Leo V the Armenian in the context of "Krum"

Bulgarian Battle of Versinikia (Bulgarian: Битката при Версиникия; Greek: Μάχη της Βερσινικίας) was a battle fought in 813 AD between the Byzantine Empire and the Bulgarian Empire, near the city of Adrianople (Edirne), in modern-day Turkey.

The Bulgarian army, led by Krum of Bulgaria, defeated the Byzantine forces. Following this defeat, Michael I Rangabe abdicated, with Leo V the Armenian taking the Byzantine throne. The battle further strengthened Bulgarian control in the region after their victory over Nikephoros I two years earlier. After the battle, the Bulgarians controlled the whole region of Eastern Thrace (until the Byzantine–Bulgarian Treaty of 815), with the exception of a few castles that remained in Byzantine control. Krum died on 13 April 814, but his muster of "5,000 iron-plated wagons [ . . . ] to carry the siege equipment" alarmed the Byzantine court to such an effect that they asked for the aid of the Carolingian Emperor Louis the Pious. These large-scale military preparations were Krum's attempt to avenge his failure at the Second Bulgarian Siege of Constantinople one year prior.

Michael II (Greek: Μιχαὴλ, Mikhaḗl; 770 – 2 October 829), called the Amorian (ὁ ἐξ Ἀμορίου, ho ex Amoríou) and the Stammerer (ὁ Τραυλός, ho Travlós or ὁ Ψελλός, ho Psellós), reigned as Byzantine emperor from 25 December 820 to his death on 2 October 829, the first ruler of the Amorian dynasty.

Born in Amorium, Michael was a soldier, rising to high rank along with his colleague Leo V the Armenian (r. 813–820). He helped Leo overthrow and take the place of Emperor Michael I Rhangabe. However, after they fell out Leo sentenced Michael to death. Michael then masterminded a conspiracy which resulted in Leo's assassination at Christmas in 820. Immediately he faced the long revolt of Thomas the Slav, which almost cost him his throne and was not completely quelled until spring 824. The later years of his reign were marked by two major military disasters that had long-term effects: the beginning of the Muslim conquest of Sicily, and the loss of Crete to Andalusian Arab pirates. Domestically, he supported and strengthened the resumption of official iconoclasm, which had begun again under Leo V.

The Treaty of 815 (Bulgarian: Договор от 815) was a 30-year peace agreement signed in Constantinople between the Bulgarian Khan Omurtag and the Byzantine Emperor Leo V the Armenian.

Theophanes the Confessor (Greek: Θεοφάνης Ὁμολογητής) or Theophanes of the Great Field (Greek: Θεοφάνης τοῦ Μεγάλου Ἄγρου; c. 759 – 817 or 818) was a member of the Byzantine aristocracy who became a monk and chronicler. He served in the court of Emperor Leo IV the Khazar before taking up the religious life. Theophanes attended the Second Council of Nicaea in 787 and resisted the iconoclasm of Leo V the Armenian, for which he was imprisoned. He died shortly after his release.

Theophanes the Confessor, venerated on 12 March in both the Catholic Church and the Eastern Orthodox Church, should not be confused with Theophanes of Nicaea, whose feast is commemorated on 11 October.

Thomas the Slav (Greek: Θωμᾶς, romanized: Thōmas, c. 760 – October 823) was a 9th-century Byzantine military commander, most notable for leading a wide-scale revolt in 821–23 against Emperor Michael II the Amorian (r. 820–829).

An army officer of Slavic origin from the Pontus region (now north-eastern Turkey), Thomas rose to prominence, along with the future emperors Michael II and Leo V the Armenian (r. 813–820), under the protection of general Bardanes Tourkos. After Bardanes' failed rebellion in 803, Thomas fell into obscurity until Leo V's rise to the throne, when Thomas was raised to a senior military command in central Asia Minor. After the murder of Leo and usurpation of the throne by Michael the Amorian, Thomas revolted, claiming the throne for himself. Thomas quickly secured support from most of the themes (provinces) and troops in Asia Minor, defeated Michael's initial counter-attack and concluded an alliance with the Abbasid Caliphate. After winning over the maritime themes and their ships as well, he crossed with his army to Europe and laid siege to Constantinople. The imperial capital withstood Thomas's attacks by land and sea, while Michael II called for help from the Bulgarian Khan Omurtag. Omurtag attacked Thomas's army, but although repelled, the Bulgarians inflicted heavy casualties on Thomas's men, who broke and fled when Michael took to the field a few months later. Thomas and his supporters sought refuge in Arcadiopolis, where he was soon blockaded by Michael's troops. In the end, Thomas's supporters surrendered him in exchange for a pardon, and he was executed.

The Amorian dynasty (or Phrygian dynasty) ruled the Byzantine Empire from 820 to 867. The Amorian dynasty continued the policy of restored iconoclasm (the "Second Iconoclasm") started by the previous non-dynastic emperor Leo V in 813, until its abolition by Empress Theodora with the help of Patriarch Methodios in 842. The continued iconoclasm further worsened relations between the East and the West, which were already bad following the papal coronations of a rival line of "Roman Emperors" beginning with Charlemagne in 800. Relations worsened even further during the so-called Photian Schism, when Pope Nicholas I challenged Photios' elevation to the patriarchate. However, the era also saw a revival in intellectual activity which was marked by the end of iconoclasm under Michael III, which contributed to the upcoming Macedonian Renaissance.

During the Second Iconoclasm, the Empire began to see systems resembling feudalism being put in place, with large and local landholders becoming increasingly prominent, receiving lands in return for military service to the central government. Similar systems had been in place in the Roman Empire ever since the reign of Severus Alexander during the third century, when Roman soldiers and their heirs were granted lands on the condition of service to the Emperor.