This created a time of relative peace, with each group largely staying out of the other's way. In the 980s, however, Danes started making raids into English territory. In 991 after the Battle of Maldon, King Æthelred the Unready paid Danegeld, a tribute to the Danes to stop their attacks.
The attacks did not stop, however, and from 997 to 1001 there were savage raids by Danes across Southern England, burning towns and killing Anglo-Saxons.
Then Æthelred learned of a rumor, that the Danes intended to kill him, all his councilors, and then possess the entire kingdom. To put it in the words of the (translated) Anglo-Saxon Chronicle:
in the same year the king gave an order to slay all the Danes that were in England. This was accordingly done on the mass-day of St. Brice; because it was told the king, that they would beshrew him of his life, and afterwards all his council, and then have his kingdom without any resistance.
Contemporary and near-contemporary and modern historians disagree on the phrase "all the Danes that were in England." Some say it was only those who were warriors, some (like the A-SC) believe it was every Dane they could find.
The massacre took place on the feast day of St. Brice, a Frankish bishop (c.370 - 444) who succeeded St. Martin as bishop of Tours. It took place in Oxford, and a royal charter issued in 1004 by Æthelred called it "a most just extermination."
Part of it involved an act of cruelty that mirrored the much later Clifford's Tower incident, an act so horrendous that it might have been the deciding factor in Sweyn's invasion. I'll explain more next time.