Way, way back in the day, the unfortunately named King Aethelred the Unready was warned that the Danish men in the Kingdom of England would “faithlessly take his life, and then all his councilors, and possess his kingdom afterwards.” So, he did the only reasonable thing and had them all slaughtered on November 13, 1002 in an event that would later be called the St. Brice’s Day Massacre.
It’s not like Aethelred didn’t have good cause to be paranoid: the Danes had been ravaging England every year from 997 to 1001, and they’d been making regular sacking visits since the 8th century. That sort of thing tends to make people nervous and upset. It’s uncertain how many died in the massacre, but most historians agree the number was significant and included many prominent Danes, such as the (possible) sister of King Sweyn of Denmark and her husband. Most of the deaths occurred in Oxford, where the remains of between 34 and 38 young men aged 16 to 25 were exhumed in 2010.
Although the massacre may have served Aethelred’s short-term goal of ridding his kingdom of Danes, it proved a poor plan in the long run. King Sweyn was sufficiently provoked to invade the kingdom in 1003, kicking off a decade of Danish raids that culminated in Aethelred fleeing to Normandy and Sweyn taking his throne. Sweyen only had a year to enjoy his conquest before he died and Aethelred returned. Aethelred died two years later. Vikings continued to attack and raid England until they were defeated in 1066, just before the Norman conquest.