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Events

August 20 - Battle of Achelous: A Byzantine expeditionary force under General Leo Phokas is routed by the Bulgarian Empire|Bulgarians] at the Achelous River near the fortress of Anchialos on the Black Sea coast. Phokas flees to Mesembria and escapes by boarding a ship. Tsar Simeon I becomes de facto ruler of the whole Balkan Peninsula, except the well-protected Byzantine capital of Constantinople and the Peloponnese.
Fall - Battle of Katasyrtai: The Bulgarian army under Simeon I marches southwards to Constantinople. Leo Phokas, who survived at Anchelous, gathers the last Byzantine troops to intercept the Bulgarians before they reach the capital. The two armies meet near the village of Katasyrtai, just outside Constantinople. After a surprise night attack, the Byzantines are completely routed from the battlefield.

Bulgarian–Serbian War: Simeon I sends a Bulgarian expeditionary force under Theodore Sigritsa and Marmais to Serbia (early medieval)|Serbia]. The two persuade Petar Gojniković, a Serbian prince who formed an anti-Bulgarian coalition, to meet for a peace agreement. They seize him and send the rebellious prince to the Bulgarian capital of Preslav, where he dies in prison. Simeon replaces Petar with Pavle Branović, a grandson of prince Mutimir, who lives in Preslav. Serbia becomes a puppet state until 921.
The Hungarians, after attacking Swabia, sack and burn Basel. They invade Lorraine in Lotharingia, destroying Verdun and Moyenmoutier, and many monasteries in Alsace. Duke Arnulf I with Hungarian military aid, reconquers his land from King Conrad I of the East Frankish Kingdom. After this event, Bavaria and Swabia agree to pay tribute to the Hungarians.
Battle of Confey: The Norse Vikings under Sigtrygg Caech defeat and kill King Augaire mac Ailella of Leinster in battle. Sigtrygg re-captures Dublin and establishes himself as king, while his kinsman Ragnall ua Ímair returns to England to become King of Northumbria.

Summer - Lady Æthelflæd of Mercia cements an alliance with King Constantine II of Scotland against Norse York. She captures the fortress at Derby, while her brother, King Edward the Elder, takes Towcester. Æthelflæd's armies also ravage Brycheiniog in revenge for killing the Mercian abbot Ecbryht.
Battle of Tempsford: The English army led by Edward the Elder defeats the Danish Vikings at Tempsford. They storm the fortified burh and kill King Guthrum II of East Anglia, along with the Danish Jarls Toglos and Manna.

Battle of San Esteban de Gormaz: Umayyad forces under Abi-Abda besiege the repoblación of San Esteban de Gormaz. King Ordoño II of León allies himself with Sancho I, king of Pamplona, and defeats the Moors. Abi-Abda is captured and executed by decapitation.

September 5 - The Great Yue Kingdom, later renamed Southern Han, is founded by Liu Yan, former governor and military advisor, in Panyu and Guangxi. Liu Yan declares himself emperor, and gives his niece Liu Hua in marriage to Wang Yanjun, a son of his rival Wang Shenzhi, to cement a relationship between the two states.