Valens Thessalonicus

In June 260, Persian king Shāpūr I defeated and took Emperor Valerian captive at Edessa in Roman Mesopotamia.

[1] Without Valerian in the east, the border populations and their legions felt vulnerable and elected their own emperors to guarantee they would have leaders against foreign threats.

After the capture of Valerian, the legions of Pannonia threw off their allegiance to Gallienus and proclaimed Ingenuus, their military commander, emperor.

After Gallienus put down this rebellion and returned to Italy to deal with the Alemannic invasion, another Roman general in Pannonia, Regalianus, would attempt to take the throne.

Ultimately, Piso journeyed only as far as Thessaly where he pronounced himself emperor assuming the name "Thessalicus," most likely a confused reference to Valens Thessalonicus.