St. Brice's Day Massacre
The St. Brice's Day massacre happened on November 13, 1002 AD when Ethelred the Unready ordered the massacre of all Danes living in England. Ethelred gave this order because England was afraid of Danish invasion and Ethelred considered the Danes to be disloyal. At least hundreds were killed, among them the sister of the current Danish king, Sweyn Forkbeard, which led to an invasion of England by Denmark in the next year. Thus, the massacre ironically triggered exactly what it was designed to prevent.