Gallienus, Publius Licinius Egnatius was Emperor of Rome from 253 until 268. He was made joint ruler on the accession of his father, Valerian, in August 253. In 258 Gallienus defeated the Alamanni, a group of German tribes who were making incursions into the Roman provinces along the Danube River. The Alamanni rose again, however, and forced their way into Italy, where Gallienus gained a second victory over them near Mediolanum.
Meanwhile, Valerian had been engaged in wars with Shapur I, the sassanid King of Persia, by whom he was taken prisoner in 260 and put to death. Gallienus then became sole emperor, but in name only, because self-appointed rulers arose in all outlying parts of the Roman Empire. To use his troops most effectively, he established a highly mobile cavalry force that was retained by his successors.