Parts of the eastern Balkan Peninsula were in antiquity inhabited by the Thracians who were a group of Indo-European tribes. The whole region as far north as the Danube River was gradually incorporated into the Roman Empire by the 1st century AD. The decline of the Roman Empire after the 3rd century AD and the continuous invasions of Goths and Huns left much of the region devastated, depopulated and in economic decline by the 5th century. The surviving eastern half of the Roman Empire, called by later historians the Byzantine Empire, could not exercise effective control in these territories other than in the coastal areas and certain cities in the interior. Nonetheless, it never relinquished the claim to the whole region up to the Danube. A series of administrative, legislative, military and economic reforms somewhat improved the situation but despite these reforms disorder continued in much of the Balkans. The reign of Emperor Justinian I (r. 527–565) saw temporary recovery of control and reconstruction of a number of fortresses but after his death the empire was unable to face the threat of the Slavs due to the significant reduction of revenue and manpower.

The Slavs, of Indo-European origin, were first mentioned in written sources to inhabit the territories to the north of the Danube in the 5th century AD but most historians agree that they had arrived earlier. The Slavic incursions in the Balkans increased during the second half of Justinian I's reign and while these were initially pillaging raids, large-scale settlement began in the 570s and 580s.

Consumed in bitter wars with the Persian Sasanian Empire in the east, the Byzantines had few resources with which to confront the Slavs. The Slavs came in large numbers and the lack of political organisation made it very difficult to stop them because there was no political leader to defeat in battle and thereby force their retreat.

The Bulgars were Turkic semi-nomadic warrior tribes that flourished in the Pontic–Caspian steppe and the Volga region during the 7th century. They became known as nomadic equestrians in the Volga-Ural region, but some researchers say that their ethnic roots can be traced to Central Asia. They spoke a form of Turkic as their main language. The Bulgars included the tribes of Onogurs, Utigurs and Kutrigurs, among others. The first clear mention of the Bulgars in written sources dates from 480, when they served as the allies of the Byzantine Emperor Zeno. In the first half of the 6th century, the Bulgars occasionally raided the Byzantine Empire.

As the power of the Western Turks faded in the 600s the Avars reasserted their domination over the Bulgars. Between 630 and 635 Khan Kubrat of the Dulo clan managed to unite the main Bulgar tribes and to declare independence from the Avars, creating a powerful confederation called Old Great Bulgaria, also known as Patria Onoguria, between the Black Sea, the Sea of Azov and the Caucasus. Kubrat, who was baptised in Constantinople in 619, concluded an alliance with the Byzantine Emperor Heraclius (r. 610–641) and the two countries remained in good relations until Kubrat's death between 650 and 665. Kubrat fought with the Khazars in the east but after his demise Old Great Bulgaria disintegrated under strong Khazar pressure in 668 and his five sons parted with their followers. The eldest Batbayan remained in his homeland as Kubrat's successor and eventually became a Khazar vassal. The second brother Kotrag migrated to the middle Volga region and founded Volga Bulgaria. The third brother Asparuh led his people west to the lower Danube. The fourth one, Kuber, initially settled in Pannonia under Avar suzerainty but revolted and moved to the region of Macedonia, while the fifth brother Alcek settled in central Italy.

The two confederations of Bulğars and Khazars fought for supremacy on the western steppeland, and with the ascendency of the latter, the former either succumbed to Khazar rule or, as under Asparukh, Kubrat's son, shifted even further west across the Danube to lay the foundations of the First Bulgarian Empire in the Balkans.

The Bulgars of Asparuh moved westwards to what is now Bessarabia, subdued the territories to the north of the Danube in modern Wallachia, and established themselves in the Danube Delta. In the 670s they crossed the Danube into Scythia Minor, nominally a Byzantine province, whose steppe grasslands and pastures were important for the large herd stocks of the Bulgars in addition to the grazing grounds to the west of the Dniester River already under their control.

The relations between the Bulgars and the local Slavs is a matter of debate depending on the interpretation of the Byzantine sources. Vasil Zlatarski asserts that they concluded a treaty but most historians agree that they were subjugated. The Bulgars were superior organisationally and militarily and came to dominate politically the new state but there was cooperation between them and the Slavs for the protection of the country. The Slavs were allowed to retain their chiefs, to abide to their customs and in return they were to pay tribute in kind and to provide foot soldiers for the army. The Seven Slavic tribes were relocated to the west to protect the frontier with the Avar Khaganate, while the Severi were resettled in the eastern Balkan Mountains to guard the passes to the Byzantine Empire. The number of Asparuh's Bulgars is difficult to estimate. Vasil Zlatarski and John Van Antwerp Fine Jr. suggest that they were not particularly numerous, numbering some 10,000, while Steven Runciman considers that the tribe must have been of considerable dimensions. The Bulgars settled mainly in the north-east, establishing the capital at Pliska, which was initially a colossal encampment of 23 km2 protected with earthen ramparts.

In 680 the Byzantine Emperor Constantine IV, having recently defeated the Arabs, led an expedition at the head of a huge army and fleet to drive off the Bulgars but suffered a disastrous defeat at the hands of Asparuh at Onglos, a swampy region in or around the Danube Delta where the Bulgars had set a fortified camp.

