Aureolus

Valerian’s son and co-emperor Gallienus fights a series of campaigns against the Goths on the Upper Rhine.

While Valerian is fighting against the Sassanid Empire and the Goths, who by this time have sacked Thrace and Asia Minor, Gallienus will be in charge of defending the Roman Empire's border.

In the Western half of the Empire, the situation is difficult.

The Danubian border resists continuous barbarian attacks.

Gallienus has to march with military reinforcements from Gaul towards Dacia and Moesia to fight the barbarians.

The situation is so severe that in 259, the legions of Pannonia and Moesia rebel and choose to make Ingenuus the emperor.

Gallienus reunites the Rhine, leaves a Roman legion their to defend it and goes off to do battle.

Within the borders of the Rhine and the Alps, a Germanic confederation, the Alamanni, who occupy a good part of the Agri Decumates (a region of the Roman Empire's provinces of Germania Superior and Raetia, covering the Black Forest, Swabian Jura, and Franconian Jura areas between the Rhine, Main, and Danube rivers), cross the Alps and attack the fertile plain of the Po river.

The sacking of the area instills terror in Rome, as the city remains unwalled.

The Senate of Rome hastily prepares a crowd of plebs for combat in an attempt to ensure that its shrinking army is capable of protecting the city.

Gallienus had just defeated the pretender Ingenuus when the news arrives of the invasion by the Alamanni.

He marches off with three legions to intercept the barbarians in Italy.

By then, according to the Byzantine historian Joannes Zonaras, the Alamanni had retreated before the unexpected resistance of the citizens of Rome and its Senate.

When Gallienus arrives in the valley of the Po, he finds the Alamanni in the vicinity of Mediolanum, present day Milan.

The victory is total: according to Zonaras three hundred thousand Alamanni fell that day and the emperor received the title Germanicus Maximus.

The Alamanni's success in reaching into the Roman Empire once more reveals the weakness of the centuries-old tradition of posting Rome's legions near the borders without providing for defense within the empire.

The battle of Mediolanum demonstrates to the Romans the value of swift, flexible military units.

Afterward, Gallienus enacts a major reform by introducing a highly mobile field army composed mainly of cavalry (vexillationes).

The main units are under the control of his general, Aureolus, and headquartered in Mediolanum, with the mission to protect Italy.

Finally, the invasion by the Alamanni demonstrates the vulnerability of Italy and especially Rome.

This later causes Emperor Aurelian to have a strong wall built to defend the capital of the Empire.

Manius Acilius Aureolus, a Roman cavalry commander under Gallienus, had in 261 defeated in Thrace the army of the usurpers Macrianus Major and Macrianus Minor, who had rebelled against Gallienus.

He is sent in 265, to dispose of the rebellion of Postumus but his carelessness allows the usurper to flee.

Zosimus reports that Aureolus and other two officers conspired against Gallienus, but that all of them were punished and submitted, except Aureolus, who retained his anger against the emperor.

Gallienus, after his failed attempt at defeating Postumus, is occupied with crises in the rest of his empire and will never challenge Postumus again.

Gallienus leads a large Roman force into Moesia to cut off the army of Goths, three hundred and twenty thousand strong, now retreating by land from its plunder of Aegean coastal cities.

The two armies meet at Naissus (modern Niš, Serbia), where the Romans slaughter the Goths by the thousands.

His attention demanded by the revolt of a usurper, Aureolus, in Italy, Gallienus returns there without further pursuit of the survivors, who flee to armed encampments to the northeast.

Mediolanum (Milan), having acquired increasing prestige and economic power over the past few centuries, has become the second city of the Empire behind Rome itself.

Manius Acilius Aureolus, commander of the field army in Mediolanum, has succeeded to recover Raetia to the central empire by 268.

In this same year, he is in Mediolanum, where he rebels against Gallienus, supporting Postumus, who has carved the Gallic Empire for himself out of the northern Roman provinces, and minting coins in his name.

Aureolus sends letters to Postumus, asking him to come and invade Italy, but Postumus refuses, and leaves Aureolus to his fate.

Gallienus, having left his “Scythian war,” returns to Italy to besiege Aureolus in Mediolanum, but is soon afterwards assassinated in a plot hatched, apparently, by his own staff officers.

After the death of the emperor, Aureolus claims the purple with the support of his army, but a large contribution to the troops secures the election of Gallienus' cavalry commander, who, as Marcus Aurelius Valerius Claudius, or Claudius II, is the first Illyrian to occupy the imperial throne.

Claudius continues the siege, rejecting Aureolus' attempts to sue for peace.

Soon after Aureolus surrenders, hoping for mercy, he is instead put to death by the Praetorian Guard, which has not forgiven his treachery.

Postumus, independent emperor in Gaul from about 258, has successfully defended the Rhine frontier and withstood Gallienus' attempts to recover Gaul.

Having declined to support the usurper Aureolus in any way, he must now deal with a rebel on his own side, Laelianus, one of his most senior military leaders, who is hailed emperor at Moguntiacum (present Mainz, Germany) by the local garrison as well as by other troops of the area.

Close by at Augusta Treverorum, Postumus acts immediately.