What was the main cause of the Lebanese civil war?

The Lebanese Civil War had a complex series of events and factors that led to its beginning. However, a simplified version of what occurred was that there was an fragile balance of power between militant groups from the Sunni Arabs and Shi'ite Arabs as well as the Maronite Christians. When the Palestinian Liberation Organization was expelled from Jordan it further unsettled the balance of power in Lebanon causing widespread conflict between these groups. In addition external powers in the region like Syria and Israel had an influence in fomenting conflict in Lebanon during this period as well.