Byzantine–Bulgarian wars
The Byzantine–Bulgarian wars were a series of conflicts fought between the Byzantines and Bulgarians which began when the Bulgars first settled in the Balkan peninsula in the 5th century, and intensified with the expansion of the Bulgarian Empire to the southwest after 680 AD. The Byzantines and Bulgarians continued to clash over the next century with variable 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, who ruled Bulgaria from 893 to 927 had multiple successful campaigns against the Byzantines. His son Peter I negotiated another long-lasting peace treaty. His rule was followed by a period of decline of the Bulgarian state.