Postumus remained Gallienus's nemesis for most of his reign and, with the aid of the traitor Aureolus, his eventual killer. Postumus came on the scene following a dispute with Saloninus over the distribution of captured war loot. Because Postumus was in command of the military, Saloninus had little to defend him except for his title and official recognition. Postumus had himself named emperor and captured and executed Saloninus sealing his fate with the rest of the empire. However, the rest of the empire could do little at the moment. Gallienus was embroiled with battling several usurpers out east and could not avenge his brother's death. Neither could Rome do anything about it. And so for the time being Postumus held the western half of the empire as de facto ruler.
When Gallienus finally returned from the east he would find Postumus entrenched in Gaul and having snatched Britain and Spain away from the empire as well. Because his power had grown during Gallienus's absence, he had had time to consolidate his leadership and posed a bigger threat. However, for one reason or another, Postumus never made preparations to attack Rome or mount an offensive against Gallienus and contented himself with this secessionist state. But he knew Gallienus was mulling over his options all the meanwhile and had on his agenda the recapturing of the lost lands. Gallienus mounted several expeditions to depose Postumus but all failed. Still, Gallienus would most likely have ultimately been the ultimate victor had Postumus not gained the aid of one of Gallienus's trusted men, Aureolus, who engineered a successful conspiracy ending with the murder of Gallienus. Postumus would meet a deadly fate himself the following year following a revolt from within instigated by one of his own earlier leading generals, Laelianus, in a move very reminiscent of his own rise to power.
RIC 57, Cohen 273 AR Antoninianus Obv: IMPCPOSTVMVSPFAVG; Radiate, cuirassed bust right.
Rev: PMTRPIIIICOSIIIPP; Mars walking right, holding spear and trophy.
RIC 287, Cohen 31 AE Antoninianus Obv: IMPCPOSTVMVSPFAVG. Radiate, draped, cuirassed bust right.
Rev: COSIIII. Victory standing right, holding wreath and palm.
RIC 106, Cohen 246 Sestertius Obv: IMPCMCASSLATPOSTVMVSPFAVG; Radiate, draped and cuirassed bust right.
Rev: PMTRPCOSIIPP S C; Postumus standing left, holding globe and spear. Lot sold for $270 4/4/01.
RIC 167 As Obv: IMPCPOSTVMVSPFAVG; Laureate, draped and cuirassed bust right.
Rev: VICTORIAAVG; Two Victories standing, attaching a shield to a trophy of captured arms, under which sit two captives.