Struggling to defend both the Rhine and Danube frontiers, Gallienus was caught off guard when the Alemanni managed to cross the Alps into Italy in 258. The Senate hurriedly organized the defence of Rome, giving Gallienus time to enter Italy and defeat the invaders at Mediolanum (Milan) in 259. The incident, however, demonstrated that now even Rome was vulnerable to barbarian incursions.
Some time between 254 and 259, during the joint rule of Valerian and Gallienus, it appears likely that the regions of Phrygia and Caria were separated from the province of Asia to create the new province of Phrygia and Caria. The evidence for this comes from four inscriptions dated to that period. This province may have been created in response to the Gothic invasions of Asia Minor in the same decade.in wikipedia
In 258 Shapur I of Persia marched through northern Cappadocia to the Black Sea, where he sacked Trapezus. Barely rebuilt after its pillage by the Goths about two years earlier, the once prosperous city would take decades to recover.in wikipedia
In 258 Gallienus’ son Valerian II died in Sirmium, Lower Pannonia, in uncertain circumstances, while Ingenuus was commander in the province. That same year, or possibly in 260, Ingenuus was proclaimed Emperor by his legions in the face of incursions by the Iazyges. Aided by his reformed cavalry arm, Gallienus marched rapidly into Lower Pannonia, where he defeated the usurper at Mursa. Ingenuus died shortly after the battle, either by flinging himself into the Danube or being killed by his own men.in wikipedia
? ?? 258–25 Apr 260 First Alemannic invasion of Italy▲
In 258 the Alemanni and Juthungi crossed the Alps at Brenner Pass, invading Italy and marching on Rome. The Senate hurriedly organized a militia to defend the capital; this proved sufficient to deter the invaders, who decided to plunder the Po Valley instead. Here they were defeated by Gallienus at Mediolanum in 259, with a further clash at Augusta Vindelicum (Augsburg) in April 260 scattering the remnants. The conflict, however, demonstrated the vulnerability of even Rome to barbarian incursions.in wikipedia
In 254 the Marcomanni and other German tribes had invaded the Roman Empire. To help resolve the crisis, Gallienus secured a treaty with King Attalus of the Marcomanni in around 258, allowing the Marcomanni to settle in parts of Upper Pannonia in return for guarding the frontier. As part of the agreement, Attalus’ daughter Pipara was given to Gallienus as either a hostage or concubine.in wikipedia