Byzantine–Bulgarian war of 913–927

The Byzantine–Bulgarian war of 913–927 (Bulgarian: Българо–византийска война от 913–927) was fought between the Bulgarian Empire and the Byzantine Empire for more than a decade. 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.

In 917, the Bulgarian army dealt a crushing defeat to the Byzantines at the Battle of Achelous, resulting in Bulgaria's total military supremacy in the Balkans. The Bulgarians again defeated the Byzantines at Katasyrtai in 917, Pegae in 921 and Constantinople in 922. The Bulgarians also captured the important city of Adrianople in Thrace and seized the capital of the Theme of Hellas, Thebes, deep in southern Greece. Following the disaster at Achelous, Byzantine diplomacy incited the Principality of Serbia to attack Bulgaria from the west, but this assault was easily contained. 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.

Simeon was aware that he needed naval support to conquer Constantinople and in 922 sent envoys to the Fatimid caliph Ubayd Allah al-Mahdi Billah in Mahdia to negotiate the assistance of the powerful Arab navy. The caliph agreed to send his own representatives to Bulgaria to arrange an alliance but his envoys were captured en route by the Byzantines near the Calabrian coast. Emperor Romanos I Lekapenos managed to avert a Bulgarian–Arab alliance by showering the Arabs with generous gifts. By the time of his death in May 927, Simeon controlled almost all Byzantine possessions in the Balkans, but Constantinople remained out of his reach.

In 927, both countries were exhausted by the huge military efforts that had taken a heavy toll on the population and economy. Simeon's successor Peter negotiated a favourable peace treaty. The Byzantines agreed to recognize him as Emperor of Bulgaria and the Bulgarian Orthodox Church as an independent Patriarchate, as well as to pay an annual tribute. The peace was reinforced with a marriage between Peter and Romanos's granddaughter Irene Lekapene. This agreement ushered in a period of 40 years of peaceful relations between the two powers, a time of stability and prosperity for both Bulgaria and the Byzantine Empire.