The Parthians launched attacks on Armenia and Mesopotamia as soon as the Roman emperor Aurelius began his reign.  The Romans counter-attacked, took back their provinces and invaded Parthia.  Around the time of the invasion, a massive epidemic de-populated Parthia and killed many Roman soldiers as well.  Without enough manpower, Rome was unable to subdue Parthia.

Notable Battles
Seleucia between Avidius Cassius and the Parthians.
Ctesiphon between Avidius Cassius and the Parthians.