The Battle of Ongal took place in the summer of 680 in the Ongal area, an unspecified location in and around the Danube delta near the Peuce Island, present-day Tulcea County, Romania. It was fought between the Bulgars, who had recently invaded the Balkans, and the Byzantine Empire, which ultimately lost the battle. The battle was crucial for the creation of the First Bulgarian Empire.

Asparuh's victory led to the Bulgarian conquest of Moesia and the establishment of some sort of alliance between the Bulgars and the local Slavic groups (described as the Severi and Seven Slavic tribes). As Asparuh commenced to raid across the mountains into Byzantine Thrace in 681, Constantine IV decided to cut his losses and conclude a treaty, whereby the Byzantine Empire paid the Bulgars an annual tribute. These events are seen in retrospect as the establishment of the Bulgarian state and its recognition by the Byzantine Empire.

To the north-east the war with the Khazars persisted and in 700 Khan Asparuh perished in battle with them. Despite this setback the consolidation of the country continued under Asparuh's successor, Khan Tervel (r. 700–721). In 705 he assisted the deposed Byzantine Emperor Justinian II in regaining his throne in return for the Zagore region of Northern Thrace, the first expansion of Bulgaria to the south of the Balkan mountains. In addition Tervel obtained the title Caesar and, having been enthroned alongside the Emperor, received the obeisance of the citizenry of Constantinople and numerous gifts. 

However, three years later, Justinian tried to regain the ceded territory by force, but his army was defeated at Anchialus. Skirmishes continued until 716 when Khan Tervel signed an important agreement with Byzantium that defined the borders and the Byzantine tribute, regulated trade relations and provided for the exchange of prisoners and fugitives. 

On 25 May 717, Leo III the Isaurian was crowned Emperor of Byzantium. During the summer of the same year the Arabs, led by Maslama ibn Abd al-Malik, crossed the Dardanelles and besieged Constantinople with a large army and navy.

Leo III made a plea to Tervel for help, relying on the treaty of 716, and Tervel agreed. The first clash between the Bulgars and the Arabs ended with a Bulgar victory. During the very first stages of the siege the Bulgars appeared in the Muslim rear and large part of their army was destroyed and the rest were trapped. The Arabs built two trenches around their camp facing the Bulgarian army and the walls of the city. They persisted with the siege despite the severe winter with 100 days of snowfall. In the spring, the Byzantine navy destroyed the Arab fleets that had arrived with new provisions and equipment, while a Byzantine army defeated Arab reinforcements in Bithynia. Finally, in early summer the Arabs engaged the Bulgars in battle but suffered a crushing defeat. According to Theophanes the Confessor, the Bulgars slaughtered some 22,000 Arabs in the battle. Shortly after, the Arabs raised the siege.

Most historians primarily attribute the Byzantine–Bulgarian victory with stopping the Arab offensives against Europe.

In 719, Tervel again interfered in the internal affairs of the Byzantine Empire when the deposed emperor Anastasios II asked for his assistance to regain the throne. Tervel provided him with troops and 360,000 gold coins. Anastasios marched to Constantinople, but its population refused to cooperate. In the meantime Leo III sent a letter to Tervel in which he urged him to respect the treaty and to prefer peace to war. Because Anastasios was abandoned by his supporters, the Bulgarian ruler agreed to Leo III's pleas and broke relations with the usurper. He also sent Leo III many of the conspirators who had sought refuge in Pliska.

According to the Nominalia of the Bulgarian khans (Imennik), Kormesiy would have reigned for 28 years and was a descendant of the royal Dulo clan. According to the chronology developed by Moskov, Kormesiy would have reigned 715–721, and the longer period reflected in the Imennik would have indicated the duration of his life or would have included a period of association with his predecessors. Other chronologies date the reign of Kormesiy to 721–738 but cannot be reconciled with the data of the Imennik.

Kormesiy is encountered in relation to the events surrounding the peace treaty between Bulgaria and the Byzantine Empire between 715 and 717 – the chronology has to be argued from the names of the Emperor and patriarch involved – for which our only source is the Byzantine chronicler Theophanes the Confessor. According to Theophanes, the treaty was signed by Kormesiy as ruler of the Bulgars.

Kormesiy is not mentioned in any other historical context. The fact that there is no record of wars between Bulgaria and the Byzantine Empire during his reign, however, implies that he sustained the peace between the two countries.

Sevar was a ruler of Bulgaria in the 8th century. The Nominalia of the Bulgarian khans states that Sevar belonged to the Dulo clan and ruled for 15 years. Some chronologies place his reign in 738–754. According to historians such as Steven Runciman and David Marshall Lang, Sevar was the last ruler of the Dulo dynasty and with Sevar died out the lineage of Attila the Hun.

With the demise of Khan Sevar the ruling Dulo clan died out and the Khanate fell into a long political crisis during which the young country was on the verge of destruction. In just fifteen years seven Khans reigned, and all of them were murdered.

