Operating under the Palestinian Liberation Organization (PLO), the Palestinian resistance grouped a number of organizations with varied ideologies, each with its own leadership and military apparatus: Fatah, Popular Front for the Liberation of Palestine (PFLP), Democratic Front for the Liberation of Palestine (DFLP), Arab Liberation Front, as-Sa'iqa, PFLP - General Command, and the Palestine Liberation Army, the official military arm of the PLO.
In 1969 an agreement signed in Cairo between the Lebanese army and the PLO officially permitted Palestinian resistance while constraining it to Lebanon's southern border. Following the expulsion of the PLO from Jordan in 1970, the organization moved its headquarters to Lebanon. South Lebanon became an important base for military operations on the border with Israel. The resistance organizations benefited from the political support of left-wing and Arab nationalist parties and held Lebanese partisans among their ranks.
The PLO was a major actor in the war as it supported the combined forces of the LNM on many levels, lending financial support, military resources and training. Military factions of the PLO participated - unofficially - in the war fronts. However, Syria's intervention in Lebanon in 1976 and its pronounced discord with the PLO produced a different political set-up that rendered the PLO openly active in the armed conflict. Following the 1982 Israeli invasion, the PLO forces were evacuated from Beirut under the supervision of multinational forces.