The Byzantine–Bulgarian wars were a series of conflicts fought between the Byzantine Empire and the Bulgarian Empire which began already in the late 5th century when the Bulgars defeated a 15.000 Roman army led by Magister Aristus (499 AD). The Bulgars permanently conquered parts of the Balkan Peninsula after 680 AD. The Byzantine and First Bulgarian Empire continued to clash over the next century with varying success, until the Bulgarians, led by Krum, inflicted a series of crushing defeats on the Byzantines. After Krum died in 814, his son Omurtag negotiated a thirty-year peace treaty. Simeon I had multiple successful campaigns against the Byzantines during his rule from 893 to 927. His son Peter I negotiated another long-lasting peace treaty. His rule was followed by a period of decline of the Bulgarian state.
In 971 John I Tzimiskes, the Byzantine emperor, subjugated much of the weakening Bulgarian Empire by defeating Boris II and capturing Preslav, the Bulgarian capital. Samuel managed to stabilize the Bulgarian state with its center around the town of Prespa. Near the end of his rule, the Byzantines regained the upper hand, and under Basil II they won the Battle of Kleidion and completely conquered Bulgaria in 1018. There were rebellions against Byzantine rule from 1040 to 1041, and in the 1070s and the 1080s, but these failed. In 1185, however, Theodore Peter and Ivan Asen started a revolt, and the weakening Byzantine Empire, facing internal dynastic troubles of its own, was unable to quash the revolt.
After the army of the Fourth Crusade conquered Constantinople in 1204, Kaloyan, the Bulgarian emperor, tried to establish friendly relations with the crusaders. However, the newly created Latin Empire spurned any offer of alliance with the Bulgarians. Because of his cold reception, Kaloyan allied with the Nicaeans, which reduced the crusaders' power in the area. Although his nephew Boril allied with the Latin Empire, Boril's successors sided with the Nicaeans, despite a few continuing attacks from them. After the Latin Empire collapsed, the Byzantines, took advantage of the Bulgarian civil war and captured portions of Thrace, however the Bulgarian emperor Theodore Svetoslav retook these lands. The Byzantine-Bulgarian relations continued to fluctuate until the Ottoman Turks captured the Bulgarian capital in 1393 and the Byzantine capital in 1453.
Asparuh's war
The Byzantines first clashed with the founders of Bulgaria (the Bulgars) when Khan Kubrat's youngest son Asparuh moved westward, occupying today's southern Bessarabia. Asparuh defeated the Byzantines, who were under Constantine IV, with a combined land and sea operation and successfully besieged their fortified camp in Ongala. Suffering from bad health, the emperor had to leave the army, which allowed itself to panic and be defeated by the Bulgars. In 681 Constantine was forced to acknowledge the Bulgarian state in Moesia and to pay protection money to avoid further inroads into Byzantine Thrace. Eight years later, Asparuh led a successful campaign against Byzantine Thrace.
Tervel's wars
Tervel, first mentioned in the Byzantine texts in 704 when the deposed emperor Justinian II came to him and asked for his aid, supported Justinian in an attempted restoration to the Byzantine throne in exchange for friendship, gifts and his daughter's hand in marriage. With an army of 15,000 horsemen provided by Tervel, Justinian suddenly advanced on Constantinople and managed to gain entrance into the city in 705. The restored emperor executed his supplanters, the emperors Leontios and Tiberios III, alongside many of their supporters. Justinian rewarded Tervel with many gifts, the title of kaisar (Caesar), which made him second only to the emperor and the first foreign ruler in Byzantine history to receive such a title, and possibly a territorial concession in northeastern Thrace, a region called Zagore. Whether Justinian's daughter Anastasia was married to Tervel as had been arranged is unknown.
A mere three years later Justinian II himself violated this arrangement and apparently commenced military operations to recover the ceded area. Tervel routed him at the Battle of Anchialus (or Ankhialo) in 708. In 711, faced by a major revolt in Asia Minor, Justinian again sought the aid of Tervel but received only lukewarm support manifested in an army of 3,000. Outmaneuvered by the rebel emperor Philippicus, Justinian was captured and executed, while his Bulgar allies were allowed to retire to their country. Tervel took advantage of the disorder in Byzantium to raid Thrace in 712, plundering as far as the vicinity of Constantinople.