The only surviving sources of this period are Byzantine and present only the Byzantine point of view of the ensuing political turmoil in Bulgaria. They describe two factions struggling for power – one that sought peaceful relations with the Empire, which was dominant until 755, and one that favoured war. These sources present the relations with the Byzantine Empire as the main issue in this internal struggle and do not mention the other reasons, which could have been more important for the Bulgarian elite. It is likely that the relationship between the politically dominant Bulgars and the more numerous Slavs was the main issue behind the struggle but there is no evidence about the aims of the rival factions. 

Kormisosh was a ruler of Bulgaria during the 8th century. The Namelist of Bulgarian Rulers states that he belonged to the Ukil (or Vokil) clan and ruled for 17 years. According to the chronology developed by Moskov, Kormisosh would have reigned from 737 to 754. Other chronologies place his reign in 753–756, but cannot be reconciled with the testimony of the "Namelist" (or would require us to assume a long period of co-regency).

The "Namelist" stresses the fact that the accession of Kormisosh represents a change of dynasty, but it remains unclear whether that was done through violence. The reign of Kormisosh inaugurated a prolonged period of war with the Byzantine Empire. The Byzantine Emperor Constantine V Kopronymos had begun to fortify the frontier and started settling Armenians and Syrians in Byzantine Thrace. In response Kormisosh demanded the payment of tribute, perhaps constituting an increase in the traditional payments. Rebuffed, Kormisosh raided into Thrace, reaching the Anastasian Wall stretching between the Black Sea and the Sea of Marmara 40 km in front of Constantinople. Constantine V marched out with his army, defeated the Bulgarians and turned them to flight.

Vineh was ruler of Bulgaria in the mid-8th century. According to the Nominalia of the Bulgarian khans, Vineh reigned for seven years and was a member of the Vokil clan. Vineh ascended the throne after the defeat of his predecessor Kormisosh by the Eastern Roman Emperor Constantine V. In c. 756 Constantine campaigned against Bulgaria by land and sea and defeated the Bulgarian army led by Vineh at Marcellae (Karnobat). The defeated monarch sued for peace and undertook to send his own children as hostages. In 759 Constantine invaded Bulgaria again, but this time his army was ambushed in the mountain passes of the Stara Planina (battle of the Rishki Pass). Vineh did not follow up his victory and sought to re-establish the peace. This won Vineh the opposition of the Bulgarian nobility, which had Vineh massacred together with his family, except Pagan of Bulgaria.

The internal instability was used by the "soldier Emperor" Constantine V, who launched nine major campaigns aiming to eliminate Bulgaria. Having contained the Arab threat during the first part of his reign, Constantine V was able to concentrate his forces on Bulgaria after 755. He defeated the Bulgarians at Marcellae in 756, Anchialus in 763 and Berzitia in 774, but lost the Battle of the Rishki Pass in 759 in addition to hundreds of ships lost to storms in the Black Sea. The Byzantine military successes further exacerbated the crisis in Bulgaria, but also rallied together many different factions to resist the Byzantines, as shown at the council of 766 when the nobility and the "armed people" denounced Khan Sabin with the words "Thanks to you, the Romans will enslave Bulgaria!". 

Between 755 and 775, the Byzantine emperor Constantine V organised nine campaigns to eliminate Bulgaria and although he managed to defeat the Bulgarians several times, he never achieved his goal.

In 759, the emperor led an army towards Bulgaria, but Khan Vinekh had enough time to bar several mountain passes. When the Byzantines reached the Rishki Pass, they were ambushed and completely defeated. The Byzantine historian Theophanes the Confessor wrote that the Bulgarians killed the strategos of Thrace Leo, the commander of Drama, and many soldiers.

Khan Vinekh did not take the favourable opportunity to advance on enemy territory and sued for peace. This act was very unpopular among the nobles and the Khan was murdered in 761.

Telets, a member of the Ugain clan, was the ruler of Bulgaria from 762 to 765. Byzantine sources indicate that Telets replaced the legitimate rulers of Bulgaria. The same sources describe Telets as a brave and energetic man in his prime (about 30 years old). Scholars have conjectured that Telets may have belonged to an anti-Slavic faction of the Bulgarian nobility.

After his accession, Telets led a well-trained and well-armed army against the Byzantine Empire and devastated the Empire's frontier zone, inviting the emperor to a contest of strength.

Emperor Constantine V Kopronymos marched north on June 16, 763, while another army was carried by a fleet of 800 ships (each carrying infantry and 12 horsemen) with the intent to create a pincer movement from the north.

The energetic Bulgarian Khan at first barred the mountain passes with his troops and some twenty thousand Slavic auxiliaries and took advantageous positions on the heights near Anchialus, but his self-confidence and impatience incited him to go down to the lowlands and charge the enemy. The battle started at 10 in the morning and lasted until sunset. It was long and bloody, but in the end the Byzantines were victorious, although they lost many soldiers, nobles, and commanders. The Bulgarians also had heavy casualties and many were captured, while Telets managed to escape. Constantine V entered his capital in triumph and then killed the prisoners. The fate of Telets was similar: two years later he was murdered because of the defeat.

