The Sabra and Shatila massacre was carried out in September 1982 by Lebanese Maronite Christian militias against refugee camps. The Maronite forces stood under the direct command of Elie Hobeika, who would later become a longtime Lebanese parliament member and in the 1990s also a cabinet minister. The number of victims of the massacre is estimated at 700-3500.

The camps were externally surrounded by Israeli soldiers throughout the incident, although the Israeli military personnel who were there claimed they had no idea of what was going on inside.

On September 1, the expulsion of the PLO fighters from Beirut was completed. Two days later, Israel deployed its armed forces around the refugee camps. The next day Ariel Sharon, Israeli Defense Minister at the time, stated that 2,000 PLO fighters had remained in Beirut. This claim was disputed by Palestinians. The Israeli Premier Menachem Begin met Gemayel in Nahariya and strongly urged him to sign a peace treaty with Israel. According to some sources, he also demanded continuing the presence of South Lebanon Army in southern Lebanon under control of Major Saad Haddad and action from Gemayel to move on the Palestinian fighters Israel claimed had remained hidden in Lebanon. Gemayel refused Israel's demands to sign the treaty or to authorize operations to seek out remaining PLO militants.

On September 14, 1982, Gemayel was assassinated in a massive explosion which demolished his headquarters. Eventually, the culprit who confessed to the crime turned out to be a member of the Syrian Social Nationalist Party and an agent of Syrian intelligence. The Palestinian and Muslim leaders denied any connection. In response, the next day, on September 15, the Israeli army reoccupied West Beirut. Estimates place casualties as high as 88 dead and 254 wounded. This Israeli action breached its agreement with the United States not to occupy West Beirut; the US had also given written guarantees that it would ensure the protection of the Muslims of West Beirut. Israel's occupation also violated its peace agreements with Muslim forces in Beirut and with Syria.

Ariel Sharon and Rafael Eitan then invited Lebanese Phalangist militia units to enter the Sabra and Shatila refugee camps to clean out the PLO fighters. Under the Israeli plan, Israeli soldiers would control the perimeters of the refugee camps and provide logistical support while the Phalangists would enter the camps, find the PLO fighters and hand them over to Israeli forces.

The Israeli military had completely surrounded and sealed off the camps and set up observation posts on the roofs of nearby tall buildings on September 15. The next day Israel announced that it controlled all key points in Beirut. The Israeli military met throughout the day with top Phalangist leaders to arrange the details of the operation.

For the next two nights, from nightfall until late into the night, people reported seeing illuminated flares above the camps, allegedly fired by the Israeli military. On the evening of September 16, 1982, the Phalangist militia, under the command of Elie Hobeika, entered the camps. For the next 36 to 48 hours, the Phalangists massacred the inhabitants of the refugee camps, while the Israeli military guarded the exits and allegedly continued to provide flares by night. A unit of 150 Phalangists was assembled at 4:00 p.m. These militiamen armed with guns, knives and hatchets entered the camps at 6:00 p.m. A Phalangist officer reported 300 killings, including civilians, to the Israeli command post at 8:00 p.m., and further reports of these killings followed through the night. Some of these reports were forwarded to the Israeli government in Jerusalem and were seen by a number of Israel's senior officials. Israeli soldiers surrounding the camps turned back Palestinians fleeing the camps, as filmed by a Visnews cameraman.

Later in the afternoon, a meeting was held between the Israeli Chief of Staff and the Phalangist staff. According to the Kahan Commission's report, the Chief of Staff concluded that the Phalange should "continue action, mopping up the empty camps south of Fakahani until tomorrow at 5:00 a.m., at which time they must stop their action due to American pressure." He claimed that he had "no feeling that something irregular had occurred or was about to occur in the camps." At this meeting, he also agreed to provide the militia with a tractor, supposedly to demolish buildings.

On Friday, September 17, while the camps still were sealed off, a few independent observers managed to enter. Among them were a Norwegian journalist and diplomat Gunnar Flakstad, who observed Phalangists during their cleanup operations, removing dead bodies from destroyed houses in the Shatila camp". The Phalangists did not exit the camps at 5:00 a.m. on Saturday as ordered. They forced the remaining survivors to march out of the camps, randomly killing individuals, and sending others to the stadium for interrogations; this went on for the entire day. The militia finally left the camps at 8:00 a.m. on September 18. The first foreign journalists allowed into the camps at 9:00 a.m. found hundreds of bodies scattered about the camp, many of them mutilated. The first official news of the massacre was broadcast around noon.