Similarly to the Isaurian emperors, Nikephoros enacted a vigorous policy of resettlement of impoverished soldiers and peasants from the themes of Asia Minor to Thrace, Greece and Macedonia.

He expanded and consolidated the imperial military forces in various themes, managing the empire's finances with strict discipline, which led to the displeasure and hostility of his subjects.

Theodore persuaded his brother, Joseph, the Archbishop of Thessalonica, to refuse to celebrate the Christmas liturgy with the Patriarch and Emperor, leading to a confrontation.

Nikephoros confirmed in a 809 synod that the emperor was not bound by canon law, and demoted Joseph, Theodore and Plato, banishing them to the Princes' Islands.

Nikephoros responded by sending a fleet, but a compromise was reached in which the pro-Frankish Doge, Obelerio degli Antenori, would accept the additional title of spatharios to confirm his imperial loyalty.

Relations between the two deteriorated, leading to a war over Venice between 807 and 810 when Obelerio began to show signs of disloyalty, and a second Byzantine fleet arrived.

[17] Shortly after Nikephoros' accession, he sent an insulting letter to Abbasid Caliph Harun al-Rashid, demanding the return of tribute which Empress Irene had paid.

Due to Bardanes' disloyalty, Nikephoros was forced to lead the military campaign himself, resulting in a severe defeat at the Battle of Krasos in Phrygia in 805.

Harun had taken measures to secure the Arab-Byzantine frontier including the construction of new fortified districts (al-Awasim) in order to more effectively gain control of Asia Minor.

However, with the Caliphate embroiled in a succession struggle following Harun al-Rashid's death in 809, Nikephoros was able to focus on dealing with Krum, the Khan of Bulgaria, who was posing a threat to the empire's northern frontiers and had recently conquered Serdica (now Sofia).

[21] In 811, Nikephoros launched an invasion of Bulgaria and was initially successful against Khan Krum and his armies, and sacked the Bulgarian capital, Pliska.

He reached the capital city of their kingdom and caused great destruction, to the point that he threw their little children on the ground and mercilessly drove over them with his threshing wagons.

"[23] As Nikephoros' forces were returning to Byzantium, the imperial army was ambushed and annihilated in the Varbitsa Pass at the Battle of Pliska by Krum.

[22] The reigns of the iconoclast emperors Leo III the Isaurian (r. 717–741) and Constantine V (r. 741–775) were militarily successful and saw the abatement of the Arab and Bulgar threat.

The validity of iconoclasm was only confirmed by the military disasters under the iconodule emperors Nikephoros I and Michael I Rhangabe (r. 811–813) as well as the success of the iconoclast Leo V the Armenian (r. 813–820) against the Bulgars.