Battle of Mediolanum

The Battle of Mediolanum took place in 259, between the Alamannic Germans and the Roman legions under the command of Emperor Gallienus.

Background
As Emperor Valerian, Gallienus´ father, was unable to be present in the battlefield, he named his son Gallienus Emperor. While Valerian was fighting against the Sassanid Empire and the Goths, who by that time had sacked Thrace and Asia Minor, Gallienus would be in charge of defending the Roman Empire's border.

In the Western half of the Empire, the situation was difficult. The Danubian border resisted continuous barbarian attacks. Gallienus had to march with military reinforcements from Gaul towards Dacia and Moesia to fight the Barbarians. The situation was so severe that in 259, the legions of Pannonia and Moesia rebelled and chose to make Ingenuus the emperor. Gallienus reunited the Rhine, left Legio II Parthica to defend it and went off to do battle.

Order of battle
Within the borders of the Rhine and the Alps, a Germanic confederation, the Alamanni, who occupied a good part of the Agri Decumates (the territory located between the mouth of the Rhine river and the Danube), crossed the Alpine steps and attacked the fertile plain of the Po river. The sacking of the area instilled terror in Rome, as it was still not a walled city. The Senate of Rome hastily prepared a crowd of plebs for combat in an attempt to ensure that its shrinking army was capable of protecting the city. Gallienus had just defeated the pretender Ingenuus when the news arrived of the invasion by the Alamanni. He marched off with the legions I Adiutrix, II Italica and II Parthica 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 arrived in the valley of the Po, he found the Alamanni in the vicinity of Mediolanum, present day Milan. The victory was total: according to Zonaras 300,000 Alamanni fell that day and the emperor received the title Germanicus Maximus.

Aftermath
The Alamanni's success in reaching into the Roman Empire once more revealed 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 demonstrated to the Romans the value of swift, flexible military units. Afterward, Gallienus enacted a major reform by introducing a highly mobile field army composed mainly of cavalry (vexillationes). The main units were under the control of his General, Aureolus, and headquartered in Mediolanum, with the mission to protect Italy.

The Roman Senate had tried to regain its authority by arming and commanding its own military forces to meet the Germanic threat. But, uncomfortable with this challenge to his power, Emperor Gallienus suppressed all of the Senate's military prerogatives.

Finally, the invasion by the Alamanni demonstrated the vulnerability of Italy and especially Rome. This later caused Emperor Aurelian to have a strong wall built to defend the capital of the Empire.