Danish raids have ravaged England every year from 997 to 1001, and in 1002 the king is told that the Danish men in England "would faithlessly take his life, and then all his councilors, and possess his kingdom afterwards."
In response, he "ordered slain all the Danish men who were in England." (Anglo-Saxon Chronicle, Abingdon ms, quoted in Ryan Lavelle, Aethelred II: King of the English, The History Press, 2008, p. 104)
Frustration and, possibly, promises of support from Normandy, lead Æthelred in 1102 to decree the so-called St. Brice’s Day Massacre, described in the chronicle of John of Wallingford.
The name St. Brice apparently refers to bishop Bricius of Tours, whose memorial day is November 13.
Historians believe there was significant loss of life.
Among those thought to have been killed were Gunhilde, who may have been the sister of King Sweyn I of Denmark.
Her husband Pallig Tokesen, the Danish Ealdorman of Devonshire, may also have died in the massacre; or according to a different version, his defection to join raiders ravaging the south coast may have played a part in provoking it.
Historians have generally viewed the massacre as a political crime which helped to provoke Sweyn's invasion of 1003.
Simon Keynes in his Oxford Online DNB article on Æthelred described it as a "so-called" massacre, a reaction of people who had been slaughtered and pillaged for a decade, directed not at the inhabitants of the Danelaw but at the mercenaries who had turned on their employers.
Æthelred's biographer, Ryan Lavelle, also questions its extent, arguing that it could not have been carried out in the Danelaw, where the Danes would have been too strong, and that it was probably confined to frontier towns such as Oxford, and larger towns with small Danish communities, such as Bristol, Gloucester and London.
He views the massacre not so much as a royally executed order as an exploitation of popular ethnic hatred and millenarianism.
Audrey MacDonald sees it as leading on to the onslaught which eventually led to Danish conquest. (Audrey MacDonald, St Brice's Day Massacre, The Oxford Companion to British History)
