Legio I Adiutrix Pia Fidelis
Following the murder of Domitian in 96, the Adiutrix, along with the Danubian army, played an important role in Roman politics, forcing Nerva to adopt Trajan as his successor. When Trajan became emperor, he gave the legion the cognomen Pia Fidelis ("loyal and faithful") to acknowledge their support. Between 101 and 106, under the new emperor's command, I Adiutrix, along with IV Flavia Felix and XIII Gemina, conquered Dacia and occupied the newly formed province. Trajan also used his Pia Fidelis in the campaign against Parthia (115–117), but they were sent back to Pannonia by his successor emperor Hadrian, with base in Brigetio.
During the next decades, I Adiutrix remained in the Danube frontier. Under Marcus Aurelius, I Adiutrix fought the war against Marcomanni commanded by Marcus Valerius Maximianus. Between 171 and 175, the commander was Pertinax, emperor for a brief period in 193. When Septimius Severus became emperor, I Adiutrix was among his supporters, following him in the march for Rome.
In the next decades, the main base was again Pannonia, but they played a part in several Parthian wars, namely the campaigns of 195 and 197–198 of Septimius Severus, 215–217 led by Caracalla and 244 by Gordian III.
The legion received the cognomen Pia Fidelis Bis ("twice loyal and faithful") and Constans ("reliable"), sometime in the 3rd century.
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