Gallienus was Roman Emperor with his father Valerian from 253 to 260, and alone from 260 to 268. He took control of the empire at a time when it was undergoing great crisis. His record in dealing with those crises is mixed, as he won a number of military victories but was unable to keep much of his realm from seceding.
Appointed regent by his father Valerian when the Germans threatened the boundaries of the empire on the Rhine and the Danube. Gallienus took the western half of the empire and his father the eastern portion, in 255. Gallienus was by nature indolent and fond of pleasure. He was cruel to the vanquished, and was unable to repel the attacks of the Frankish invaders of Gaul, but bribed their chieftains to undertake the wardenship of the Rhenish borderline. When the Alemanni burst through the limes Rhaticus, or Rhætian barrier, and invaded Upper Italy, the senate armed the Roman burgesses for the first time in thirty years and raised a force of troops on its own responsibility. Gallienus defeated the enemy at Milan, but made an alliance with one of the chiefs of the Marcomanni, and gave him Upper Pannonia. He forbade the senators to enter the military service, to have anything to do with the army, and excluded them from the administration of the provinces. In consequence of this decree, the former distinction between imperial and senatorial provinces disappeared. During the wars against the Germans many distinguished Roman officers were proclaimed emperors in the various provinces. The most successful of these was Aurelian, who later became sole emperor. In consequence of the withdrawal of the troops from the eastern boundaries, the countries near the Bosphorus and the Black Sea were laid open to pillage at the hands of the Goths. Simultaneously the Persians under Sapor I swooped down on Asia Minor. Valerian led an army against them, but was betrayed and captured. His servitude lasted until his death in 260.
Gallienus thereupon became sole ruler. A bloody persecution of the Christians broke out in 257- 258, instigated by imperial edicts; they were accused of failure to take up arms in defence of the empire from its invaders. Whoever refused to take part in the Roman pagan rites was first exiled, then slain. One of the first victims was St. Cyprian, Bishop of Carthage, who was executed 14 September, 258; at Rome Sixtus II and his deacon St. Lawrence suffered martyrdom. After the death of his father, Gallienus granted liberty of worship to the Christians. He recognized as his deputy in the East Odenanthus, ruler of the commercial city of Palmyra and energetic conqueror of Sapor I, King of Persia. Afterwards he made him emperor. In the course of the wars against the enemies of the empire, the soldiers at various times proclaimed eighteen of their generals provincial emperors. These men were also called "The Thirty Tyrants". Among them were Postumus in Gaul, and Ingenuus in Pannonia, over whom Gallienus won a partial victory, with the help of Aureolus, the commander-in-chief of the imperial armies. When the troops in Italy acclaimed Aureolus "imperator", he tried to make himself master of Italy and Rome, but was defeated by Gallienus on the Adda and shut up in Milan. Gallienus was assassinated by his officers while this siege was going on.