On this day the 13th of November 1002, King Æthelred the Unready ordered a mass execution of all Danes in England, in response to a perceived threat on his life.
This came after a few decades of yearly Danish raiding, a subsequent "tribute" (protection money) to the Danish King and marriage to a Norman with Danish heritage.
After raids continued in 997 - 1002, it seemed that Æthelred was fed up. He was told that settled Danish men "would faithlessly take his life, his councillors lives, and possess his kingdom afterwards." So, logically, he ordered the deaths of all Danes living in England. Smart move, man. A "most just extermination" occurred in Oxford, and probably many other sites across his territory.
Obviously this didn't take well with the Danes. Some historians believe this event may have contributed to the accession of King Cnut in 1016, after Æthelred's death.
Moral of the story: if you've been threatened, maybe explore other avenues before resorting to racial genocide? Just maybe?