Vologases supported Emperor Pescennius Niger () in his struggle for the Roman throne against Emperor Septimius Severus () in 192–193, during the Year of the Five Emperors. Furthermore, he also intervened in the affairs of the Roman vassal states in northern Mesopotamia—Adiabene and Osroene. Because of this, Septimius Severus, who emerged victorious in the struggle, attacked the Parthian Empire in 195. Severus advanced into Mesopotamia, made Osroene a Roman province, and captured the Parthian capital Ctesiphon in 199. At the same time, revolts were occurring in the Parthian provinces of Media and Pars. Septimius Severus now declared himself "Parthicus Maximus" ("great victor in Parthia"). He was, however, unable to maintain his conquests, due to lack of food supplies and reinforcements. As a result, he withdrew his forces; during his withdrawal, he attempted in vain to conquer the Arab fortress of Hatra twice, later withdrawing his forces to Syria.