Sabra and Shatila massacre was the 16–18 September 1982 killing of between 1300 and 3500 civilians—mostly Palestinians and Lebanese Shias—in the city of Beirut during the Lebanese Civil War. It was perpetrated by the Lebanese Forces, one of the main Christian militias in Lebanon, and supported by IOF that had surrounded Beirut’s Sabra neighbourhood and the Shatila refugee camp.
Here’s recent US secretary of state Antony Blinken’s thoughts on it in the 80s: