Trajan's successor Hadrian ("r". 117–138 AD) reaffirmed the Roman-Parthian border at the Euphrates, choosing not to invade Mesopotamia due to Rome's now limited military resources. Parthamaspates fled after the Parthians revolted against him, yet the Romans made him king of Osroene. Osroes I died during his conflict with Vologases III, the latter succeeded by Vologases IV of Parthia ("r". c. 147–191 AD) who ushered in a period of peace and stability. However, the Roman–Parthian War of 161–166 AD began when Vologases invaded Armenia and Syria, retaking Edessa. Roman emperor Marcus Aurelius ("r". 161–180 AD) had co-ruler Lucius Verus ("r". 161–169 AD) guard Syria while Marcus Statius Priscus invaded Armenia in 163 AD, followed by the invasion of Mesopotamia by Avidius Cassius in 164 AD. The Romans captured and burnt Seleucia and Ctesiphon to the ground, yet they were forced to retreat once the Roman soldiers contracted a deadly disease (possibly smallpox) that soon ravaged the Roman world. Although they withdrew, from this point forward the city of Dura-Europos remained in Roman hands. When Roman emperor Septimius Severus ("r". 193–211 AD) invaded Mesopotamia in 197 AD during the reign of Vologases V of Parthia ("r". c. 191–208 AD), the Romans once again marched down the Euphrates and captured Seleucia and Ctesiphon. After assuming the title "Parthicus Maximus", he retreated in late 198 AD, failing as Trajan once did to capture Hatra during a siege.