The Eastern Roman Empire, centered on Constantinople and ruled from 820 by the Amorian, or Phrygian, dynasty, is on the decline: it loses Sicily and Crete to Muslim polities that will retain them into the next age.

The early career of Abdallah, the second son of the Persian general Tahir ibn Husayn, governor of Khurasan, had consisted of serving with his father in pacifying the lands of the Abbasid Caliphate following the civil war between al-Amin and al-Ma'mun.

He later succeeded his father as governor of Al-Jazira, with the task of defeating the rebel Nasr ibn Shabath, and earlier in 824 had persuaded Nasr to surrender.

He had next been sent to Egypt, where he successfully ends an uprising led by 'Abd-Allah ibn al-Sari.

He also recovers Alexandria, which had been seized seven years before by Andalusian Muslim refugees.

Abd-ar-Rahman, after his Damascus-based dynasty, the Umayyads, lost the position of Caliph in 750, had run from Abbasid persecutors for six years before arriving in Spain intent on regaining a position of power.

Defeating the existing Islamic rulers of the area,  Abd-ar-Rahman had united various local fiefdoms into an emirate in 756 to become Emir of Córdoba in the Al-Andalus (Moorish Iberia).

His grandson, Al-Hakam, Umayyad Emir from 796 until 822, had crushed a rebellion led by clerics in a suburb called al-Ribad on the south bank of the Guadalquivir river and punished the inhabitants by exiling them by ship.

Eventually reaching Alexandria, they had dominated the city until their expulsion in 824, following which the refugees head to Crete.

Crete, as the target of Muslim attacks since the first wave of the Muslim conquests in the mid-seventh century, had in 654 suffered a first raid in 674/675 another; parts of the island had been temporarily occupied between 705 and 715 during the reign of the Umayyad Caliph al-Walid I.

The island had never been conquered, however, and despite occasional raids in the eighth century it has remained securely in imperial hands; a quiet cultural backwater, Crete is too distant from the Arab naval bases in the Levant for an effective expedition against it to be undertaken.

A group of Andalusian exiles at some point in the second half of the reign of Emperor Michael II, who reigns from 820 to 829, land on Crete and begin its conquest.

These exiles, with a long history of wanderings behind them, are the survivors of a failed revolt in 818 against the emir Al-Hakam I of Córdoba.

In the aftermath of its suppression, the citizens of the Cordovan suburb of al-Rabad had been exiled en masse.

Some had settled in Fez in Morocco, but others, numbering over ten thousand, had taken to piracy, probably joined by other Andalusians.

Some of the latter group, under the leadership of Umar ibn Hafs ibn Shuayb ibn Isa al Balluti, commonly known as Abu Hafs, had landed in Alexandria and had taken control of the city until 827, when they had been besieged and expelled by the Abbasid general Abdullah ibn Tahir al-Khurasani.

The exact chronology of their landing in Crete is uncertain.

Following the Muslim sources, it is usually dated to 827 or 828, after the Andalusians' expulsion from Alexandria.

Byzantine sources, however seem to contradict this, placing their landing soon after the suppression of the large revolt of Thomas the Slav (821–823).

Further considerations regarding the number and chronology of the campaigns launched against the invaders and prosopographical questions of the imperial generals that headed them have led other scholars like Vassilios Christides and Christos Makrypoulias to propose an earlier date, around 824.

The Andalusians and their families, under the terms of their agreement with Ibn Tahir, had left Alexandria in forty ships.

Historian Warren Treadgold estimates them at some twelve thousand people, of whom about three thousand would be fighting men.

The Andalusians according to Byzantine historians were already familiar with Crete, having raided it in the past.

They also claim that the Muslim landing was initially intended as a raid, and was transformed into a bid for conquest when Abu Hafs himself set fire to their ships.

This is probably later invention, however, as the Andalusian exiles had brought their families along.

The Andalusians' landing-place is also unknown; some scholars think that it was at the north coast, at Suda Bay or near where their main city and fortress Chandax (”Castle of the Moat", modern Heraklion) will later be built, but others think that they most likely landed on the south coast of the island, then moved to the more densely populated interior and the northern coast.

Archbishop Cyril of Gortyn is assassinated and his city so thoroughly devastated it will never be reoccupied.

Emperor Michael II, immediately upon learning of the Arab landing and before the Andalusians have secured their control over the entire island, reacts decisively and sends successive expeditions to recover the island.

Losses suffered during the revolt of Thomas the Slav have hampered Constantinople's ability to respond, however, and if the landing occurred in 827/828, the diversion of ships and men to counter the gradual conquest of Sicily by the Tunisian Aghlabids also interfered.

The first expedition, under Photeinos, strategos of the Anatolic Theme, and Damian, Count of the Stable, had been defeated in open battle, in which Damian had been killed.

The next expedition had been sent a year later and comprised seventy ships under the strategos of the Cibyrrhaeots Krateros.

It is initially victorious, but the overconfident imperial navy is then routed in a night attack.

Krateros manages to flee to Kos, but here he is captured by the Arabs and crucified.

