The Slavs in the vicinity of Thessalonica rebel against the Empire in 837, soon after Presian's accession.

Emperor Theophilos seeks Bulgarian support in putting down the rebellion, but simultaneously arranges for his fleet to sail through the Danube delta and undertake a clandestine evacuation of some of the Greek captives settled in trans-Danubian Bulgaria by Krum and Omurtag.

The refugees from Sozopetra begin arriving at Mu'tasim's capital, Samarra, while Theophilos returns home to celebrate a triumph and be acclaimed in the Hippodrome of Constantinople as the "incomparable champion".

The caliphal court is outraged by the brutality and brazenness of the raids: not only have the Greeks acted in open collusion with the Khurramite rebels, but during the sack of Sozopetra—which some sources claim as Mu'tasim's own birthplace—all male prisoners were executed and the rest sold into slavery, and some captive women were raped by Theophilos's Khurramites.

Al-Mu'tasim decides to respond with a major punitive expedition against Constantinople, aiming to capture the two major imperial cities of central Anatolia, Ancyra and Amorion.

The latter is probably the largest city in Anatolia at this time, as well as the birthplace of the reigning Amorian dynasty and consequently of particular symbolic importance; according to the chronicles, al-Mu'tasim's soldiers painted the word "Amorion" on their shields and banners.

The Empire’s wars with the Arabs had been waged on and off for almost two centuries when the young Theophilos ascended the throne in Constantinople in 829.

An ambitious man and a convinced iconoclast, Theophilos seeks to bolster his regime and support his religious policies by military success against the Abbasid Caliphate, the Empire's major antagonist.

Theophilos launches a series of moderately successful campaigns against the Caliphate throughout the 830s, allowing him to portray himself in the traditional Roman way as a victorious emperor.

He breaks the brief truce with the caliphate by giving imperial support to the Khurramite rebel Babak al-Khorammi.

Theophilos decides in 837—at the urging of the hard-pressed Babak—to take advantage of the Caliphate's preoccupation with the suppression of the Khurramite revolt and personally lead a major campaign against the frontier emirates.

He assembles a very large army, some seventy thousand fighting men and one hundred thousand in total according to al-Tabari, and invades Arab territory around the upper Euphrates almost unopposed.

The imperial army takes the towns of Sozopetra and Arsamosata, ravages and plunders the countryside, extracts ransom from several cities in exchange for not attacking them, and defeats a number of smaller Arab forces.

Theophilos also forces Melitene to pay tribute and deliver hostages in return for being spared.

Isbul, in retaliation for Theophilus’s clandestine rescue mission to trans-Danubian Bulgaria, campaigns along the Aegean coasts of Thrace and Macedonia and captures the city of Philippi, where he sets up a surviving memorial inscription set up in a local church.

Isbul's campaign may have resulted in the establishment of Bulgarian suzerainty over the Slavic tribe of the Smoljani.

Umayyad emir Abd-ar-Rahman II, the successor to al-Hakam of Córdoba, in 837 crushes a second revolt, similar to that of 815, by Spanish Christians and Jews in Toledo.

The Duchy of Naples does not hesitate to ally itself with infidels if it proves advantageous to do so: in 836, for example, Naples asks for support from the Saracens in order to repel the siege of Lombard troops coming from the neighboring Duchy of Benevento.

The city of Koblenz, with the fall of the Western Roman Empire, had been conquered by the Franks and become a royal seat.

After the division of Charlemagne's empire in 814, it had been included in the lands of his son Louis the Pious.

It is assigned in 837 to Charles the Bald.

The church of Saint Castor (Kastorkirche) is constructed here in 836-37.

Viking attacks in Ireland accelerate, particularly in the 830s, under the semilegendary chief Turgeis, who raids Armagh and Ulster, Connacht, and Meath, sailing in 837 up the River Liffey.

Danish raids on England increase in the 830s.

Rebellious Britons, aided by Danish Vikings, battle the West Saxons at Hingston Down, Cornwall.

Egbert of Wessex, victorious, annexes the Cornish territory.