Battle of Brissarthe: The Franks led by Robert the Strong are defeated by a joint Breton-Viking army.

The Battle of Brissarthe was fought on 2 July 866, between the West Franks and a joint Breton-Viking army near Brissarthe, Neustria. It was marked by the death of Robert the Strong, the Neustrian margrave, and Ranulf I, the duke of Aquitaine, and an ancestor of all the kings of France from 987 to 1848.

The Kingdom of the Franks, also known as the Frankish Kingdom, or just Francia, was the largest post-Roman kingdom in Western Europe. It was established by the Franks, one of the Germanic peoples. Its founder was king Clovis I (481–509), who united Frankish tribes, and expanded the Frankish realm into the Roman Gaul. During the Early Middle Ages, the Kingdom was ruled by the Merovingian and Carolingian dynasties. In 800, it evolved into the Carolingian Empire, thus becoming the longest lasting Germanic kingdom from the era of Great Migrations.

Robert the Strong was the father of two kings of West Francia: Odo and Robert I of France. His family is named after him and called the Robertians. In 853, he was named missus dominicus by Charles the Bald, King of West Francia. Robert the Strong was the great-grandfather of Hugh Capet and thus the ancestor of all the Capetians.

The Bretons are an ethnic group native to Brittany, north-western France. Originally, the demonym designated groups of Brittonic speakers who emigrated from southwestern Great Britain, particularly Cornwall and Devon, mostly during the Anglo-Saxon settlement of Britain. They migrated in waves from the 3rd to 9th century to Armorica. The region was subsequently named after them, as were the inhabitants of Armorica as a whole.