Upon coming to the throne, the Byzantine emperor Nikephoros I (r. 802–811) ceased paying the tribute agreed to by his predecessors with the Caliphate, and launched attacks on the Abbasid frontier regions.

The Abbasid army set out from Raqqa on 11 June 806, crossed the coastal region of Cilicia and the Taurus Mountains, and invaded the Byzantine province of Cappadocia.

The Byzantine losses forced Nikephoros to seek peace terms in which he offered a resumption of tribute payments in exchange for the Abbasids' withdrawal.

Smaller-scale raids continued on both sides, but the Abbasid civil war, which began after 809, and the Byzantine preoccupation with the Bulgars contributed to a cessation of large-scale Arab–Byzantine conflict for the next two decades.

The deposition of Byzantine empress Irene of Athens in October 802 and the accession of Nikephoros I in her place marked the start of a more violent phase in the long history of the Arab–Byzantine wars.

[4] In retaliation for the cessation of tribute and the violation of the peace agreement concluded with Irene, Harun launched a raid under his son al-Qasim in spring 803.

[1][18][19] At the same time, another Byzantine force raided the Upper Mesopotamian thughur and unsuccessfully besieged the fortress of Melitene, while a Byzantine-instigated rebellion against the local Arab garrison began in Cyprus, which for over a century had been an Arab–Byzantine condominium.

[b][18] This sudden resumption of Byzantine offensive activity greatly alarmed Harun, especially as he received reports that Nikephoros was planning similar attacks for the next year, which this time would aim at the full reoccupation of these frontier territories.

On the other hand, Nikephoros was certainly aware of the huge superiority of the Caliphate in men and resources, and it is more likely that he intended this campaign simply as a show of strength and a test of his enemy's resolve.

[21] Having settled matters in Khurasan by confirming Ibn Mahan in his governorship,[22] Harun returned to the west in November 805 and prepared a huge retaliatory expedition for 806, drawing men from Syria, Palestine, Persia, and Egypt.

As the historian Marius Canard remarks, "for the Arabs the capture of Herakleia had an impact as profound as the Sack of Amorium in 838", which is completely at odds with the city's actual importance.

He campaigned himself at the head of his army and seemingly won a few minor engagements against isolated detachments, but stayed well clear of the main Abbasid forces.

In the end, with the harrowing possibility of the Arabs wintering on Byzantine soil in Tyana, he sent three clerics as ambassadors: Michael, the bishop of Synnada, Peter, the abbot of the monastery of Goulaion, and Gregory, the steward of the metropolis of Amastris.

The agreement of peace terms was followed by a friendly exchange between the two rulers, related by al-Tabari: Nikephoros asked Harun for a young Byzantine woman, one of the candidate brides for his son Staurakios, who had been taken captive when Herakleia fell, and for some perfume.

[43][44][45] The failure of the year's Abbasid efforts were compounded by the outbreak of Rafi ibn al-Layth's rebellion in Khurasan, which forced Harun to depart again for the East.

[d][55] In that regard, the Abbasid campaign was certainly a success: after 806, the Byzantine ruler abandoned whatever expansionist plans he may have had for the eastern border and focused his energy on his fiscal reforms, the recovery of the Balkans, and his wars there against the Bulgars, which would end with his death in the disastrous Battle of Pliska in 811.

[47][56][57] On the other hand, the historian M. A. Shaban considers the campaign a "limited success" at best, and criticizes Harun's "single-minded" attention to the Byzantines as a "totally misguided effort".

[43][59] Isolated raids and counter-raids continued at land as well as at sea, and, independently of the Abbasids, local Muslim leaders conquered Crete and launched the conquest of Sicily in the 820s.

The account of the 17th-century Ottoman traveller Evliya Çelebi mixes the events of Harun's 782 campaign, when the Arab army had reached the Bosporus, with those of 806, as well as introducing clearly fictional elements such as the manner of Nikephoros' death.

The first time the Caliph withdrew, after securing as much land as an oxhide could cover and building a fortress there, in an imitation of the ancient tale of Queen Dido.