Makrypoulias suggests that these campaigns must have taken place before the Andalusians completed their construction of Chandax, where they transfer the capital from the inland site of Gortyn.

Candia (Chandax, modern Heraklion), a city built by the transplanted Iberians, is made the new capital of the island.

They build a moat around the city for protection, and name the city rabḍ al-ḫandaq ("Castle of the Moat").

The Saracens allow the port to be used as a safe haven for pirates who operate against Imperial shipping and raid Imperial territory around the Aegean.

Abu Hafs has repulsed the early attacks by the imperial navy and slowly consolidated control of the entire island.

He recognizes the suzerainty of the Abbasid Caliphate, but he rules as a de facto independent prince.

The conquest of the island is of major importance as it transforms the naval balance of power in the Eastern Mediterranean and opens the hitherto secure Aegean Sea littoral to frequent and devastating raids.

The Andalusians also occupy several of the Cyclades during these early years, but Michael II organizes another large-scale expedition, recruiting an entire new marine corps, the Tessarakontarioi, and building new ships.

Under the admiral Ooryphas, this fleet manages to evict the Arabs from the Aegean islands but fails to retake Crete, which will survive as a piratical emirate until its reconquest in 961 by Constantinople.

Michael II's successor Theophilos sends an embassy to Abd ar-Rahman II of Córdoba proposing a joint action against the Andalusian exiles, but beyond Abd ar-Rahman giving his assent to any imperial action against Crete, this comes to nothing.

The Battle of Thasos is fought on October 829 between the fleets of the Empire and the newly founded Emirate of Crete.

The Cretan Arabs score a major victory: Theophanes Continuatus records that almost the entire imperial fleet was lost.

This success opens up the Aegean to the Saracens' raids.

The Cyclades and other islands are pillaged, and Mount Athos is so devastated that it will be deserted for long time.

The Cretan Saracens have attacked Euboea between 835 and 840, Lesbos in 837, and the coasts of the Thracesian Theme, whose governor (strategos), Constantine Kontomytes, inflicts a severe defeat on the Cretan Saracens in 841 when they raid the rich monastic community of Mount Latros.

Shortly before or soon after, Constantine's daughter marries the magistros Bardas, who is the nephew of Empress Theodora on his mother's side and of Patriarch John VII the Grammarian on his father's.

Bardas will later assume his father-in-law's surname.

New measures to confront the Cretan threat in 842 after the death of Theophilos are undertaken by the new imperial regime:a new maritime theme, that of the Aegean Sea, is established in 843 to better deal with the Saracen raids, and another expedition to recover Crete is launched under the personal leadership of the powerful logothetes and regent Theoktistos.

It succeeds in occupying much of the island, but Theoktistos has to abandon the army due to political intrigues in Constantinople, and the troops left behind are slaughtered by the Arabs.

The imperial government, headed by the Empress-regent Theodora and the logothetes Theoktistos, have embarked on a sustained assault on the Empire's main political and ideological foe, the Abbasid Caliphate and its dependencies, following the restoration of the veneration of icons in March 843.

This aggressive stance is on the one hand facilitated by the internal stability that the end of the Iconoclasm controversy has brought, and on the other encouraged by a desire to vindicate the new policy through military victories against the Muslims.

The first such campaign, an attempted reconquest of the Emirate of Crete led by Theoktistos in person, had made initial gains, but ultimately ended in disaster.

After scoring a victory over the Arabs in Crete, Theoktistos had learned of a rumor that Theodora intended to name a new emperor, possibly her brother Bardas.

Theoktistos had hurried back to Constantinople, where he discovered that the rumor was false, but in his absence, the imperial army in Crete had been slaughtered by the Arabs.

Theoktistos, according to Byzantine sources, learns in 844 of an Arab invasion of imperial territory in Asia Minor, led by a certain 'Amr, probably the semiautonomous emir of Malatya, Umar al-Aqta.

The Arab sources do not make explicit mention of this campaign.

The Russian scholar Alexander Vasiliev, however, identified it with an expedition recorded in the poems of Abu Tammam and Buhturi, which was led by general Abu Sa'id and took place during the regency of Theodora.

Umar al-Aqta's participation is likely, as he often aided the Abbasids in their raids against the imperial forces.

According to Arab accounts, the troops led by Abu Sa'id comprised men from the border emirates of Qaliqala (Erzurum) and Tarsus.

The Arab forces united at Ardandun (possibly the border fort of Rhodandos) before raiding through the imperial themes of Cappadocia, Anatolikon, Boukellarion, and Opsikion.

Sa'id's troops sacked Dorylaion and even reached the shore of the Bosporus.

Theoktistos leads the imperial army in against the invaders, but is heavily defeated at Mauropotamos ("Black River").

The location of the latter, if indeed it is a river and not a simple toponym, is disputed; it was most likely a tributary of the Sangarius in Bithynia or of the Halys in Cappadocia.

Not only do the Greeks suffer heavy casualties, but many senior officials defect to the Arabs.

Theoktistos returns to Constantinople.