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.

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.

Furious, Krum ravaged the environs of Constantinople and headed home, capturing Adrianople[12] en route and transplanting its inhabitants (including the parents of the future Emperor Basil I) across the Danube.

Emperor Theophilos sought Bulgar support in putting down the rebellion, but he simultaneously arranged for his fleet to sail through the Danube delta and undertake a clandestine evacuation of some of the Byzantine captives settled in trans-Danubian Bulgaria by Krum and Omurtag.

In retaliation Isbul campaigned along the Aegean coasts of Thrace and Macedonia and captured the city of Philippi, where Theophilos set up a surviving memorial inscription in a local church.

Despite his able diplomacy, statesmanship, and his importance in the process of converting Bulgaria to Christianity, Boris I was not a particularly successful leader in war, being at various times defeated by the Franks, Croats, Serbs, and Byzantines.

This gave the Bulgarian ruler a great opportunity to attempt a campaign towards the Byzantine capital, so he attacked in full force in late July or August 913, reaching Constantinople without any serious resistance.

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).

In the Battle of Achelous (or Anchialus) on 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.

Meanwhile, the admiral Romanos Lekapenos replaced Zoe as regent of the young Constantine VII in 919 and advanced himself to the rank of co-emperor in December 920, effectively assuming control of the empire.

Simeon's forces appeared before Constantinople in 921, when they demanded the deposition of Romanos and captured Adrianople; in 922 they were victorious at Pigae, burning much of the Golden Horn and seizing Bizye.

He conversed with Romanos on the Golden Horn on 9 September 924 and arranged a truce, according to which Byzantium would pay Bulgaria an annual tax, but would be ceded back some cities on the Black Sea coast.

Peter also gained a Byzantine bride, Maria Lecapena, granddaughter of Romanus I, an annual tribute, and recognition of his title of tsar and of the autocephalus status of the Bulgarian church.

This involved allowing Boris II and his brother Roman to escape from their honorary captivity at the Byzantine court, in the hope that their arrival in Bulgaria would cause a division between the Cometopuli and other Bulgarian leaders.

Although the Byzantines sacked Ohrid, they failed to take Pernik and received troubling intelligence that Ivan Vladislav was attempting to induce the Pechenegs to come to his aid, following up the general practice of his predecessors.

After his death much of the Bulgarian nobility and court, including his widow Maria and his sons, submitted to the advancing Basil II in exchange for guarantees of the preservation of their lives, status, and property.

As the Bulgarian and Byzantine troops were preparing for battle, Alusian deserted to the enemy and headed for Constantinople, where his possessions and lands were restored to him, and he was rewarded with the high courtly rank of magistros.

Meanwhile, though blind, Peter II Delyan resumed command of the Bulgarian forces, but the Byzantine Emperor Michael IV determined to take advantage of the situation and advanced against them.

Norse sagas refer to the participation of the future Norwegian King Harald Hardråda, who allegedly cut down Peter II in the field of battle as a member of the Varangian Guard.

When Isaac II Angelos penetrated into Moesia again in 1187 he failed to capture either Tărnovo or Loveč, and he signed a treaty effectively recognizing the Second Bulgarian Empire, but neither side had any intention of keeping the peace.

When the Third Crusade led by Frederick I, the Holy Roman Emperor, was advancing towards Constantinople, representatives of Peter IV and Ivan Asen approached him with offers of military assistance against the latently hostile Isaac II Angelos at Niš and again at Adrianople.

In charge of the defense of the Tărnovo, Ivan Asen I incited the Byzantine emperor to a hasty retreat by spreading rumors about the arrival of a great Cuman army to the relief of the besieged city.

Success now definitely swung in favor of the Bulgarians, who captured the areas of Sredec (Sofia) and Niš in 1191, of Belgrade in 1195, of Melnik and Prosek in 1196, while raiding parties reached as far south as Serres.

The war between the Bulgarians and the Byzantines was stalled in 1204, when Catholic forces of the Fourth Crusade captured and sacked Constantinople and created the Latin Empire, electing as emperor Baldwin I of Flanders.

In the meantime, a Byzantine force besieged the Bulgarian capital and, after hearing a rumor of Ivailo's death in battle, the local nobility surrendered and accepted Ivan Asen III as emperor in 1279.

Desperate for relief, Ivan Asen III fled Tărnovo in 1280, and his brother-in-law, George Terter I, seized the throne, uniting the factious aristocracy and gradually taking away the support for Ivailo.

As a consequence of his success, Theodore felt secure enough to take the offensive by 1303 and captured several fortresses in northeastern Thrace, including Mesembria (Nesebăr), Ankhialos (Pomorie), Sozopolis (Sozopol), and Agathopolis (Ahtopol) in the following year.

Taking advantage of the situation, George invaded Byzantine Thrace and, encountering little, if any, resistance, conquered the major city of Philippopolis (Plovdiv) and part of the surrounding area in 1322 or 1323.

At the same time, a Byzantine-sponsored pretender, Vojsil, brother of the former Bulgarian emperor Smilec, ensconced himself in Krăn, assuming control of the valley between the Balkan mountains and Sredna Gora.

Another showdown in front of Adrianople in 1328 ended without battle and with the renewal of the peace treaty, after which Michael Asen III returned to his country, but not before securing a large payoff.