Full-scale civil war broke out in Lebanon in April 1975. After shots were fired at a church, gunmen in Christian East Beirut ambushed a busload of Palestinians. Palestinian forces joined predominantly leftist-Muslim factions as the fighting persisted, eventually spreading to most parts of the country and precipitating the President's call for support from Syrian troops in June 1976. In fall of 1976, Arab summits in Riyadh and Cairo set out a plan to end the war. The resulting Arab Deterrent Force, which included Syrian troops already present, moved in to help separate combatants. As an uneasy quiet settled over Beirut, security conditions in the south began to deteriorate.
In March 1978, following a PLO attack on a bus in northern Israel, Israel launched Operation Litani, which was aimed at driving the PLO outside southern Lebanon. In response, the UN Security Council passed Resolution 425 calling for the immediate withdrawal of Israeli forces and creating the UN Interim Force in Lebanon (UNIFIL), charged with maintaining peace. Israeli forces withdrew later in 1978, turning over positions inside Lebanon along the border to a Lebanese ally, the South Lebanon Army (SLA) under the leadership of Maj. Saad Haddad, thus informally setting up a 12-mile wide "security zone" to protect Israeli territory from crossborder attack.
In 1981 heavily armed forces of the PLO occupied large areas of southern Lebanon and used it as a base to attack Israel with rockets and artillery. PLO militants fought with Lebanese forces, and killed many thousands of Lebanese citizens. Due to continued civil war since 1975, Lebanon had no effective central government at the time.