Bagsecg (also known as Bægsecg or Bagsec) (died 8 January 871) was a Viking leader commonly referred to as a king.  In 870 or 871 he led The Great Summer Army to England. His forces joined those of the Great Danish Army which had already overrun much of England. He and Halfdan Ragnarsson became the leaders of a joint invasion of the Kingdom of Wessex. He was killed at the Battle of Ashdown, fighting a West Saxon Army led by King Ethelred and his younger brother, the future Alfred the Great.
Little is known about Bagsecg prior to his arrival in England. He arrived in either 870 or 871 in East Anglia as the leader of a massive army known as The Great Summer Army which arrived in the summer, so the likely date when he arrived could be 870. By this time the Viking forces controlled much of the north and the east of England, including Kent, and were advancing slowly west and south west towards Wessex. As early as 871, Bagsecg and Halfdan Ragnarsson dispatched a few forces to attack the Kingdom of Wessex, which remained vulnerable to raids. They captured Reading, Berkshire, and set up camp within the town. On 4 January 871, Alfred the Great (he was not yet king) attempted to attack the camp; however Bagsecg won a great victory at the Battle of Reading inflicting terrible losses on Alfred's army.
On 8 January 871, both forces met on the North Wessex Downs in Berkshire (possibly the part now in Oxfordshire). The Vikings were commanded by Bagsecg and Halfdan and five other Danish earls. The Viking army itself was outnumbered in comparison to the West Saxons led by Alfred. 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 all 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. The Battle of Ashdown itself was a limited West Saxon victory.
According to local folklore in Berkshire, Bagsecg was buried at Waylands Smithy and his earls at The Seven Barrows. This is however quite doubtful. Waylands Smithy dates back to Neolithic times and The Seven Barrows dates back to the Bronze Age. It is always possible that the Barrows could have been reused for burial over the course of time. However, on the downs above the village of Harwell there once stood a succession of trees known as "Bag’s or Bagg’s Tree" , perhaps an indication of the death site or burial site of Bagsecg. This trees no longer exists, but the location is still marked on Ordnance Survey maps of the area.
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- Whitelock, Dorothy (1996) English Historical Documents, 500-1042 (Psychology Press) ISBN 9780415143660