The Empire, following the rapid Muslim conquests of the seventh century, had found itself confined to Asia Minor, the southern coasts of the Balkans, and parts of Italy.

As Constantinople remains the Caliphate's major infidel enemy, Arab raids (razzias) into Asia Minor continue throughout the eighth and ninth centuries.

These expeditions, launched from bases in the Arab frontier zone on an almost annual basis, have over time cquired a quasi-ritualized character.

The imperial forces during this period have generally been on the defensive, and have suffered some catastrophic defeats such as the razing in 838 of Amorium, the home city of the Empire’s reigning dynasty.

With the waning of the Abbasid Caliphate's power after 842 and the rise of semi-independent emirates along the Empire’s eastern frontier, however, Constantinople is increasingly able to assert its own power.

The most persistent threats to the Empire in the 850s had been the emirate of Melitene (Malatya) under Umar al-Aqta, the emirate of Tarsus under Ali ibn Yahya ("Ali the Armenian"), the emirate of Qaliqala (Theodosiopolis, modern Erzurum) and the Paulicians of Tephrike under their leader Karbeas.

Melitene, in particular, is a major threat to the Empire as its location on the western side of the Anti-Taurus range allows direct access to the Anatolian plateau.

An indication of the threat posed by these states comes in 860, when Umar and Karbeas raid deep into Asia Minor and return with much plunder; they are followed shortly after by another raid by the forces of Tarsus under Ali, while a naval attack from Syria sacks the major imperial naval base at Attaleia.

Umar strikes again in the summer of 863, joining forces with the Abbasid general Ja'far ibn Dinar al-Khayyat (probably the governor of Tarsus) for a successful raid into Cappadocia.

The Arabs cross he Cilician Gates into imperial territory, plundering as they go, until they reach a place near Tyana.

Here, the Tarsian army returns home, but Umar obtains Ja'far's leave to press on into Asia Minor.

Umar's forces represen the bulk of his emirate's strength, but their size is unknown: the contemporary Muslim historian Ya'qubi claims that Umar had eight thousand men at his disposal, while the Byzantine historians Genesius and Theophanes Continuatus inflate the numbers of the Arab army to forty thousand men.

The Byzantinist John Haldon considers the former number to be closer to reality, and estimates the size of the combined Arab force at fifteen to twenty thousand men.

It is likely that a Paulician contingent under Karbeas was present as well.

Emperor Michael III had assembled an imperial army to counter the Arab raid, and meets them at a battle in an area called Marj al-Usquf ("Bishop's Meadow") by Arab sources, a highland near Malakopeia, north of Nazianzus.

The battle is bloody with many casualties on both sides; according to the Persian historian al-Tabari, only a thousand of Umar's army survived.

Nevertheless, ...

…the Arabs manage to escape the imperial forces and continue their raid north into the Armeniac Theme, eventually reaching the Black Sea and capturing and sacking the port city of Amisos.

The Byzantine historians report that Umar, enraged at the sea blocking his advance, ordered it to be lashed, but this is most likely inspired by the similar account of Xerxes during the Persian Wars.

Umar plunders the regions of Paphlagonia and Galatia, then withdraws his forces toward the Anti-Taurus Mountains.

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.