Legions of the Parthian Wars
Parthia had always been a thorn in the side of the Roman Empire. The initial campaigns by Crassus and Mark Antony were total failures, and although Trajan and Syrian governor Cassius made some progress in the 2nd century CE, both failed to eliminate the Parthians as a viable threat. The last big clash came in 198 CE under Septimius Severus, which ultimately achieved nothing but left both empires weakened.