According to the chronological information of the Imennik, Tervel died in 715. However, the Byzantine chronicler Theophanes the Confessor ascribes Tervel a role in an attempt to restore the deposed Emperor Anastasius II in 718 or 719. If Tervel did survive this long, he was the Bulgarian ruler who concluded a new treaty with Emperor Theodosius III in 716 (confirming the annual tribute paid by the Byzantines to Bulgaria and the territorial concessions in Thrace, as well as regulating commercial relations and the treatment of political refugees), and he was also the Bulgarian ruler who helped relieve the second Arab siege of Constantinople in 717–718 by land. According to Theophanes, the Bulgars slaughtered some 22,000 Arabs in the battle.
Constantine V's wars
After the death of Sevar, Bulgaria descended into a long period of crisis and unrest, while the Byzantines consolidated their positions. Between 756 and 775, the new Byzantine Emperor Constantine V led nine campaigns against his northern neighbour to establish a Byzantine border on the Danube. Due to the frequent changes of rulers (eight khans held the throne in twenty years) and the constant political crisis, Bulgaria was on the verge of destruction.
In his first campaign in 756, Constantine V was successful and managed to defeat the Bulgars twice, but in 759, Vinekh, the Bulgar Khan, defeated the Byzantine army comprehensively in the Battle of the Rishki Pass. Vinekh then sought to make peace with the Byzantines but was assassinated by Bulgar nobles. The new ruler, Telets, was defeated at the Battle of Anchialus in 763. During their next campaigns, both sides failed to gain significant success because the Byzantines could not pass through the Balkan Mountains, and their fleet was destroyed twice in heavy storms (2,600 ships sank in just one of the storms in 765). In 774, they defeated an inferior Bulgarian force at Berzitia, but this was the last success of Constantine V: as a result of their defeat, the Bulgars took serious precautions to get rid of the Byzantine spies in Pliska. Khan Telerig sent a secret emissary to Constantine V indicating his intention to flee Bulgaria and seek refuge with the emperor, and seeking assurances of hospitality. Telerig succeeded in having the emperor betray his own agents in Bulgaria, who were duly rounded up and executed. In response, Constantine V invaded Bulgaria once again, in 775, but became ill and died on his return journey to Constantinople.
Constantine VI's failed retaliations
In 791, the Byzantine emperor Constantine VI embarked on an expedition against Bulgaria, in retaliation for Bulgarian incursions in the Struma valley since 789. Kardam pre-empted the Byzantine invasion and met the enemy near Adrianople in Thrace. The Byzantine army was defeated and turned to flight.
In 792, Constantine VI led another army against the Bulgars and encamped at Marcellae, near Karnobat, which he proceeded to fortify. Kardam arrived with his army on July 20 and occupied the neighboring heights. After some time passed with the two forces sizing up each other up, Constantine VI ordered the attack, but in the resulting Battle of Marcellae the Byzantine forces lost formation and were once again defeated, fleeing the field. Kardam captured the imperial tent and the emperor's servants. After his return to Constantinople, Constantine VI signed a peace treaty and agreed to pay an annual tribute to Bulgaria.
By 796, the imperial government became recalcitrant and Kardam found it necessary to demand the tribute while threatening to devastate Thrace if it were not paid. According to the chronicler Theophanes the Confessor, Constantine VI mocked the demand by having dung sent instead of gold as "fitting tribute" and promising to lead a new army against the elderly Kardam at Marcellae. Once again the emperor's army headed north, and once again it encountered Kardam in the vicinity of Adrianople. The armies faced each other for 17 days without entering into battle, while the two monarchs probably engaged in negotiations. In the end, conflict was averted and the peace resumed on the same terms as in 792.
