Bray conjectures that Saloninus's appointment as Caesar, like that of his elder brother, Valerian II, in Illyria, was made at the instigation of Valerian I who was, simultaneously, the senior Emperor and grandfather of the two young Caesars and, as head of the Licinius clan, exercised also the patria potestas over all members of the Imperial family, including his son Gallienus, his co-Emperor. Bray suggests that Valerian's motive in making these appointments was securing the succession and establishing a lasting imperial dynasty. We do not know how Valerian envisaged his grandson interacting with the existing governors and military commanders of the Gallic provinces. There is no reason to suppose that he ever thought the thing through as systematically as Diocletian when he established the Tetrarchy some thirty years later. However, Silvanus must have been a seasoned soldier and administrator, and he does seem to have harboured the notion that, as guardian of Saloninus, he should exercise real authority in Gaul. This was demonstrated by the circumstances in which he fell out with the usurperPostumus. In 260 Silvanus ordered Postumus to hand over some booty that Postumus's troops had seized from a German warband which had been on its way home from a successful raid into Gaul. However, Postumus's men took violent exception to this attempt to enforce the rights of the representative of a distant emperor who was manifestly failing in his duty to protect the Gallic provinces. Asserting what was probably the prevailing custom of the frontier, they turned on Saloninus and Silvanus, who had to then flee to Cologne with some loyal troops. It was probably at this time that Postumus was acclaimed emperor by his army. Riding the tiger of military discontent which he could barely control, Postumus then besieged Saloninus and Silvanus in Cologne.
Death
Gallienus, who was fully engaged elsewhere – probably campaigning on the middle Danube – could do nothing to save his son. Saloninus's troops, in their desperation, finally proclaimed him emperor, perhaps hoping that this would induce Postumus's army to desert him and join them in a bid for Empire – i.e., against Valerian and Gallienus. If this was indeed their hope, they were to be disappointed in the event, for Postumus's army pressed on with the siege and, about one month later, the citizens of Colonia Agrippina handed Saloninus and his guardian over to their enemy. Postumus was then unable to prevent his army from murdering them. Despite his public protestations of regret, it seems in fact unlikely that Postumus made a serious effort to resist this course of events. Whether or not Gallienus ever concurred with Valerian's dynastic experiment is not known. Certainly the murder of Saloninus, so soon after the suspicious death of Valerian II, seems to have cured Gallienus of any ambition in this regard. We may assume that Valerian's mother, Salonina, would have been most unhappy: the death of her elder son, Valerian II, in Illyria under the tutelage of Ingenuus must have seemed to her to have confirmed her worst fears of this sort of arrangement. It had certainly proved to be folly to set up inexperienced boys as hostages to fortune and hope that their relationship to the imperial family would quell provincial resentment at the perceived inability of the central government to secure the frontiers from barbarian attack. Throughout the period of his sole reign, Gallienus made no effort to elevate his third son, Egnatius Marinianus, to the purple or associate him in any way with his government of the Empire – although he did allow him to be elected to the largely ceremonial office of Consul in 268.
