Early historians regarded the reign of Gallienus as one of the most disastrous in the history of Rome, but later ones have taken a different line and given him credit for some successes during a very difficult period of Roman history.
Probably born in 218, Publius Licinius Valerianus Egnatius Gallienus was the son of a general who became Emperor Valerian in 253. When this happened, Gallienus was made co-Emperor by his father, taking care of the western empire while Valerian concentrated on the eastern side.
<a href="https://commons.wikimedia.org/wiki/File:0253_Portraitb%C3%BCste_Kaiser_Gallienus_anagoria.JPG" target="_blank" rel="noopener">Source</a>
At the time of his accession as co-Emperor, Gallienus had three sons of his own, two of whom, Valerianus and Saloninus, were given the title of Caesar despite probably being aged only 15 and 11. The boys were despatched to Illyria and Gaul respectively to create an imperial presence, but neither lived long enough to prove their worth.
Valerianus had been entrusted to the care of a commander named Ingenuus, but after Valerianus died he declared himself to be emperor. Gallienus clearly could not accept this, especially as it was almost certain that Ingenuus had been responsible for his son’s death. Battle was joined in 260. Gallienus was victorious and Ingenuus took his own life.
A general in Dacia, Regalianus, tried to seize power, also in 260, but was killed by his own troops.
In Gaul, Marcus Postumus declared himself “Emperor of the Gauls” and was able to maintain that stance right through Gallienus’s reign. Gallienus suspected that Marcus Postumus had something to do with the death of Saloninus but could never find a way of proving it.
Gallienus was never secure on his throne. An ancient text, the “Historia Augusta”, listed “thirty tyrants” who claimed the title of emperor at various times, although this was almost certainly an exaggeration.
In 260 Gallienus became sole emperor when Valerian was captured by the Sassanid emperor Shapur and disappeared from the scene. He soon realised that the empire had reached the stage where the old method of ruling from the centre was no longer appropriate and a lot more delegation had to take place.
The myth of the “thirty tyrants” may well have to do with this new policy, in that more governors and commanders were given positions of real authority, this being seen mistakenly by some historians as posing challenges to the rule of the emperor.
A good example of how Gallienus organised the government of the empire can beseen in Palmyra (modern Syria). The local prince, Septimius Odaenathus, was also under threat from the Sassanids and saw that an alliance with Rome was in his best interests. With Roman help he was able to keep Shapur at bay, and he gave himself the title “King of Kings” on the strength of this. However, he was always loyal to Gallienus and cannot therefore count as one of the “Tyrants”.
However, some of the threats were real enough, and one them finally succeeded. In 268 a plot was hatched by some of Gallienus’s senior officers who launched a revolt in northern Italy. Gallienus was killed while besieging Milan, but the siege was eventually successful and one of Gallienus’s loyal generals took over the role of emperor, being known to history as Claudius II.