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 ( vexillatio nes ) . 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 .