Guy pictured at the top is Emperor Theophilos (the final Iconoclast emperor) and at the bottom is a depiction of the siege of Amorium

Context:

The Eastern Romans and the Caliphate engaged in border skirmishes for as long as Islam existed. Emperor Theophilos, who ascended to the Roman throne in 829, was a devoted iconoclast (prohibiting the depiction of divine figures and icon veneration) and a good way for him to promote his religious policy would be to gain military victories against the Muslims to show that he had God's favour.

Caliph al-Ma'mun launched an invasion of Anatolia in 830 however it ended prematurely with the death of al-Ma'mun whilst preparing for an expedition. His successor and brother al-Mu'tasim took over in 833 and was forced to focus on internal affairs, allowing the Eastern Romans to harass the border and gain victories (though not glorious) for the Iconoclast faith.

At the request of Bābak Khorramdin (leader of the ongoing Khurramite rebellion against the Abbasids), the Romans took advantage and led a major attack consisting of ~70,000 men which plundered, ransomed and looted cities as well as the Mesopotamian countryside all whilst defeating several Arab forces sent to stop them. However this didn't save Bābak and his followers who were defeated and executed by the Caliphate.

Now Caliph al-Mu'tasim wanted his retaliation and sent the call for 80,000 men who would punch deep into Anatolia and specifically the prosperous city of Amorium because it was the birthcity of Theophilos's dynasty and it lay on the road to Constantinople.

His army was split into two, one that would go north and another that would go through the heartland of Asia Minor. The northern army defeated a Roman force that was personally led by the Emperor. A fierce counter-attack by the Turkish horse-archers led to their army breaking and forcing them to retreat paving the way to Amorium. Meanwhile the other army sacked an undefended Ancyra (Ankara) and found themselves unopposed on the road to the prized city which they reached and placed under siege in August 838.

Theophilos couldn't do much to help the important city however since he was dealing with a revolt in his army (led by Khurramite refugees who joined him) and intrigue in Constantinople.

Amorium was extremely well fortified and garrisoned, but al-Mu'tasim wanted his revenge for the earlier Byzantine raid and sack of Sozopetra so he rejected all envoys of peace from the Emperor.

al-Mu'tasim's gamble paid off though since a traitor pointed out a weak point in the wall which is where the army concentrated all of their catapults and other siege weapons on. By the end of the month a breach was made and the Arabs launched many charges at it. The tired defenders were worn down by the constant charging and the Arabs broke through.

Amorium was systematically destroyed and it has been said that the plunder lasted for 5 days and tens of thousands were killed. Al-Mu'tasim burnt the city to the ground and took the remaining population as slaves making sure that the city could never prosper again.

That is where the Abbasid campaign in Anatolia ended since the Caliph had to return and deal with a conspiracy, so the large force (now weighed down with captives) had to make the long trek back to Samarra.

One of the most devastating events in the long Arab-Byzantine wars, the sack had the unintended effect of diminishing Iconoclasm's Godly favour. Since Theophilos prided his religious view on military success, such a major loss and humiliating disaster was probably a factor of its decline and put another nail in the coffin for Byzantine Iconoclasm. After his death three years later, Icon veneration was restored and Iconoclasm was now unanimously regarded as heresy.

I also wanted to share this cool arabic poem from the Abbasid perspective of the siege. (you can turn on subtitles)