Simplify the following text for a Grade 12 native English-speaking person.
Original: In 163 , while Statius Priscus was occupied in Armenia , the Parthians intervened in Osroene , a Roman client state in upper Mesopotamia , just east of Syria , with its capital at Edessa .
Simplified: In 163 , while Statius Priscus was occupied in Armenia , the Parthians intervened in Osroene , a Roman client in upper Mesopotamia , just east of Syria , with its capital at Edessa .