Then Maximian returned into Gaul; and after having made some stay in those quarters, he went to Galerius, the enemy of his son, that they might confer together, as he pretended, about the settlement of the commonweal; but his true purpose was, under colour of reconciliation, to find an opportunity of murdering Galerius, and of seizing his share of the empire, instead of his own, from which he had been everywhere excluded.
Diocles was at the court of Galerius when Maximian arrived; for Galerius, meaning now to invest Licinius with the ensigns of supreme power in the room of Severus, had lately sent for Diocles to be present at the solemnity. So it was performed in presence both of him and of Maximian; and thus there were six who ruled the empire at one and the same time.
Now the designs of Maximian having been frustrated, he took flight, as he had done twice before, and returned into Gaul, with a heart full of wickedness, and intending by treacherous devices to overreach Constantine, who was not only his own son-in-law, but also the child of his son-in-law; and that he might the more successfully deceive, he laid aside the imperial purple. The Franks had taken up arms. Maximian advised the unsuspecting Constantine not to lead all his troops against them, and he said that a few soldiers would suffice to subdue those barbarians. He gave this advice that an army might be left for him to win over to himself, and that Constantine, by reason of his scanty forces, might be overpowered. The young prince believed the advice to be judicious, because given by an aged and experienced commander; and he followed it, because given by a father-in-law. He marched, leaving the most considerable part of his forces behind. Maximian waited a few days; and as soon as, by his calculation, Constantine had entered the territory of the barbarians, he suddenly resumed the imperial purple, seized the public treasures, after his wont made ample donatives to the soldiery, and feigned that such disasters had befallen Constantine as soon after befell himself. Constantine was presently informed of those events, and, by marches astonishingly rapid, he flew back with his army. Maximian, not yet prepared to oppose him, was overpowered at unawares, and the soldiers returned to their duty. Maximian had possessed himself of Marseilles (he fled thither), and shut the gates. Constantine drew nigh, and seeing Maximian on the walls, addressed him in no harsh or hostile language, and demanded what he meant, and what it was that he wanted, and why he had acted in a way so peculiarly unbecoming him. But Maximian from the walls incessantly uttered abuse and curses against Constantine. Then, of a sudden, the gates on the opposite side having been unbarred, the besiegers were admitted into the city. The rebel emperor, and unnatural parent and a perfidious father-in-law, was dragged into the presence of Constantine, heard a recital made of his crimes, was divested of his imperial robe, and, after this reprimand, obtained his life.