Despite being able to defeat the Bulgarians several times the Byzantines were able neither to conquer Bulgaria, nor to impose their suzerainty and a lasting peace, which is a testimony to the resilience, fighting skills and ideological coherence of the Bulgarian state. The devastation brought to the country by the nine campaigns of Constantine V firmly rallied the Slavs behind the Bulgars and greatly increased the dislike of the Byzantines, turning Bulgaria into a hostile neighbour. The hostilities continued until 792 when Khan Kardam achieved an important victory in the battle of Marcellae, forcing the Byzantines once again to pay tribute to the Khans. As a result of the victory, the crisis was finally overcome, and Bulgaria entered the new century stable, stronger, and consolidated.

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, 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. 

The Battle of Pliska was one of the worst defeats in Byzantine history. It deterred Byzantine rulers from sending their troops north of the Balkans for more than 150 years afterwards, which increased the influence and spread of the Bulgarians to the west and south of the Balkan Peninsula, resulting in a great territorial enlargement of the First Bulgarian Empire. This was the first time a Byzantine Emperor was killed in battle since the Battle of Adrianople in 378.

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". 

Infuriated by the treachery of the Byzantines, Krum ordered all churches, monasteries, and palaces outside Constantinople to be destroyed, the captured Byzantines were slain and the riches from the palaces were sent to Bulgaria on carts. After that all enemy fortresses in the surroundings of Constantinople and Marmara Sea were seized and razed to the ground. The castles and settlements in the hinterland of Eastern Thrace were looted and the whole region devastated. Then Krum returned to Adrianople and strengthened the besieging forces. With the help of mangonels and battering rams he forced the city to surrender. The Bulgarians captured 10,000 people who were resettled in Bulgaria across the Danube. A further 50,000 from other settlements in Thrace were deported there. During the winter Krum returned to Bulgaria and launched serious preparation for the final assault on Constantinople. The siege machines had to be transported to Constantinople by 5,000 iron-covered carts hauled by 10,000 oxen. However, he died during the height of the preparations on 13 April 814.

Krum's successor Khan Omurtag (r. 814–831) concluded a 30-year peace treaty with the Byzantines, thus allowing both countries to restore their economies and finance after the bloody conflicts in the first decade of the century, establishing the border along the Erkesia trench between Debeltos on the Black Sea and the valley of the Maritsa River at Kalugerovo. 

To the west the Bulgarians were in control of Belgrade 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. Omurtag started in 814 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 under Omurtag's successors under the guidance of the capable kavhan (First Minister) Isbul.

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.

Despite a number of military setbacks, the reign of Boris I was marked with significant events that shaped Bulgarian and European history. With the Christianization of Bulgaria in 864 paganism (i.e. Tengrism) was abolished. A skillful diplomat, Boris I successfully exploited the conflict between the Patriarchate of Constantinople and the Papacy to secure an autocephalous Bulgarian Church, thus dealing with the nobility's concerns about Byzantine interference in Bulgaria's internal affairs.

When in 885 the disciples of Saints Cyril and Methodius were banished from Great Moravia, Boris I gave them refuge and provided assistance which saved the Glagolithic and later promoted the development of the Cyrillic script in Preslav and the Slavic literature. After he abdicated in 889, his eldest son and successor tried to restore the old pagan religion but was deposed by Boris I. During the Council of Preslav which followed that event, the Byzantine clergy was replaced with Bulgarians, and the Greek language was replaced with what is now known as Old Church Slavonic.

After the successful war against Rascia, a medieval Serbian state, Bulgaria's ongoing expansion to the west reached Croatian borders. Bulgarian forces invaded Croatia approximately in 853 or 854 in northeastern Bosnia, where Croatia and Bulgaria bordered at the time.

According to available sources, there was only one major battle between the Bulgarian army and the Croatian forces. Sources say that the invading army led by the powerful Bulgarian Khan Boris I fought Duke Trpimir's forces on the mountainous territory of present-day northeastern Bosnia and Herzegovina in 854. The exact place and time of the battle is not known due to the lack of contemporary accounts of the battle. Neither Bulgarian nor Croatian side emerged victorious. Very soon afterward, both Boris of Bulgaria and Trpimir of Croatia turned to diplomacy and reached a peace treaty. Negotiations resulted in a long term establishment of peace with the border between the Duchy of Croatia and the Bulgarian Khanate stabilized at the Drina River (between modern-day Bosnia and Herzegovina and the Republic of Serbia).

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.

During Simeon's rule, Bulgaria spread over a territory between the Aegean, the Adriatic and the Black Sea. The newly independent Bulgarian Orthodox Church became the first new patriarchate besides the Pentarchy, and Bulgarian Glagolitic and Cyrillic translations of Christian texts spread all over the Slavic world of the time. It was at the Preslav Literary School in the 890s that the Cyrillic alphabet was developed. Halfway through his reign, Simeon assumed the title of Emperor (Tsar), having prior to that been styled Prince (Knyaz).

The Golden Age of Bulgaria is the period of the Bulgarian cultural prosperity during the reign of emperor Simeon I the Great. The term was coined by Spiridon Palauzov in the mid 19th century. During this period there was an increase of literature, writing, arts, architecture and liturgical reforms.

