Having settled matters in Khurasan by confirming Ibn Mahan in his governorship, Harun returned to the west in November 805 and prepared a huge retaliatory expedition for 806, drawing men from
Syria,
Palestine,
Persia, and
Egypt. According to
al-Tabari, his army numbered 135,000 regular troops and additional volunteers and
freebooters. These numbers are easily the largest ever recorded for the entire Abbasid era, and about half as many as the estimated strength of the entire
Byzantine army. Although they—and the even more fantastic claims of the Byzantine chronicler
Theophanes the Confessor of 300,000 men—are certainly exaggerated, they are nevertheless indicative of the size of the Abbasid force. At the same time, a
naval force under his admiral
Humayd ibn Ma'yuf al-Hajuri was prepared to raid Cyprus. , where the Abbasid campaign of 806 took place The huge invasion army departed Harun's residence of
Raqqa in northern Syria on 11 June 806, with the Caliph at its head. Al-Tabari reports that Harun put on a cap with the inscription "Warrior for the Faith and Pilgrim" (in Arabic, ", "). The Abbasids crossed Cilicia, where Harun ordered Tarsus to be rebuilt, and entered Byzantine
Cappadocia through the
Cilician Gates. Harun marched to
Tyana, which at the time seems to have been abandoned. There, he began to establish his base of operations, ordering
Uqbah ibn Ja'far al-Khuza'i to refortify the town and erect a
mosque. Harun's lieutenant
Abdallah ibn Malik al-Khuza'i took Sideropalos. From there, Harun's cousin Dawud ibn Isa ibn Musa moved to pillage central Cappadocia, with half the Abbasid army—some 70,000 men according to al-Tabari. Another of Harun's generals,
Sharahil ibn Ma'n ibn Za'ida, captured the so-called "Fortress of the Slavs" () and the recently rebuilt town of Thebasa, while Yazid ibn Makhlad captured the "Fort of the Willow" () and
Malakopea.
Andrasos was captured and
Kyzistra was placed under siege, while raiders reached as far as Ancyra, which they did not capture. Harun himself, with the other half of his forces, went west and captured the strongly fortified city of
Herakleia after a month-long siege in August or September. The city was plundered and razed, and its inhabitants enslaved and deported to
slavery in the Abbasid Caliphate. The fall of Herakleia was considered by the Arab chroniclers the most significant achievement of Harun's expeditions against the Byzantines, and is the central event in the narratives of Harun's retaliatory campaign against Nikephoros. 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. Indeed, the Byzantine sources do not place any particular emphasis on the fall of Herakleia compared to the other fortresses captured during Harun's 806 campaign. At the same time, on Cyprus, Humayd ravaged the island and took some 16,000 Cypriots, including the
local archbishop, captive to Syria, where they were sold as slaves. Nikephoros, outnumbered and threatened by the
Bulgars in his rear, could not resist the Abbasid onslaught. 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. Harun agreed to peace in exchange for the payment of an annual tribute (30,000
gold nomismata, according to Theophanes, 50,000 according to al-Tabari), but the Emperor and his son and heir,
Staurakios, were to pay a humiliating personal poll-tax () of three gold coins each to the Caliph (four and two respectively, in Tabari's version), thereby acknowledging themselves as the Caliph's subjects. In addition, Nikephoros promised not to rebuild the dismantled forts. Harun then recalled his forces from their various sieges and evacuated Byzantine territory. ==Aftermath==