By the late 9th and the beginning of the 10th century, Bulgaria extended to Epirus and Thessaly in the South, Bosnia in the West and controlled the whole of present-day Romania and Eastern Hungary to the North. The Serbian state came into existence in the mid-9th century as a response to the Bulgarian expansion West of the Morava. Switching loyalties between Bulgaria and the Byzantines, the Serb rulers successfully resisted several Bulgarian invasions until 924 AD, when it was fully subordinated under the general and possibly Count of Sofia Marmais. Under Tsar Simeon I (Simeon the Great), who was educated in Constantinople, Bulgaria became again a serious threat to the Byzantine Empire and reached its greatest territorial extension. Simeon I hoped to take Constantinople and fought a series of wars with the Byzantines throughout his long reign (893–927). The border close to the end of his rule reached the Northern limits of Attica in the South. Simeon I styled himself "Emperor (Tsar) of the Bulgarians and Autocrat of the Greeks", a title which was recognized by the Pope, but not by the Byzantine Emperor nor the Ecumenical Patriarch of the Eastern Orthodox Church. He was recognized "Emperor (Tsar) of the Bulgarians" by the Byzantine Emperor and the Patriarch only at the end of his rule.
Between 894 and 896 he defeated the Byzantines and their allies the Magyars in the "Trade War", so-called because the pretext of the war was the shifting of the Bulgarian market from Constantinople to Solun. In the decisive battle of Bulgarophygon, the Byzantine army was routed, and the war ended favourably for Bulgaria, though the peace was often violated by Simeon I. In 904 he captured Solun, which was previously looted by the Arabs, and returned it to the Byzantines only after Bulgaria received all Slavic-populated areas in Macedonia and 20 fortress in Albania, including the important town Drach.
After the unrest in the Byzantine Empire following the death of Emperor Alexander in 913, Simeon I invaded Byzantine Thrace, but he was persuaded to stop in return for official recognition of his Imperial title and marriage of his daughter to the infant Emperor Constantine VII. Simeon I was supposed to become regent of the Emperor and to temporarily rule the Byzantine Empire. However, after a plot in the Byzantine court, Empress Zoe, mother of Constantine VII, rejected the marriage and Simeon's title, and both sides prepared for a decisive battle. By 917 Simeon I broke every attempt of the Empire to form an alliance with the Magyars, the Pechenegs, and the Serbs, and the Byzantines were forced to fight alone. On 20 August the two armies clashed at Anchialus in one of the greatest battles in the Middle Ages. The Byzantines suffered an unprecedented defeat, leaving 70,000 killed on the battlefield
; the pursuing Bulgarian forces defeated the remainder of the enemy armies at Katasyrtai. Constantinople was saved by a Serb attack from the West; the Serbs were thoroughly defeated, but their attack allowed the Byzantine admiral and later Emperor Romanos Lakepanos to prepare the defense of the city. In the following decade the Bulgarians gained control of the whole Balkan peninsula with the exception of Constantinople and Pelopones.
BTW Amazon thanks for this response: https://bitsharestalk.org/index.php?topic=4958.msg65412#msg65412