What events led to the start of the Lebanese Civil War in 1975?

The Lebanese Civil War (1975–1990) was rooted in the deterioration of the Lebanese state and the inability of its security apparatus to maintain a monopoly on force amidst rising social and political tensions.12

Key factors leading to the conflict included:

• Political and Social Polarization: The presidency of Suleiman Franjieh (beginning in 1970) saw a weakening of state security, which failed to manage the growing presence of Palestinian guerrillas. This deepened the divide between the Maronite Christian community, represented by the Phalangists, and the predominantly Muslim Lebanese National Movement (LNM).23
• Militia Emergence: As the state weakened, various sectarian-based militias formed to provide security, effectively challenging the state's authority.12
• The Catalyst: The war is typically dated to April 13, 1975, when Phalangists attacked a bus carrying Palestinians to a refugee camp at Tell al-Zaatar. This incident escalated intermittent communal violence into a general conflict between the Phalangists and the LNM.423