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.