The Sabra and Shatila massacre (also known as the Sabra and Chatila massacre) was the killing of between 460 and 3,500 civilians, mostly Palestinians and Lebanese Shiites, by the militia of the Lebanese Forces, a Maronite Christian Lebanese right-wing party, under the command of Elie Hobeika, in the Sabra neighborhood and the adjacent Shatila refugee camp in Beirut, Lebanon. President Bachir Gemayel had been assassinated two days earlier and the Phalangists sought revenge. From approximately 18:00 on 16 September to 08:00 on 18 September 1982, a widespread massacre was carried out by the militia, while the Israeli Defence Forces (IDF) had the camp surrounded. The militia had been ordered by the IDF to clear Palestine Liberation Organization (PLO) fighters out of Sabra and Shatila, as part of the IDF's maneuvering into West Beirut. As the massacre unfolded, the IDF received reports of atrocities but did not take any action to prevent or stop the massacre.In June 1982, the IDF had invaded Lebanon with the intention of rooting out the PLO. By 30 August 1982, under the supervision of the Multinational Force, the PLO withdrew from Lebanon following weeks of battles in West Beirut and shortly before the massacre took place. Various forces Israeli, Lebanese Forces and possibly also the South Lebanon Army (SLA) were in the vicinity of Sabra and Shatila at the time of the slaughter, taking advantage of the fact that the Multinational Force had removed barracks and mines that had encircled Beirut's predominantly Muslim neighborhoods and kept the Israelis at bay during the Beirut siege. The Israeli advance over West Beirut in the wake of the PLO withdrawal, which enabled the Lebanese Forces raid, was in violation of the ceasefire agreement between the various forces. The Israeli Army surrounded Sabra and Shatila and stationed troops at the exits of the area to prevent camp residents from leaving and, at the request of the Lebanese Forces, fired illuminating flares at night.According to Alain Menargues, the direct perpetrators of the killings were the "Young Men", a gang recruited by Elie Hobeika (the Lebanese Forces intelligence chief and liaison officer with Mossad), from men who had been expelled from the Lebanese Forces for insubordination or criminal activities. The killings are widely believed to have taken place under Hobeika's direct orders. Hobeika's family and fiance had been murdered by Palestinian militiamen and their Lebanese allies during the 1976 Damour massacre, itself a response to the Karantina massacre of Palestinians and Lebanese Muslims at the hands of Christian militants two days before. In all 300400 militiamen were involved, including some from Sa'ad Haddad's South Lebanon Army.In February 1983, a commission chaired by Sen MacBride, the assistant to the UN Secretary General and President of United Nations General Assembly at the time, looking into reported violations of International Law by Israel concluded that Israel, as the camp's occupying power, bore responsibility for the violence. The commission also concluded that the massacre was a form of genocide.In February 1983, the Israeli Kahan Commission, appointed to investigate the incident, found that Israeli military personnel, aware that a massacre was in progress, had failed to take serious steps to stop it. The commission deemed Israel indirectly responsible, and Ariel Sharon, then Defense Minister, bore personal responsibility "for ignoring the danger of bloodshed and revenge", forcing him to resign.

The 1982 Lebanon War, dubbed Operation Peace for Galilee (Hebrew: מבצע שלום הגליל, or מבצע של"ג Mivtsa Shlom HaGalil or Mivtsa Sheleg) by the Israeli government, later known in Israel as the Lebanon War or the First Lebanon War (Hebrew: מלחמת לבנון הראשונה, Milhemet Levanon Harishona), and known in Lebanon as "the invasion" (Arabic: الاجتياح, Al-ijtiyāḥ), began on 6 June 1982, when the Israel Defense Forces (IDF) invaded southern Lebanon, after repeated attacks and counter-attacks between the Palestine Liberation Organization (PLO) operating in southern Lebanon and the IDF that had caused civilian casualties on both sides of the border. The military operation was launched after gunmen from Abu Nidal's organization attempted to assassinate Shlomo Argov, Israel's ambassador to the United Kingdom. Israeli Prime Minister Menachem Begin blamed Abu Nidal's enemy, the PLO, for the incident, and used the incident as a casus belli for the invasion.After attacking the PLO – as well as Syrian, leftist, and Muslim Lebanese forces – the Israeli military, in cooperation with their Maronite allies and the self-styled Free Lebanon State, occupied southern Lebanon, eventually surrounding the PLO and elements of the Syrian Army. Surrounded in West Beirut and subjected to heavy bombardment, the PLO forces and their allies negotiated passage from Lebanon with the aid of United States Special Envoy Philip Habib and the protection of international peacekeepers. The PLO, under the chairmanship of Yasser Arafat, had relocated its headquarters to Tripoli in June 1982. By expelling the PLO, removing Syrian influence over Lebanon, and installing a pro-Israeli Christian government led by President Bachir Gemayel, Israel hoped to sign a treaty which Menachem Begin promised would give Israel "forty years of peace".Following the assassination of Gemayel in September 1982, Israel's position in Beirut became untenable and the signing of a peace treaty became increasingly unlikely. Outrage following the IDF's role in the Phalangist-perpetrated Sabra and Shatila massacre of Palestinians and Lebanese Shias, as well as Israeli popular disillusionment with the war, led to a gradual withdrawal from Beirut to the areas claimed by the Free Lebanon State in southern Lebanon (later to become the South Lebanon security belt), which was initiated following the 17 May Agreement and Syria's change of attitude towards the PLO.

After Israeli forces withdrew from most of Lebanon, the War of the Camps broke out between Lebanese factions, the remains of the PLO and Syria, in which Syria fought its former Palestinian allies. At the same time, Shi'a militant groups began consolidating and waging a low-intensity guerrilla war over the Israeli occupation of southern Lebanon, leading to 15 years of low-scale armed conflict. The Lebanese Civil War would continue until 1990, at which point Syria had established complete dominance over Lebanon.