866 Vikings captured York

Angus McBride:9th century viking warriors
A York Penny
In the autumn the viking army under the leadership of Ivar the boneless, who wanted to revenge the murder of his father Ragnar Lodbrok moved from East Anglia into Northumbria, Where king Aella just had driven the former king Osbryth away from the throne. The internal power struggle had drained Northumbria for ressources and had made any serious resistance impossible. The viking army won the coastal area between the Tyne river and Firth of Forth (Bernica) and in November York was captured and in the south Ethelred became the king of Wessex.
The Battle of Brissarthe was fought on the 2 July 866, between the Franks and a combined Breton and Viking army near Brissarthe, Neustria. It was marked by the two deaths of Robert the Strong, the Neustrian count, and Ranulf I, the duke of Aquitaine.

In 866, Salomon, Duke of Brittany, allied with Hastein (Hasting), a Danish chieftain, formed an expedition into Anjou, Maine, and Touraine. In the course of the campaign, Le Mans was sacked. Robert, commander of the local area, assembled a large army to expel them. He was joined by Ranulf of the region of Poitou and Gauzfrid and Hervé of Maine.
The Frankish army succeeded in intercepting the Danes before they reached their dragon ships on the Loire River.

The Danes attempted to take refuge in a local church, but the Franks besieged them. During the night, the Danes attempted to break out. During the ensuing battle, Robert the strong was killed, Ranulf was mortally wounded by arrows, and Hervé injured. and the Franks had to retreat. Vikings escaped.
King Aed Findiliath destroyed viking camps in Northern Ireland.
Irish was on the offensive against pirate bases on the north coast of Ireland between Donegal and Antrim, A Viking Camp at Youghal(Cork) and camps between Cenel neogain and Araide(uncertain location) Port Manann(uncertain location)
Harald Fairhair (850–933) won a decisive battle in his quest to become king of  whole Norway.