Константин V
The Byzantine emperor Constantine V had used the weakness of the Umayyad caliphate to regain land from Muslim rulers. After the Umayyad caliphate was defeated by Al-Manur's predecessor As-Saffah, Constantine V invaded Armenia and occupied parts of it throughout 751 and 752. Under Al-Mansur's rule Muslim armies conducted raids on Byzantine territory. Al-Mansur was the first Abbasid caliph to hold a ransom meeting with the Byzantine Empire. Diplomats in the service of Constantine V and Al-Mansur first negotiated the exchange of prisoners in 756.