The capital Preslav was built in Byzantine fashion to rival Constantinople. Among the city's most remarkable edifices were the Round Church, also known as the Golden Church, and the imperial palace. At that time was created and painted Preslavian pottery, which followed the most prestigious Byzantine models. A chronicle of the 11th century testified that Simeon I had built Preslav for 28 years.

Simeon I gathered around himself the so-called Simeon's circle, that included some of the most prominent literary authors in medieval Bulgaria. Simeon I himself is alleged to have been active as a writer: works that are sometimes credited to him include Zlatostruy (Golden stream) and two of Simeon (Svetoslavian) collections. The most important genres were Christian edifying oratory eulogies, lives of saints, anthems and poetry, chronicles, and historical narratives.

In Bulgaria, Clement of Ohrid and Naum of Preslav created (or rather compiled) the new alphabet which was called Cyrillic and was declared the official alphabet in Bulgaria in 893. The Slavic language was declared as official in the same year. In the following centuries this alphabet was adopted by other Slavic peoples and states. The introduction of Slavic liturgy paralleled Boris' continued development of churches and monasteries throughout his realm. 

The Byzantine–Bulgarian war of 894–896 (Bulgarian: Българо–византийска война от 894–896) was fought between the Bulgarian Empire and the Byzantine Empire as a result of the decision of the Byzantine emperor Leo VI to move the Bulgarian market from Constantinople to Thessaloniki which would greatly increase the expenses of the Bulgarian merchants.

Following the defeat of the Byzantine army in the initial stages of the war in 894 Leo VI sought aid from the Magyars who at the time inhabited the steppes to the north-east of Bulgaria. Aided by the Byzantine navy, in 895 the Magyars invaded Dobrudzha and defeated the Bulgarian troops. Simeon I called for truce and deliberately protracted the negotiations with the Byzantines until securing the assistance of the Pechenegs. Cornered between the Bulgarians and the Pechanegs, the Magyars suffered a crushing defeat at the hands of the Bulgarian army and had to migrate westwards, settling in Pannonia.

With the Magyar threat eliminated, Simeon led his hosts south and routed the Byzantine army in the battle of Boulgarophygon in the summer of 896, which forced Byzantium to agree to the Bulgarian terms. The war ended with a peace treaty which restored the Bulgarian market in Constantinople and confirmed Bulgarian domination in the Balkans. The Byzantine Empire was obliged to pay Bulgaria an annual tribute in exchange for the return of captured Byzantine soldiers and civilians. Under the treaty, the Byzantines also ceded an area between the Black Sea and the Strandzha mountains to Bulgaria. Despite several violations, the treaty formally lasted until Leo VI's death in 912.

Having dealt with the pressure from the Magyars and the Byzantines, Simeon was free to plan a campaign against the Magyars looking for retribution. He negotiated a joint force with the Magyars' eastern neighbours, the Pechenegs.

Using a Magyar invasion in the lands of the neighbouring Slavs in 896 as a casus belli, Simeon headed against the Magyars together with his Pecheneg allies, defeating them completely in the Battle of Southern Buh and making them leave Etelköz forever and settle in Pannonia. Following the defeat of the Magyars, Simeon finally released the Byzantine prisoners in exchange for Bulgarians captured in 895.

The Battle of Boulgarophygon was fought in the summer of 896 near the town of Bulgarophygon, modern Babaeski in Turkey, between the Byzantine Empire and the First Bulgarian Empire. The result was an annihilation of the Byzantine army which determined the Bulgarian victory in the trade war of 894–896.

Despite the initial difficulties in the war against the Magyars, who acted as Byzantine allies, the battle of Boulgarophygon proved to be the first decisive victory of the young and ambitious Bulgarian ruler Simeon I against the Byzantine Empire. Simeon would go on to inflict a number of defeats on the Byzantines in pursuit of his ultimate goal, the throne in Constantinople. The peace treaty that was signed as a result of the battle confirmed the Bulgarian domination in the Balkans.

Although the war was provoked by the Byzantine emperor Alexander's decision to discontinue paying an annual tribute to Bulgaria, the military and ideological initiative was held by Simeon I of Bulgaria, who demanded to be recognized as Tsar and made it clear that he aimed to conquer not only Constantinople but the rest of the Byzantine Empire, as well.

The Bulgarian–Serbian wars of 917–924 were a series of conflicts fought between the Bulgarian Empire and the Principality of Serbia as a part of the greater Byzantine–Bulgarian war of 913–927. After the Byzantine army was annihilated by the Bulgarians in the battle of Achelous, the Byzantine diplomacy incited the Principality of Serbia to attack Bulgaria from the west. The Bulgarians dealt with that threat and replaced the Serbian prince with a protégé of their own. In the following years the two empires competed for control over Serbia. In 924 the Serbs rose again, ambushed and defeated a small Bulgarian army. That turn of events provoked a major retaliatory campaign that ended with the annexation of Serbia in the end of the same year. The Bulgarian advance in the Western Balkans were checked by the Croats who defeated a Bulgarian army in 926.