Khan Krum's wars
Khan Krum engaged in an aggressive policy within the Balkans, raiding along the Struma valley in 807, where he defeated a Byzantine army and captured an enormous amount of gold intended as wages for the whole Byzantine army. In 809, Krum besieged and forced the surrender of Serdica (Sofia), slaughtering the Byzantine garrison in spite of his promise of safe conduct. This provoked Byzantine Emperor Nikephoros I to settle Anatolian populations along the frontier to protect it and to attempt to retake and refortify Serdica, although this enterprise ultimately failed.
Conflict with Nikephoros I
In early 811, Nikephoros I undertook a massive expedition against Bulgaria, advancing to Marcellae (near Karnobat). Here Krum attempted to negotiate on July 11, 811, but Nikephoros was determined to continue his advance. His army managed to avoid Bulgar ambushes in the Balkan Mountains and defeated an army of 12,000 that tried to block their advance into Moesia. Another hastily assembled army of 50,000 was defeated before the walls of the Bulgarian capital, Pliska, which fell to the emperor on July 20. Here Nikephoros, who had been a financial minister before becoming emperor, helped himself to the treasures of Krum, while setting the city ablaze and turning his army on the population. A new diplomatic tentative from Krum was rebuffed. Nikephorus showed great cruelty, ordering his army to kill the population of the capital.
Increasingly concerned about the breakdown of discipline in his army, Nikephoros finally began to retreat towards Thrace. In the meantime, Krum had mobilized as many of his subjects as he could (including the women) and had begun to set traps and ambushes for the retreating imperial army in the mountain passes. At dawn on July 26 the Byzantines found themselves trapped against a moat and wooden wall in the Vărbica pass. Nikephoros was killed in the ensuing battle along with many of his troops, while his son Staurakios was carried to safety by the imperial bodyguard after receiving a paralyzing wound to his neck. According to tradition, Krum had the Emperor's skull lined with silver and used it as a drinking cup. This enhanced his reputation for brutality and won him the nickname "New Sennacherib".
Conflict with Michael I Rangabe
Staurakios was forced to abdicate after a brief reign (he died from his wound in 812) and was succeeded by his brother-in-law, Michael I Rangabe. In 812 Krum invaded Byzantine Thrace, taking Develt and scaring the population of nearby fortresses to flee towards Constantinople. From this position of strength, Krum offered a return to the peace treaty of 716. Unwilling to compromise his regime by weakness, the new Emperor Michael I refused to accept the proposal, seemingly opposing the clause for exchange of deserters. To apply more pressure on the emperor, Krum besieged and captured Mesembria (Nesebar) in the fall of 812.
In February 813, the Bulgars raided into Thrace but were repelled by the emperor's forces. Encouraged by this success, Michael I summoned troops from the entire empire and headed north, hoping for a decisive victory. Krum led his army south towards Adrianople and pitched camp near Versinikia. Michael I lined up his army against the Bulgars, but neither side initiated an attack for two weeks. Finally, on June 22, 813, the Byzantines attacked but were immediately turned to flight. With Krum's cavalry in pursuit, the rout of Michael I was complete, and Krum advanced on Constantinople, which he besieged by land. Discredited, Michael was forced to abdicate and become a monk—the third Byzantine emperor undone by Krum in as many years.
Conflict with Leo V the Armenian
The new emperor, Leo V the Armenian, offered to negotiate and arranged for a meeting with Krum. As Krum arrived, he was ambushed by Byzantine archers and was wounded as he made his escape. Furious, Krum ravaged the environs of Constantinople and headed home, capturing Adrianople en route and transplanting its inhabitants (including the parents of the future Emperor Basil I) across the Danube. In spite of the approach of winter, Krum took advantage of good weather to send a force of 30,000 into Thrace, capturing Arkadioupolis (Lüleburgaz) and carrying off some 50,000 captives. The loot from Thrace was used to enrich Krum and his nobility and included architectural elements utilized in the reconstruction of Pliska, perhaps largely by captured Byzantine artisans.
Krum spent the winter preparing for a major attack on Constantinople, where rumor reported the assemblage of an extensive siege park to be transported on 5,000 carts. He died before he set out, however, on April 13, 814, and was succeeded by his son Omurtag.