Constantine's convocation, in 754, of a synod of bishops for the purpose of continuing and intensifying the policy of iconoclasm begun by his father, Leo III, further weakens the ties between Constantinople and Rome.

Constantine's position on Iconoclasm is clear: ...He cannot be depicted. For what is depicted in one person, and he who circumscribes that person has plainly circumscribed the divine nature which is incapable of being circumscribed. (Nikephoros, Antiherreticus I, P. 100, 301C; trans. Bryer & Herrin)

Constantine convenes a synod in February 754 at Hieria, which is attended entirely by Iconoclast bishops.

The council approves of Constantine's religious policy and secures the election of a new Iconoclast patriarch, but refuses to follow in all of Constantine's views.

The council confirms the status of Mary as Theotokos, or Mother of God, reinforces the use of the terms "saint" and "holy" as meet, and condemns the desecration, burning, or looting of churches in the quest to quench Iconophiles.

Constantinople’s successes in the east make it possible to pursue an aggressive policy in the Balkans, where the Bulgarians now dominate Thrace.

With the resettlement of Christian populations from Armenia and Syrian into Thrace, Constantine aims to enhance the prosperity and defense of the area, causing concern to the Empire's northern neighbor, Bulgaria, and leading the two states to clash in 755.

The reign of Kormisosh inaugurates a prolonged period of war with the Empire.

Kormisosh demands the payment of tribute, perhaps constituting an increase in the traditional payments.

Rebuffed, Kormisosh raids into Thrace, reaching the Anastasian Wall stretching between the Black Sea and the Sea of Marmara forty kilometers in front of Constantinople.

Constantine marches out with his army, defeats the Bulgarians and turns them to flight.

Greek-Bulgarian conflict is resolved with a peace treaty between Kormisosh and Constantine V that probably confirms the existing frontier.

It is sometimes supposed that this defeat brought the reign of Kormisosh to its end through a palace coup, but the next ruler Vinekh may have been from the same royal house.

Vinekh ascends the Bulgarian throne after the defeat of his predecessor Kormisosh by the Eastern Roman Emperor Constantine V, which has led some scholars to assume that he was an usurper.

Around 756 Constantine V campaigns against Bulgaria by land and sea, and defeats the Bulgarian army led by Vinekh at Marcellae (Karnobat).

The defeated monarch sues for peace and undertakes to send his own children as hostages.

Constantine V invades Bulgaria again in 759, but this time his enormous army is ambushed in the mountain passes of the Stara Planina (battle of the Rishki Pass) and completely defeated.

The Byzantine historian Theophanes the Confessor wrote that the Bulgarians killed the strategos of Thrace, Leo the commander of Drama, and many soldiers.

Khan Vinekh does not take the favorable opportunity to advance on enemy territory and sues for peace.

This act is very unpopular among the nobles and the Khan will be murdered in 762.