After summer 197 Septimius Severus headed east, relieving the Parthian siege of Nisibis and invading the Parthian clients of Adiabene and Araba. Before invading Parthia, he attempted to capture Hatra—a mountain fortress and capital of Araba—but, after losing many men to both missiles and disease, eventually called off the siege. Severus made a second attempt at Hatra after returning from Parthia in early 198, then a final effort the following winter, but was repelled with heavy losses in both cases.in wikipedia
In winter 197 Septimius Severus led a sudden invasion of Parthia from Araba, sailing down the Euphrates in quickly-constructed boats. The move caught the Parthians by surprise, allowing Severus to seize Seleucia and Babylon without resistance. He then routed the Parthian army and plundered Ctesiphon, before returning north via the Tigris.in wikipedia
With his victory against Parthia, Septimius Severus annexed territory in the upper Tigris and Euphrates to the Roman Empire. Part of this was reorganized into a new province of Mesopotamia, with its capital at Nisibis. Although Severus’ Mesopotamia was far smaller than Trajan’s attempt in 116–17, its strategic location posed a considerable threat to Ctesiphon, the capital of Parthia (and later, Sasanian Persia).in wikipedia
