The invading Arabs threaten the Empire by land and sea (they had penetrated as far as Galatia in 714), and Anastasios has attempted to restore peace by diplomatic means.

His emissaries having failed in Damascus, he undertakes the restoration of Constantinople's walls and the construction of a new fleet.

However, the death of the Caliph al-Walid I in February 715 has given Anastasius an opportunity to turn the tables on the enemy.

He has his fleet concentrate on Rhodes with orders not only to resist the approach of the enemy but to destroy their naval stores, and he dispatches an army under Leo the Isaurian, afterwards emperor, to invade Syria.

The troops of the Opsikian theme, resenting the Emperor's strict measures, mutiny, slay the admiral John, and proclaim as emperor Theodosios, a financial officer and tax collector of lowborn extraction in the southern portion of the theme of Opsikion.

According to one theory, he was the son of the former Emperor Tiberius III.

He did not readily accept this choice and, according to the chronicler Theophanes the Confessor, had even attempted to hide in the forests near Adramyttium.

Eventually he was found and was acclaimed emperor in May 716.

Theodosius and his troops immediately lay siege to Constantinople.

The eunuch churchman Germanus, made bishop of Cyzicus in about 705, had been pressured by Philippikos to sign a decree in 712 rehabilitating Monothelite teachings.

Anastasios upholds the decisions of the Sixth Ecumenical Council and deposes the Monothelete Patriarch John VI of Constantinople, replacing him with Germanus in August 715.

Germanus pronounces the orthodox creed and once again repudiates Monothelitism at a local council this same year.

This also puts an end to the short-lived local schism with the Church in Rome.

The rebels, after besieging the city for six months, gain entry in November.

Theodosius shows himself remarkably moderate in his treatment of his predecessor and his supporters.

Through the intercession of Patriarch Germanus I of Constantinople, Anastasios II is persuaded to abdicate and become a monk in Thessalonica.

Little is known of the short reign of Theodosios III, who immediately faces an Arab invasion deep into Anatolia and the advance of the Arab fleet.

Theodosius turns out to be quite unsuited to imperial office.

Arabic expansion at the expense of the Empire presumably precipitates the famous Roman-Bulgar treaty of 716, under which Constantinople agrees to pay annual tribute to khan Tervel (whom Justinian, prior to his second deposition, had made his son-in-law and named Caesar), further Roman-Bulgar commercial relations, and recognize the border in Thrace, where the Bulgars are to retain Zagora.

(This policy will pay off in 719 when they help relieve the Arabs’ second siege of Constantinople.)

Leo, in alliance with Artavasdos, the commander of the Armeniakon theme (the second largest in Asia Minor), refuses to recognize the new emperor and continues to champion the cause of Anastasios.

Theodosios' son had been captured by Leo in Nicomedia.

The feeble emperor, realizing that opposition is futile,chooses to resign the throne on March 25, 717.

He and his son subsequently enter the clergy; Theodosios will end his life in a monastery at Ephesus.

Leo now heads an empire humiliated by the presence of pagan barbarians upon Balkan soil rightfully considered “Roman,” threatened by an attack upon its Anatolian heartland and its capital, and reduced, finally, in the West to Sicily and the remnants of the Ravenna exarchate.

Leo's first task is the organization of the defense of Constantinople against eighty thousand Arab troops under Maslama ibn Abd al-Malik, who has angrily perceived his client’s deception.

Approaching Constantinople from Pergamon (Bergama), the Arabs besiege the Roman capital by land and sea from August 15, 717.

Leo's military prowess, augmented by the skillful deployment of Greek fire, forces the invaders from the city walls.

Caliph Sulayman joins the attack with two thousand warships and another eighty thousand troops, but upon landing is driven back by the Roman army.

The caliphate's forces camp for the winter.

Arab armies have meanwhile invaded Asia Minor.

Leo the Isaurian deceives them into believing that he will subjugate the empire for them, and thus wins their goodwill and support for his own attempt to seize the throne.

After persuading them to spare Roman territory, he rebels against Theodosius' rule in collusion with Artabasdos, the strategos of the Armeniac Theme, and marches on Constantinople.

Anastasios heads a revolt in 719 against Leo the Isaurian, who had succeeded Theodosius as Leo III, receiving considerable support, including auxiliaries reportedly provided by Tervel of Bulgaria.

However the chronicler Theophanes the Confessor, who offers this information elsewhere, confuses Tervel with his eventual successor Kormesiy, so perhaps Anastasios was allied with the younger ruler.

In any case, the rebel forces advance on Constantinople.

The enterprise fails, and Anastasios falls into Leo's hands and is put to death by his orders.