Excerpts from Books and Wikipedia
"His reign saw a consolidation of Byzantine security from external threats. As an able military leader, Constantine took advantage of Muslim disunity to make limited offensives on the Arab frontier. With this eastern frontier secure, he undertook repeated campaigns against the Bulgars in the Balkans. His military activity, and policy of settling Christian populations from the Arab frontier in Thrace, made Byzantium's hold on its Balkan territories more secure. Religious strife and controversy was a prominent feature of his reign. His fervent support of iconoclasm and opposition to monasticism led to his vilification by later Byzantine historians and writers." [Wikipedia] "In three decades on the Byzantine throne, Constantine V had presided over the ongoing quarrels between icon-lovers and icon-haters; he had seen the end of the Umayyad caliphate and the beginning of the Abbasid rule; he had watched Byzantine rule in Italy fail . . . ; he had heard news of the crowning of Pippin the Youngers the first Carolingian king and the rise of Charlemagne's power. He had married a Khazar wife and been troubled by the raids of Avars and the invasions of Arab armies." [Bauer: Medieval World, p. 387]