In 917, a particularly strong Byzantine army led by Leo Phokas the Elder, son of Nikephoros Phokas, invaded Bulgaria accompanied by the Byzantine navy under the command of Romanos Lekapenos, which sailed to the Bulgarian Black Sea ports. En route to Mesembria (Nesebǎr), where they were supposed to be reinforced by troops transported by the navy, Phokas' forces stopped to rest near the river of Acheloos, not far from the port of Anchialos (Pomorie). Once informed of the invasion, Simeon rushed to intercept the Byzantines, and attacked them from the nearby hills while they were resting disorganized. In the Battle of Acheloos of 20 August 917, one of the largest in medieval history, the Bulgarians completely routed the Byzantines and killed many of their commanders, although Phokas managed to escape to Mesembria. Decades later, Leo the Deacon would write that "piles of bones can still be seen today at the river Acheloos, where the fleeing army of the Romans was then infamously slain".

While the victorious Bulgarian army was marching southwards, the Byzantine commander Leo Phokas, who survived at Achelous, reached Constantinople by sea and gathered the last Byzantine troops to intercept his enemy before reaching the capital. The two armies clashed near the village of Katasyrtai just outside the city and after a night fight, the Byzantines were completely routed from the battlefield.

The last Byzantine military forces were literally destroyed and the way to Constantinople was opened, but the Serbs rebelled to the west and the Bulgarians decided to secure their rear before the final assault of the Byzantine capital which gave the enemy precious time to recover.

The Battle of Pegae was fought between 11 and 18 March 921 in the outskirts of Constantinople between the forces of the Bulgarian Empire and the Byzantine Empire during the Byzantine–Bulgarian war of 913–927. The battle took place in a locality called Pegae (i.e. "the spring"), named after the nearby Church of St. Mary of the Spring. The Byzantine lines collapsed at the very first Bulgarian attack and their commanders fled the battlefield. In the subsequent rout most Byzantine soldiers were killed by the sword, drowned or were captured.

In 922 the Bulgarians continued their successful campaigns in Byzantine Thrace, capturing a number of towns and fortresses, including Adrianople, Thrace's most important city, and Bizye. In June 922 they engaged and defeated yet another Byzantine army at Constantinople, confirming the Bulgarian domination of the Balkans. However, Constantinople itself remained outside their reach, because Bulgaria lacked the naval power to launch a successful siege. The attempts of the Bulgarian emperor Simeon I to negotiate a joint Bulgarian–Arab assault on the city with the Fatimids were uncovered by the Byzantine and countered.

In 924, the Serbs ambushed and defeated a small Bulgarian army on its way to Serbia, provoking a major retaliatory campaign that ended with Bulgaria's annexation of Serbia at the end of that year.

Desperate to conquer Constantinople, Simeon planned a large campaign in 924 and sent envoys to the Fatimid caliph Ubayd Allah al-Mahdi Billah, who possessed a powerful navy which Simeon needed. The caliph agreed and sent his own representatives back with the Bulgarians to arrange the alliance. However, the envoys were captured by the Byzantines at Calabria. Romanos offered peace to the Arabs, supplementing this offer with generous gifts, and ruined their union with Bulgaria.

Simeon's aim was to defeat the Byzantine Empire and conquer Constantinople. To achieve his aim, Simeon overran the eastern and central Balkans several times, occupied Serbia and finally attacked Croatia. The result of the battle was an overwhelming Croatian victory.

In 926, Simeon's troops under Alogobotur invaded Croatia, at the time a Byzantine ally, but were completely defeated by the army of King Tomislav in the Battle of the Bosnian Highlands.

Peter I negotiated a peace treaty with the Byzantine government. The Byzantine Emperor Romanos I Lakapenos eagerly accepted the proposal for peace and arranged for a diplomatic marriage between his granddaughter Maria and the Bulgarian monarch. In October 927 Peter arrived near Constantinople to meet Romanos and signed the peace treaty, marrying Maria on 8 November in the church of the Zoödochos Pege. To signify the new era in Bulgaro-Byzantine relations, the princess was renamed Eirene ("peace"). The extensive Preslav Treasure is thought to represent part of the dowry of the princess. The treaty of 927 actually represents the fruit of Simeon's military successes and diplomatic initiatives, ably continued by his son's government. Peace was obtained with the frontiers restored to those defined in treaties of 897 and 904. The Byzantines recognised the Bulgarian monarch's title of emperor (basileus, tsar) and the autocephalus status of the Bulgarian patriarchate, while the payment of an annual tribute to Bulgaria by the Byzantine Empire was renewed.

Despite the treaty and the largely peaceful era that followed, the strategic position of the Bulgarian Empire remained difficult. The country was surrounded by aggressive neighbours – the Magyars to the north-west, the Pechenegs and the growing power of Kievan Rus' to the north-east, and the Byzantine Empire to the south, which proved to be an unreliable neighbour.

Bulgaria suffered several devastating Magyar raids between 934 and 965. 

