The Palestine Liberation Organization (PLO) were based in Lebanon for a significant period of time (1960s-1982), using their set-up in the country to expand as an organization, gathering support and maintaining their armed struggle with Israel. Arguably, the Lebanon period was the most significant time in PLO's existence, both for reasons of political gain and international recognition – though it also involved a great deal of violence, displacement of civilians and economic instability. PLO were able to maintain a strong presence, particularly in Southern Lebanon for a number of years and at times were able to have a positive impact on the local population, but due to religious tensions and a confusion of structure were often the cause of dissatisfaction and fear amongst Lebanese citizens.
The origins of PLO's presence in Lebanon goes right back to the late 1960s, following the 1967 Six-Day War. There were already large areas of Lebanon populated by Palestinian exile communities, or around 12% of the total population. Lebanese president Charles Helou was anxious about the prospect of nationalist sentiment causing turbulence in Lebanon following the war, which had ignited mass support from Arab communities of the Palestinian cause. Nevertheless, guerrilla networks were increasingly active throughout the late 1960s and were given even more freedom after the Cairo agreement in 1969, in which refugee camps in Lebanon were placed under the command of Palestinian forces, instead of the heavy-handed Deuxième Bureau.
The establishment of PLO as being based wholly in Lebanon came after king Hussein of Jordan's decision to expel the group from his country in 1970. The Black September movement which formed as the result of the hijacking of planes, which were brought into Jordan. Much of 1970s Lebanon exists in history under the shadow of the lead-up to and subsequent bloody realization of the civil war, which PLO became embroiled in after initial reluctance of any involvement. It was not in the interests of PLO to get involved in a conflict which would drain resources and detract focus from the goal of planning insurgencies against Israel.