In spring 809, Khan Krum seized the major Byzantine fortress of Serdica (modern Sofia), strengthening Bulgarian control over key routes in the central Balkans and escalating the northern frontier conflict with Byzantium.
On 26 July 811, Krum’s forces ambushed the retreating imperial army in the Balkan passes after the Byzantines sacked Pliska; Emperor Nikephoros I was killed, producing one of Byzantium’s most severe defeats and reshaping strategy on the Danubian frontier.
On 22 June 813, Krum defeated Emperor Michael I Rangabe near Adrianople (Versinikia). The loss destabilized the Byzantine regime and enabled Bulgarian forces to advance deep into Thrace.
On 17 July 813, Krum reached Constantinople and began a failed siege effort. Although the walls held, the campaign intensified pressure on Byzantium and led to widespread devastation in the surrounding regions.
Khan Krum died on 13 April 814 while preparing another major offensive against Constantinople, ending a period of exceptionally aggressive Bulgarian operations on the northern frontier.
In early 815, Khan Omurtag and Emperor Leo V concluded a 30-year peace agreement, stabilizing the frontier after years of major campaigns and enabling both states to recover from sustained warfare.
On 20 August 917, Simeon I won a decisive victory over the main Byzantine field army near the Achelous River (Anchialus), one of the largest and most consequential battles of the wars, leaving Byzantium militarily weakened in the Balkans.
In fall 917, shortly after Achelous, Simeon’s forces defeated the remaining Byzantine troops at Katasyrtai outside Constantinople, further reducing Byzantine capacity to stop Bulgarian operations near the capital.
In 927, Byzantium and Bulgaria concluded a major peace agreement that ended the war of 913–927; the settlement recognized the Bulgarian ruler’s imperial title and affirmed the standing of the Bulgarian Church, creating a new diplomatic balance on the northern frontier.
In March 970, a Byzantine army under Bardas Skleros defeated a Rus’ force (including Bulgarian and steppe allies) near Arcadiopolis, during the struggle that followed Sviatoslav’s intervention in Bulgaria and threatened Byzantium’s European provinces.
In 971, Emperor John I Tzimiskes besieged and defeated the Rus’ at Dorostolon (Silistra). The outcome ended the Rus’–Byzantine conflict in the lower Danube region and altered the strategic environment for subsequent Byzantine–Bulgar struggles.
On 29 July 1014, Basil II defeated Tsar Samuel’s army at Kleidion (Belasitsa). The victory broke major Bulgarian defensive positions in the southwest Balkans and accelerated the collapse of Bulgarian resistance over the following years.
In autumn 1017, Basil II defeated Bulgarian forces led by Ivan Vladislav near Setina in Macedonia, continuing a campaign of attrition and territorial pressure that narrowed Bulgaria’s remaining options for resistance.
In 1018, after sustained military and diplomatic pressure, Bulgarian resistance collapsed and the First Bulgarian Empire was incorporated into the Byzantine Empire, ending this phase of the Byzantine–Bulgar wars and transforming the empire’s northern frontier.
Byzantine–Bulgar Wars and Northern Frontier Conflicts (811–1018)