Relations with the Byzantine Empire worsened after the death of Peter's wife in the mid-960s. Victorious over the Arabs, Emperor Nikephoros II Phokas refused to pay the annual tribute to Bulgaria in 966, complaining of the Bulgarian alliance with the Magyars, and he undertook a show of force at the Bulgarian border. Dissuaded from a direct attack against Bulgaria, Nikephoros II dispatched a messenger to the Rus prince Sviatoslav Igorevich to arrange a Rus attack against Bulgaria from the north.

Sviatoslav readily launched a campaign with a vast force of 60,000 troops, routed the Bulgarians on the Danube, and defeated them in a battle near Silistra, seizing some 80 Bulgarian fortresses in 968. The Byzantines encouraged the Rus' ruler Sviatoslav to attack Bulgaria, leading to the defeat of the Bulgarian forces and the occupation of the northern and north-eastern part of the country by the Rus' for the following two years.

The Battle of Silistra occurred in the spring of 968 near the Bulgarian town of Silistra, but most probably on the modern territory of Romania. It was fought between the armies of Bulgaria and Kievan Rus' and resulted in a Rus' victory. Upon the news of the defeat, the Bulgarian emperor Peter I abdicated. The invasion of the Rus' prince Sviatoslav was a heavy blow for the Bulgarian Empire.

Stunned by the success of his ally and suspicious of his actual intentions, Emperor Nikephoros II hastened to make peace with Bulgaria and arranged the marriage of his wards, the underage emperors Basil II and Constantine VIII, to two Bulgarian princesses. Two of Peter's sons were sent to Constantinople as both negotiators and honorary hostages. In the meantime Peter managed to secure the retreat of the Rus forces by inciting Bulgaria's traditional allies, the Pechenegs, to attack Kiev itself.

In spite of this temporary success and the reconciliation with Byzantium, Bulgaria faced a new invasion by Sviatoslav in 969. The Bulgarians were defeated again, and Peter suffered a stroke, which led him to abdicate and become a monk. He died on 30 January 970.

Sviatoslav's brief sojourn into the south awakened in him the desire to conquer these fertile and rich lands. In this intention he was apparently encouraged by the former Byzantine envoy, Kalokyros, who coveted the imperial crown for himself. Thus, after defeating the Pechenegs, he set up viceroys to rule Russia in his absence and turned his sights southward again.

In summer 969, Sviatoslav returned to Bulgaria in force, accompanied by allied Pecheneg and Magyar contingents. In his absence, Pereyaslavets had been recovered by Boris II; the Bulgarian defenders put up a determined fight, but Sviatoslav stormed the city. Thereafter Boris and Roman capitulated, and the Rus' rapidly established control over eastern and northern Bulgaria, placing garrisons in Dorostolon and the Bulgarian capital of Preslav. There Boris continued to reside and exercise nominal authority as Sviatoslav's vassal. In reality he was little more than a figurehead, retained in order to lessen Bulgarian resentment at and reaction to the Rus' presence. Sviatoslav appears to have been successful in enlisting Bulgarian support. Bulgarian soldiers joined his army in considerable numbers, tempted partly by the prospects of booty, but also enticed by Sviatoslav's anti-Byzantine designs and probably mollified by a shared Slavic heritage. The Rus' ruler himself was careful not to alienate his new subjects: he forbade his army from looting the countryside or plundering cities that surrendered peacefully.

Thus Nikephoros' scheme had backfired: Instead of a weak Bulgaria, a new and warlike nation had been established at the Empire's northern border, and Sviatoslav showed every intention of continuing his advance south into Byzantium. 

From ca. 970 until 1018, a series of conflicts between the Bulgarian Empire and the Byzantine Empire led to the gradual reconquest of Bulgaria by the Byzantines, who thus re-established their control over the entire Balkan peninsula for the first time since the 7th-century Slavic invasions. The struggle began with the incorporation of eastern Bulgaria after the Russo-Byzantine War (970–971). Bulgarian resistance was led by the Cometopuli brothers, who – based in the unconquered western regions of the Bulgarian Empire – led it until its fall under Byzantine rule in 1018.

In early 970, a Rus' army, with large contingents of Bulgarians, Pechenegs, and Magyars, crossed the Balkan Mountains and headed south. The Rus' stormed the city of Philippopolis (now Plovdiv), and, according to Leo the Deacon, impaled 20,000 of its surviving inhabitants. Skleros, with an army of 10,000–12,000 men, confronted the Rus' advance near Arcadiopolis (now Luleburgaz) in early spring 970. The Byzantine general, whose army was considerably outnumbered, used a feigned retreat to draw the Pecheneg contingent away from the main army into a prepared ambush. The main Rus' army panicked and fled, suffering heavy casualties at the hands of the pursuing Byzantines. The Rus' withdrew north of the Balkan mountain range, which gave Tzimiskes time to deal with internal unrest and to assemble his forces.

After the death of John Tzimiskes in 976, the Cometopuli launched a joint offensive into Byzantine territory. The former Bulgarian heartland to the northeast rose up in revolt and overthrew the Byzantine administration. The uprising was led by the local nobles Peter and Boyan, who became allies of the Cometopuli and submitted to their rule. The newly liberated territories remained in Bulgarian hands until 1001.

