A senior general of Armenian origin, Leo distinguished himself under Nikephoros I and Michael I Rhangabe, eventually becoming the stratēgos of the Anatolic Theme.

In 811, when Nikephoros was planning his major campaign against the Bulgars (which was to end disastrously), Arab raiders captured and destroyed the city of Euchaita in the Armeniac Theme—a humiliating defeat in which the salaries of the thematic units were also lost.

[9] Recalled by Michael I Rhangabe in 811, Leo became governor of the Anatolic Theme and conducted himself well in a war against the Arabs in 812, defeating the forces of the Cilician thughur under Thabit ibn Nasr.

Leo survived the Battle of Versinikia in 813 by abandoning the battlefield, but nevertheless took advantage of this defeat to force the abdication of Michael I in his favor on 11 July 813.

With this moment of respite, Leo divorced his allegedly adulterous wife and married the daughter of the patrikios Arsaber, the well-regarded Armenian noblewoman Theodosia, crowning her first son, the ten-year-old Symbatios, co-emperor, and renaming him Constantine, recalling the militarily successful iconoclast emperors of the eighth century, Leo III the Isaurian and Constantine V. In 814, Krum sacked Arcadiopolis and other Thracian towns, and planned a full-scale siege of Constantinople, but died of a stroke before he could begin, causing the Bulgar threat to finally recede.

In turn, Constans and Leo appealed to the legacy of Constantine the Great in convoking the First Council of Nicaea, and believed their actions were only a re-assertion of this legitimate imperial authority in spiritual matters.

When called to comment, the iconodule Patriarch Nikephoros I of Constantinople rejected the report and argued that the biblical passages quoted as condemning iconodulia were not dealing with "holy images" but with idols.

Leo asked the commission to produce a more carefully researched document, and enlisted the help of a major bishop, Antony, later Patriarch of Constantinople from 821.

In the second document, the commission stated that, since the military blamed their defeats by the Bulgars on iconodulia, a compromise must be sought in which only images hanging low in churches (which are therefore able to be venerated) would be removed, but Nikephoros resisted again.

Some soldiers of the imperial guard threw stones and mud at the icon of Christ over the Chalke Gate, shouting iconoclastic slogans, prompting Leo to have it replaced with a cross.

The next day, Leo requested the iconodule clergy to engage in a debate with the commission, and kissed a small icon to demonstrate his desire for compromise.

After several weeks of correspondence and attempts by Leo to persuade clergy of the compromise proposal, Nikephoros remained unwavering in his views.

The central argument was that holy images were unsuitable if placed in areas that would implicitly or explicitly earn them the devotion and adoration which is due to God alone.

[14]: 371–374 Leo's compromise policy resulted in great success, and Theodore of Stoudion reports in a letter that virtually the whole clergy of Constantinople adopted the imperial position, along with many bishops and laymen throughout the Empire.

For example, in the Life of Nicetas of Medikion, it is reported that Patriarch Theodotos gave communion to iconodule monks while they were proclaiming "Anathema to those who do not venerate the icon of Christ".

[14]: 380 Shortly after the reinstitution of iconoclasm, the new Khan of Bulgaria, Omurtag, executed his Byzantine captives who refused to renounce Christianity, raided Thrace and ignored an imperial embassy, forcing Leo to fight.

Theodore the Stoudite pre-empted these efforts through his correspondence with Paschal beforehand, in which he criticised the iconoclast patriarch and directly appealed to the papacy to restore orthodoxy in the Byzantine Empire.

In 817, he sent fleets to raid the weakened Abbasid Caliphate and led an army to retake the frontier fortress of Kamachon which was lost under Constantine VI.

In 818, the Rus' raided the Empire for the first time, specifically the northern coast of Anatolia, which Leo responded to by reinforcing the Armeniac Theme with naval squadrons.

[20] In the dim light they mistook the officiating priest for the Emperor and the confusion allowed Leo to snatch a heavy cross from the altar and defend himself.

He called for his guards, but the conspirators had barred the doors and within a few moments a sword stroke had severed his arm, and he fell before the communion-table, where his body was hewed in pieces.

Unfortunately for them Leo had hidden the key on his person, and since it was too early in the morning to find a blacksmith, Michael was hastily crowned as emperor with the iron clasps still around his legs.

The military especially was clearly inspired by the heroic image of the iconoclast Constantine V. Leo found more allies in the clergy, especially the learned figure of John VII of Constantinople, his appointee.

After the institution of the liturgical Feast of Orthodoxy on 11 March 843, now a symbol of the Eastern Orthodox faith, iconoclasm was very rarely defended by any Byzantine clergy.