871 Harsh battle between Danes and Wessex

England 
In January the battles began in the borderlands of Wessex. The defense of Wessex was led by Aethelred and his brother, who later became known as Alfred the great and on the the 8th January the English won their first victory over the vikings at Asdown.
The vikings was not shaken by the defeat and when Alfred the great became king when Aethelred died in April, king Alfred the great had to pay a large sum of money to secure a peace treaty.
The sons of Ragnar Lothbrok managed to capture London and hold it for several years, here Halfdan began to mint coins.
9 battles were fought between the Danes and king Alfred the great of Wessex. Some of them the place and date are not recorded, the others are:
January 4 – Reading  : Viking leader Bagsecg won a great victory at the Battle of Reading inflicting heavy losses on Alfred's army.
January 8 – Ashdown : probably  North Wessex Downs in Berkshire. The Vikings leaders were Bagsecg and Halfdan and five more Danish earls. The Viking army itself was outnumbered in comparison to the West Saxons led by Alfred the great. This battle would determine the fate of Wessex and its king. Alfred's elder brother King Æthelred of Wessex was busy praying in a church, and refused to fight until his other army arrived. This left Alfred in command, and the West Saxon and Viking armies met, the battle itself lasting the whole day.
Bagsecg was killed along with his five earls. According to the Anglo-Saxon Chronicle, Bagsecg was slain by a sword while Halfdan fled from the battlefield with the rest of the army back to Reading
January 22 – Basing
March 22 – Merton
May – Wilton
After the battle at Wilton, peace was made, with the Danes and Alfred splitting England.
The viking army captured Toksey in Lindsey and Reading.
Ireland
Irish Annals record that Óláf Cuaran and Ívar Boneless returned to Dublin after their raids against the Strathclyde Welsh, Albans, and West Saxons with 200 hundred ships and English, Welsh, and Pictish captives to be sold into slavery. Ivar Boneless ruled as king of Dublin 871-873
Norway
Tønsberg was founded.