCULIUS, Roman Emperor, was adopted by Diocletian and named his co-regent in 285, because by this division of the sovereignty the danger of the warriors' mutiny, the ambitious efforts of the usurpers, and the attacks of foreign enemies seemed to be prevented in the surest way. Diocletian gave him, who had been hitherto his brother-in-arms and was now his fellow regent, the surname Herculius, in remembrance of the help which the mythological Hercules rendered his father Jupiter in the latter's struggle against the giants. Like Diocletian, Maximianus came from Illyria, from the neighborhood of Sirmium; as the son of a simple peasant, he possessed only very little education; he was violent and brutal, but was a brave fighter. For this reason, when Diocletian was struggling with the Persians in Asia, Maximianus was entrusted with the leadership of the punitive expedition against the peasants and field slaves (Bagaudans) in Gaul who, driven by economical causes, had risen against Diocletian. The new emperor soon restored peace, and received from Diocletian, in token of the ratter's gratitude, the title of Augustus on April 1, 286. However, only the administration of the empire was divided; the sovereignty remained centralized now as ever, and the will of the emperor-in-chief, Diocletian, was absolute. While Maximianus, having established his headquarters at Mainz, was successful in the struggles with the Burgundians and the Alamanni, who had crossed the frontier and the Rhine, he found many obstacles in repulsing the Menapian pirate chief Carausius. Originally commander-in-chief of the Roman navy, Carausius had pursued and conquered the pi-rates of the German ocean; then, driven by greed and ambition, he had forced Britain to do homage to him, and seized the whole trade in Gaul and Britain. In 286 he even appropriated the title of Augustus, and caused coins to be struck which bore his own portrait. Even Diocletian, by a compromise in 290, was forced to recognize Carausius as the legal emperor, while the latter agreed to supply Diocletian with corn, as had been the custom.
As Diocletian left Syria to enter the countries of the Lower Danube, he met Maximianus, and both the emperors crossed the Alps in the beginning of 291 in order to attend a conference at Milan, there to discuss the better administration of the empire and the improvement of the constitution. Henceforward two substitutes, called Caesars, were to supplement the two governing emperors. Constantius and Galerius were proclaimed Caesars March 1, 293; the first was forced to marry the stepdaughter of Maximianus, Theodora, after the exile of his mother Helena. Maximianus now took charge of the administration of Italy, Africa, and Spain. His residence was Milan, where he was surrounded by 6000 Illyrian picked troops, called Herculians. Constantius on his part was now successful in his struggle with Carausius. The war came quickly to an end, as Carausius was assassinated by Allectus, prefect of his guard, in 293. Constantius then reunited Britain with the Roman Empire, while Maximianus protected the frontiers of Gaul against the Teutons on the Upper Rhine. When Constantius had returned from Britain, Maximianus went in 297 to Africa, where he sucessfully made war upon rebellious tribes of the Moors, and sent a great many captives into the other provinces. In 302 he celebrated a great triumph with Diocletian in Rome; seventeen times he had borne the title of Imperator. The persecution of the Christians, which Diocletian had conducted with reckless brutality in the East since 303, was also taken up by Maximianus in the western provinces, of which he was governor.
It is said that during these persecutions—it is impossible to state the time correctly—the Christian soldiers of the Theban legion also suffered martyrdom in Agaunum (St-Maurice, Canton of Valais, Switzerland) in the then Diocese of Octodurum. The Christian soldiers of this legion refused to execute his orders when Maximianus, on a march over what is now the Great St. Bernard, commanded them to punish the Christians living in these districts; for this refusal the legion was twice decimated by the sword, and, as the survivors held out to the last, all the soldiers were massacred by order of the emperor. Because Rome was degraded by Diocletian more and more to the position of a provincial town, and because Galerius's new and hard system of taxes was to be extended also to Italy and to Rome, the senators and the pretorians proclaimed as Caesar M. Aurelius Maxentius, the son of Maximianus; the latter laid down the purple at Milan. But the new emperor proved to be incapable of governing, and Maximlanus, who was popular with the army, was recalled to restore order for the new Augustus. This he did not accomplish, and the old Diocletian, living as a private person in Salona, called a meeting of all the members of the dynasties at Carnuntum for the end of the year 307. Maximianus had to renounce the purple for the second time. He now went to Gaul, and gave his youngest daughter Fausta in marriage to Constantine. As his hope to regain his former imperial dignity failed here also, he returned to his son Maxentius in Italy. Repulsed by the latter and spurned by Galerius on account of his ambitions, he departed once more for Gaul and donned the imperial purple for the third time. When the news of Constantine's approach reached his own soldiers, they surrendered him to his rival and opponent at Marsilia. Although Constantine in his generosity pardoned him, he returned to the forging of nefarious schemes against his son-in-law, and finally was compelled to take his own life in 310.