When imperial forces move east under the command of Emperor Lucius Verus after the forces of Vologases IV of Parthia attack Armenia, the Romans' defense of the eastern territories is hampered when large numbers of troops succumb to what will become known as the Antonine Plague.

The Parthian capitals of Seleucia and Ctesiphon had been captured by the Roman general Avidius Cassius in 165 or 166.

Most likely around the same time, Roman legions invade Media and Adiabene.

However, the Romans are forced to withdraw in 166, due to suffering heavy losses by a plague that has erupted in Seleucia.

The war soon ends afterwards, with Vologases IV having lost a vast part of northern Mesopotamia to the Romans.