Celts advance into northern Italy, taking the city of Melpum (present Milan) in 396 and founding other settlements, including …

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.

Gallienus defeats the Alamanni at Milan in 262.

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.

The Alamanni, allied with the Juthungi, had again invaded northern Italy in late 270.

Aurelian, almost immediately after being proclaimed emperor, quick-marches an army north to Mediolanum to confront the invaders at …

Maximian, who is responsible for the West, is installed at Milan in northern Italy, in order to prevent German invasions.

As an Augustus, Maximian has thus become in theory the colleague of Diocletian, but his role will always be subordinate.

Diocletian and Maximian meet in Milan on the five-year anniversary of their rule, either in late December 290 or January 291, to discuss their successes and failures.

The meeting is undertaken with a sense of solemn pageantry; the Emperors spend most of their time in public appearances.

It has been surmised by David Potter that the ceremonies were arranged to demonstrate Diocletian's continuing support for his faltering colleague.

(Potter, David S. The Roman Empire at Bay: AD 180–395.

New York: Routledge, 2005) A deputation from the Roman Senate meets with the Emperors, renewing that body's infrequent contact with the Imperial office.

The choice of Milan over Rome further snubs the capital's pride, but it is already a long established practice that Rome itself is only a ceremonial capital, as the actual seat of the Imperial administration is determined by the needs of defense.

Long before Diocletian, Gallienus (r. 253–68) had already chosen Milan as the seat of his headquarters.

If the panegyric detailing the ceremony implies that the true center of the Empire is not Rome, but where the Emperor sits ("...the capital of the Empire appeared to be there, where the two emperors met"), (Panegyrici Latini 11(3)12, qtd.

in Williams, 57.)

it simply echoes what had already been stated by the historian Herodian in the early third century: "Rome is where the emperor is".

During the meeting, decisions on matters of politics and war are probably made, but they are made in secret.

The Augusti will not meet again until 303.

Maximian, residing in Milan, continues to govern Italy, Sicily, Spain, and Africa.

Maximian had returned to the north of Italy after his Mauretanian campaign, living a life of leisure in palaces in Milan and Aquilea, and leaving warfare to his subordinate Constantius.

Maximian is more aggressive in his relationship with the Senate than Constantius, and Lactantius contends that he terrorized senators, to the point of falsely charging and subsequently executing several, including the prefect of Rome in 301/2.

Maximian (although he will long be viewed by future Christians as a persecutor of their religion) seems to have done no more than obediently execute in his part of the empire the first edict of Diocletian, which ordered the burning of the Scriptures and the closing of the churches.