410:

Visigoths Sack Rome

Under command of King Alaric, the Visigoths besieged Rome when negotiations with the empire for land and supplies broke down. On August 24, allies of Alaric within the city opened the gates, and the Visigoths plundered the capital for three days. They treated the inhabitants humanely, burning few buildings, but nevertheless the incident was shocking, for it had been almost 800 years since a foreign force had occupied Rome.

This incident is sometimes regarded as the beginning of the Middle Ages.

1572:

St. Bartholomew's Day Massacre

The massacre of Huguenots in Paris by Catholic noblemen and other citizens focused the attention of all Europe on the wars of religion that beset 16th-century France.

An eyewitness account of the event can be found at Hanover College.