The Battle of the Gates of Trajan was a battle between Byzantine and Bulgarian forces in the year 986. It was the largest defeat of the Byzantines under Emperor Basil II. After the unsuccessful siege of Sofia he retreated to Thrace, but was surrounded by the Bulgarian army under the command of Samuil in the Sredna Gora mountains. The Byzantine army was annihilated and Basil himself barely escaped.

As a response, a Byzantine army under Nikephorus Uranos was sent after the Bulgarians, who returned north to meet it. The two armies met near the flooded river of Spercheios. The Byzantines found a place to ford, and on the night of 19 July 996 they surprised the unprepared Bulgarian army and routed it in the battle of Spercheios. Samuel's arm was wounded and he barely escaped captivity; he and his son allegedly feigned death After nightfall they headed for Bulgaria and walked 400 kilometres (249 mi) home. 

The battle was a major defeat of the Bulgarian army. At first Samuil showed readiness for negotiations but upon the news of the death of Bulgaria's official ruler Roman in prison, he proclaimed himself the sole legitimate tsar and continued the war. 

In 1003, Basil II launched a campaign against the First Bulgarian Empire and after eight months of siege conquered the important town of Vidin to the north-west. The Bulgarian counter strike in the opposite direction towards Odrin did not distract him from his aim and after seizing Vidin he marched southwards through the valley of the Morava destroying the Bulgarian castles on his way. Eventually, Basil II reached the vicinity of Skopje and learned that the camp of the Bulgarian army was situated very close on the other side of the Vardar river.

Samuil of Bulgaria relied on the high waters of the river of Vardar and did not take any serious precautions to secure the camp. Strangely the circumstances were the same as at the battle of Spercheios seven years earlier, and the scenario of the fight was similar. The Byzantines managed to find a fjord, crossed the river and attacked the heedless Bulgarians at night. Unable to resist effectively the Bulgarians soon retreated, leaving the camp and Samuil's tent in the hands of the Byzantines. During this battle Samuil managed to escape and headed east.

The Battle of Kleidiontook place on July 29, 1014, between the Byzantine Empire and the Bulgarian Empire. It was the culmination of the nearly half-century struggle between the Byzantine Emperor Basil II and the Bulgarian Emperor Samuel in the late 10th and early 11th centuries. The result was a decisive Byzantine victory.

The battle took place in the valley between the mountains of Belasitsa and Ograzhden, near the modern Bulgarian village of Klyuch. The decisive encounter occurred on July 29 with an attack in the rear by a force under the Byzantine general Nikephoros Xiphias, who had infiltrated the Bulgarian positions. The ensuing battle was a major defeat for the Bulgarians. Bulgarian soldiers were captured and reputedly blinded by order of Basil II, who would subsequently be known as the "Bulgar-Slayer". Samuel survived the battle, but died two months later from a heart attack, reportedly brought on by the sight of his blind soldiers.

Although the engagement did not end the First Bulgarian Empire, the Battle of Kleidion reduced its ability to resist Byzantine advances, and it has been considered the pivotal encounter of the war with Byzantium.

Resistance continued for four more years under Gavril Radomir (r. 1014–1015) and Ivan Vladislav (r. 1015–1018) but after the demise of the latter during the siege of Dyrrhachium the nobility surrendered to Basil II and Bulgaria was annexed by the Byzantine Empire. The Bulgarian aristocracy kept its privileges, although many noblemen were transferred to Asia Minor, thus depriving the Bulgarians of their natural leaders. Although the Bulgarian Patriarchate was demoted to an archbishopric it retained its sees and enjoyed a privileged autonomy. Despite several major attempts at restoring its independence, Bulgaria remained under Byzantine rule until the brothers Asen and Peter liberated the country in 1185, establishing the Second Bulgarian Empire.

The Bulgarian state existed before the formation of the Bulgarian people. Prior to the establishment of the Bulgarian state the Slavs had mingled with the native Thracian population. The population and the density of the settlements increased after 681 and the differences among the individual Slavic tribes gradually disappeared as communications became regular among the regions of the country. By the second half of the 9th century, Bulgars and Slavs, and romanized or hellenized Thracians had lived together for almost two centuries and the numerous Slavs were well on the way to assimilating the Thracians and the Bulgars. Many Bulgars had already started to use the Slavic Old Bulgarian language while the Bulgar language of the ruling caste gradually died out leaving only certain words and phrases.The Christianization of Bulgaria, the establishment of Old Bulgarian as a language of the state and the church under Boris I, and the creation of the Cyrillic script in the country, were the main means to the final formation of the Bulgarian nation in the 9th century; this included Macedonia, where the Bulgarian khan, Kuber, established a state existing in parallel with Khan Asparuh's Bulgarian Empire. The new religion dealt a crushing blow to the privileges of the old Bulgar aristocracy; also, by that time, many Bulgars were presumably speaking Slavic. Boris I made it a national policy to use the doctrine of Christianity, that had neither Slavic nor Bulgar origin, to bind them together in a single culture. As a result, by the end of the 9th century the Bulgarians had become a single Slavic nationality with ethnic awareness that was to survive in triumph and tragedy to present.