Michael, immediately upon learning of the fall of Amisos, had ordered a huge force to be assembled (al-Tabari gives its size at fifty thousand men) under his uncle Petronas, the Domestic of the Schools, and Nasar, the stratēgos of the Bucellarian Theme.

Al-Tabari records that the Emperor himself assumed command of these forces, but this is not supported by Byzantine sources.

Given the bias against Michael by the historians writing during the Macedonian dynasty, this may be a deliberate omission.

The forces assembled come from all over the Empire.

Three separate armies are formed and converge on the Arabs: a northern force composed of the forces from the Black Sea themes of the Armeniacs, Bucellarians, Koloneia and Paphlagonia; a southern force, probably the one that had already fought at the Bishop's Meadow and had kept shadowing the Arab army, composed from the Anatolic, Opsician and Cappadocian themes, as well as the kleisourai (frontier districts) of Seleukeia and Charsianon; and the western force, under Petronas himself, comprising the men of the Macedonian, Thracian and Thracesian themes and of the imperial tagmata from the capital.

The coordination of all these forces is not easy, but the imperial armies, marching from three directions, are able to converge on the same day (September 2) and surround Umar's smaller army at a location called Poson or Porson near the Lalakaon River.

The exact location of the river and the battle site have not been identified, but most scholars agree that they lay near the river Halys, some one hundred and thirty kilometers (eighty-one miles) southeast of Amisos.

With the approach of the imperial armies, the only open escape route left to the Emir and his men is dominated by a strategically located hill.

During the night, both Arabs and Greeks endeavor to occupy it, but the imperial forces emerge victorious from the ensuing fight.

On the next day, September 3, Umar decides to throw his entire force towards the west, where Petronas is located, attempting to achieve a breakthrough.

The imperial troop stand firm, though, giving the other two imperial wings time to close in and attack the Arab army's exposed rear and flanks.

The rout is complete, as the larger part of the Arab army falls on the field, including Umar himself.

Casualties possibly included the Paulician leader Karbeas: although the latter's participation in the battle is uncertain, it is recorded that he died in that year.

Only the Emir's son, at the head of a small force, manages to escape the battlefield, fleeing south towards the border area of Charsianon.

He is, however, pursued by Machairas, the kleisourarchēs of Charsianon, and is defeated and captured with many of his men.

The importance of Constantinople’s victories of 863 does not go unnoticed at the time: the Empire’s citizens hail them as revenge for the sack of Amorium twenty-five years earlier, the victorious generals are granted a triumphal entry into Constantinople, and special celebrations and services ware held.

Petronas is awarded the high court title of magistros, and the kleisoura of Charsianon is raised to a full theme.

The imperial forces move quickly to take advantage of their victory: an imperial army invades Arab-held Armenia, and sometime in October or November, defeats and killed the emir Ali ibn Yahya.

Thus, within a single campaigning season, the Empire has eliminated the three most dangerous opponents on its eastern border.

In retrospect, these successes will prove decisive, as the battle has permanently destroyed the power of Melitene.

The imperial victory at Lalakaon has altered the strategic balance in the region, and heralds the beginning of Constantinople’s centurylong offensive in the East.

Nikephoros Phokas the Elder, the son of the founder of the Phokas family, a middle-ranking military officer (tourmarches) from Cappadocia named Phokas, attested on about, had begun his military career under Basil, probably at the same time as his father was appointed to the post of tourmarches around 872.

Nikephoros had originally been appointed to the guard corps of the Manglabitai, and had possibly participated in Basil's 873 campaign against Samosata.

Shortly after, at any rate before 878, Nikephoros is promoted to the rank of protostrator and the post of commanding general (strategos) of the theme of Charsianon.