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.
