In June 1978, Prime Minister Begin, under intense American pressure, withdrew Israel's Litani River Operation forces from southern Lebanon. They were replaced by UNIFIL, a UN force to restore peace and help the Lebanonese government re-establish its authority, as authorized by UN Resolution 425. The withdrawal of Israeli troops without having removed the PLO from its bases in southern Lebanon became a major embarrassment to the Begin government, maintaining pressure for Israel to return. UNIFIL was unable to prevent terrorists from reinfiltrating the region and introducing new, more dangerous arms. Cross-border conflict between Israel and the various forces in Southern Lebanon continued at differing levels of intensity after 1978. Civilians on both sides, and UNIFIL peacekeepers, were killed as the fighting ebbed and flowed. Israel increased its support of the Lebanese Christian Militia in the south, under Major Saad Haddad, who regularly fought armed PLO fighters but also caused casualties among non-combatants. The US government during the Carter administration (1976-1980) had several times joined in UN condemnations of Israeli raids and reprisals in South Lebanon, always condemning simultaneously PLO terrorist cross-border activities (generally not condemned by the UN).
In July of 1981 Lebanese-American Philip Habib was sent by the Reagan Administration to negotiate a more lasting cease-fire between Lebanon and Israel. On July 24 Habib announced agreement that all hostile military action between Lebanese and Israeli territory in either direction would cease. For the next eleven months the cease-fire was in effect as a formality, but the PLO repeatedly violated the agreement. Israel charged that the PLO staged 270 terrorist actions in Israel, the West Bank and Gaza, and along the Lebanese and Jordanian borders. Twentynine Israelis died and more than 300 were injured in the attacks. In April 1982, after a landmine killed an Israeli officer, the rocket attacks and air strikes recommenced.
Israeli strikes and commando raids were unable to stem the growth of the PLO army which built camps, trained thousands of fighters, and stockpiled arms in south Lebanon. The situation in the Galilee became intolerable as the frequency of attacks forced thousands of Israeli residents to flee their homes or to spend large amounts of time in bomb shelters. Israel was not prepared to wait for more deadly attacks to be launched against its civilian population before acting against the PLO terrorists.
The final provocation occurred in June 3, 1982 when a Palestinian terrorist group led by Abu Nidal attempted to assassinate Israel's Ambassador to Great Britain, Shlomo Argov. The IDF subsequently bombed PLO bases and ammunition dumps in Beirut and attacked other targets in Lebanon on June 4-5, 1982. The PLO responded with a massive artillery and mortar attack on the Israeli population of the Galilee. It was the PLO shelling, and not directly the Argov shooting as is sometimes assumed, that triggered the Israeli invasion of Lebanon.
On June 6, 1982, under the direction of Defense Minister Ariel Sharon, Israel invaded Lebanon with a massive force, called Operation Peace for the Galilee, driving all the way to Beirut and putting the PLO and residents, as well as the Lebanese civilian population of that city, under siege. Israel justified its breech of the Habib cease-fire by citing the attempted assassination of the Israeli ambassador in London and a build-up of PLO armaments in South Lebanon. Israel was also concerned by increasing Syrian involvement in the Lebanese civil war and wanted to forestall a hostile, Syrian-backed government developing in Lebanon.
Former US Secretary of State Henry Kissinger said of the operation:
No sovereign state can tolerate indefinitely the buildup along its borders of a military force dedicated to its destruction and implementing its objectives by periodic shellings and raids. (Washington Post, June 16, 1982)