It's been forever since I've posted in /r/AskHistorians

So you're asking wtf went down? Not completely my area of expertise, but I did gloss over it while studying. My focus was on Central Asia in college, but I took a few islamic/arabic studies classes that lead to me reading a few books on the matter. Here is what I got:

Background/tl;dr

So, Lebanon is/was made of several tribal factions. The major groups are the Maronite Christians, the Druse, and the Shi’ite and Sunni Muslims. When Lebanon was first created, the majority of the population was Christian. For this reason, the Christians were given majority control of the government. In an unwritten agreement known as the National Pact, the Lebanese President would always be a Maronite. Parliament would have a 6:5 ratio of Christians to Muslim. The Prime Minister a Sunni Muslim and the Speaker of the Parliament a Shiite.

This tricky balance was first tested in the Lebanese Crisis of 1958, when the President of Lebanon had decided to maintain friendly terms with western nations despite their attacks on Egypt, their Arab compatriot by way of the National Pact, over the Suez Canal. Christians supported the president and his pro-western views while Muslims felt betrayed by the President’s crossing of the National Pact’s agreements. Tensions arose and a civil war seemed imminent until the United States relieved pressure by a display of force in the form of deploying troops in order to scare the Muslims into submission. Although a confrontational plan, it worked and the Muslims felt satisfied when the President stepped down from office; he was replaced by Fuad Chehab, a moderate, who was the last president to successfully placate the Christians and Muslims before the civil war that erupted in 1975.

The make up of the population had shifted drastically by the late '60s due to the Arab-Israeli wars and other conflicts. These led to a lowering of the percent of the population that was Maronite to approximately 30%. The Muslims, wanting equal or majority power, demanded reforms, while the Maronites wanted to hold onto what was promised to them on the outset of the creation of Lebanon.

Arguably the final nail in the imbalance coffin: In 1970 the PLO gets kicked out of Jordan and they pour into Lebanon. Even more imbalance in the original ratio and Arab nationalists have even more power. This led to the hiring of private militias. The entire country soon became enveloped in what is essentially a gang war over territory. Not a true war in the classical All Quiet on the Western Front sense, but sporadic assassinations and killings here and there.

I highly suggest you read From Beirut to Jerusalem if you have the time for a 500pg behemoth. Interesting read written by a journalist rather than a historian. Biased? yes, but not at all dry. Also, watch Waltz with Bashir. Fantastic animated documentary.

That's the beginning and that's it in its simplest form. The civil war turns into a complete shit show from here. I suggest you only read the rest if you have 30+ minutes of free time. The top is my TLDR and the rest I'm taking from a paper I wrote about 5 years ago where I compare multiple books on the subject. If you have specific questions or want a specific answer, just reply to one of my posts and I'll get back to you.

The following are the books I will cover in the next two posts:

Abraham, A. J. The Lebanon War. Westport, Conn.: Praeger, 1996.

Gilmour, David. Lebanon, the fractured country. 1st Edition. ed. Oxford: M. Robertson, 1983

Hanf, Theodor. Co-Existence in Wartime Lebanon: Decline of a State and Rise of a Nation. illustrated edition ed. London: I. B. Tauris, 1994.

Johnson, Michael. All Honourable Men: The Social Origins of War in Lebanon (I.B.Tauris in Association With the Centre of Lebanese Studies). New York, N.Y.: I. B. Tauris, 2001.

O'Ballance, Edgar. Civil War in Lebanon, 1975-92. New York, N.Y.: Palgrave Macmillan, 1998.