Byzantine–Bulgarian wars: Battle of Kleidion: Byzantine emperor Basil II inflicts a decisive defeat on the Bulgarian army, and his subsequent treatment of 15,000 prisoners reportedly causes Tsar Samuil of Bulgaria to die of a heart attack less than three months later, on October 6.

The Byzantine–Bulgarian wars were a series of conflicts fought between the Byzantine Empire and the Bulgarian Empire which began after the Bulgars 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.