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Territorial expansion

During the reign of Krum (r. 803–814) Bulgaria doubled in size and expanded to the south, west and north, occupying the vast lands along the middle Danube and Transylvania, becoming European medieval great power during the 9th and 10th century along with the Byzantine and Frankish Empires. Between 804 and 806 the Bulgarian armies thoroughly eliminated the Avar Khaganate, which had suffered a crippling blow by the Franks in 796, and a border with the Frankish Empire was established along the middle Danube or Tisza. Prompted by the Byzantine moves to consolidate their hold on the Slavs in Macedonia and northern Greece and in response to a Byzantine raid against the country, the Bulgarians confronted the Byzantine Empire. In 808 they raided the valley of the Struma River, defeating a Byzantine army, and in 809 captured the important city of Serdica (modern Sofia).

In 811 the Byzantine Emperor Nicephorus I launched a massive offensive against Bulgaria and seized, plundered and burned down the capital Pliska, but on the way back the Byzantine army was decisively defeated in the battle of the Varbitsa Pass. Nicephorus I himself was slain along with most of his troops, and his skull was lined with silver and used as a drinking cup. Krum took the initiative and in 812 moved the war towards Thrace, capturing the key Black Sea port of Messembria and defeating the Byzantines once more at Versinikia in 813 before proposing a generous peace settlement. However, during the negotiations the Byzantines attempted to assassinate Krum. In response, the Bulgarians pillaged Eastern Thrace and seized the important city of Adrianople, resettling its 10,000 inhabitants in " Bulgaria across the Danube ". Krum made extensive preparations to capture Constantinople: 5,000 iron-plated wagons were built to carry the siege equipment; the Byzantines even pleaded for help from the Frankish Emperor Louis the Pious. Due to the sudden death of Krum on 14 April 814, however, the campaign was never launched. Khan Krum implemented legal reforms and issued the first known written law code of Bulgaria that established equal rules for all peoples living within the country's boundaries, intending to reduce poverty and to strengthen the social ties in his vastly enlarged state.

Krum's successor Khan Omurtag (r. 814–831) concluded a 30-year peace treaty with the Byzantines establishing the border along the Erkesia trench between Debeltos on the Black Sea and the valley of the Maritsa River at Kalugerovo, thus allowing both countries to restore their economies and finance after the bloody conflicts in the first decade of the century. To the west the Bulgarians were in control of Belgrade (whose modern name was first known as Alba Bulgarica) by the 820s, and the northwestern boundaries with the Frankish Empire were firmly settled along the middle Danube by 827. To the north-east Omurtag fought the Khazars along the Dnieper River, which was the easternmost limit of Bulgaria. Extensive building was undertaken in the capital Pliska, including the construction of a magnificent palace, pagan temples, ruler's residence, fortress, citadel, water-main, and bath, mainly from stone and brick. In 814 Omurtag began the persecution of Christians, in particular against the Byzantine prisoners of war settled north of the Danube.

The expansion to the south and south-west continued with Omurtag's successors under the guidance of the capable kavhan (First Minister) Isbul. During the short reign of Khan Malamir (r. 831–836), the important city of Philippopolis (Plovdiv) was incorporated into the country. Under Khan Presian (r. 836–852), the Bulgarians took most of Macedonia, and the borders of the country reached the Adriatic Sea near Valona and Aegean Sea. Byzantine historians do not mention any resistance against the Bulgarian expansion in Macedonia, leading to the conclusion that the expansion was largely peaceful. With this, Bulgaria had become the dominant power in the Balkans. The advance further west was blocked by the development of a new Slavic state under Byzantine patronage, the Principality of Serbia. Between 839 and 842 the Bulgarians waged war on the Serbs but did not make any progress. Historian Mark Whittow asserts that the claim for a Serb victory in that war in De Administrando Imperio was wishful Byzantine thinking, but notes that any Serb submission to the Bulgarians went no further than the payment of tribute.

The reign of Boris I (r. 852–889) began with numerous setbacks. For ten years the country fought against the Byzantine Empire, Eastern Francia, Great Moravia, the Croats and the Serbs, forming several unsuccessful alliances and changing sides. Around August 863 there was a period of 40 days of earthquakes and there was a lean harvest, which caused famine throughout the country. To cap it all, there was an incursion of locusts. Yet, despite all the military setbacks and natural disasters, the skilful diplomacy of Boris I prevented any territorial losses and kept the realm intact. In this complex international situation Christianity had become attractive as a religion by the mid 9th-century because it provided better opportunities for forging reliable alliances and diplomatic ties. Taking this into account, as well as a variety of internal factors, Boris I converted to Christianity in 864, assuming the title Knyaz (Prince). Taking advantage of the struggle between the Papacy in Rome and the Ecumenical Patriarchate of Constantinople, Boris I brilliantly manoeuvred to assert the independence of the newly established Bulgarian Church. To check the possibility of Byzantine interference in the internal matters of Bulgaria, he sponsored the disciples of the brothers Cyril and Methodius to create literature in Old Bulgarian language. Boris I dealt ruthlessly with the opposition to the Christianisation of Bulgaria, crushing a revolt of the nobility in 866 and overthrowing his own son Vladimir (r. 889–893) after he attempted to restore the traditional religion. In 893 he convened the Council of Preslav where it was decided that the capital of Bulgaria was to be moved from Pliska to Preslav, the Byzantine clergy was to be banished from the country and replaced with Bulgarian clerics, and Old Bulgarian language was to replace the Greek in liturgy. Bulgaria was to become the principal threat to the stability and security of the Byzantine Empire in the 10th century.