Why were East Beirut and West Beirut associated with different religious communities during the civil war?

During the Lebanese Civil War (1975–1990), Beirut became a divided city, split by the Green Line into Christian-controlled East Beirut and Muslim-controlled West Beirut. This division reflected deep-seated sectarian tensions that had been exacerbated by rapid urbanization, socioeconomic disparities, and the influx of Palestinian refugees following the 1948 Arab-Israeli war.1234

As the city grew, it failed to achieve a balanced integration of its diverse groups. Migrants from rural areas, who were often underprivileged and mainly Muslim, settled in impoverished areas surrounding the city, creating a "poverty belt" that intensified social and political polarization. These tensions intersected with existing confessional grievances, leading to a situation where each quarter of the city increasingly took on a distinct religious affiliation.4

When open hostilities erupted in 1975, these sectarian loyalties solidified into geographic control. Militias aligned with these communities took charge of their respective sectors, turning the city into a battleground where the Green Line served as a dangerous barricade separating the factions. This division was further entrenched by the presence of armed groups, including the Palestine Liberation Organization, which found common cause with Lebanese Muslims and leftists.124