From Beirut 2020: Diary of the Collapse, by Charif Majdalani (Other Press, 2021), Kindle pp. x-xii (preface to the English-language edition, which provides very helpful context for the diary entries, which I will refrain from excerpting):
All this explains why the tensions between the large religious groups remained very strong, in particular because the constitution created in 1945 implicitly gave more power to the roles reserved for Christians than to those accorded to Muslims. The Muslims demanded reforms, but the Christians, fearing for their status and survival and continuing to believe that Lebanon was created for them, refused. Moreover, the Christians held great fears at the prospect of the rise in power and militarization of the Palestinian organizations that had sprung from the refugee communities in Lebanon in 1948, and that started demanding to play a role in internal Lebanese politics in 1969 and 1970. The strategy of these organizations consisted in giving their support to Lebanese Muslims. Faced with this coalition of Islamic-Palestinian interests, the Lebanese Christians took fright and armed themselves in turn, leading inevitably to the Lebanese civil war, which lasted from 1975 to 1990.
This was indeed a civil war, in that most of the fighting was between the Lebanese people themselves, but it was also very much a foreign war, because the Palestinians, Syrians, and Israelis were also involved. In 1982 the Palestinian militias were forced out of Lebanon by the Israeli invasion. But the Israelis had to evacuate the invaded Lebanese territories and confine themselves to the southern border regions adjacent to Israel. This opened Lebanon’s doors to the Syrians, who allied themselves with the Lebanese Muslims and Druzes, and with war chiefs such as the Druze Walid Jumblatt or the Shiite Nabih Berri, as well as with the Shiite Hezbollah organization, which was engaged in a war with Israel in the regions it still occupied. For their part, the Christians resisted the Syrians for years, under the command of men such as Bashir Gemayel and Samir Geagea. In 1989, the reckless and unruly Christian general Michel Aoun took it into his head to unite the Christian ranks, and threw himself into devastating wars against his rivals on the same side, notably Samir Geagea, which led to the collapse of the Christian camp in 1990 and to the entire country falling to Syrian control.
This marked the end of the civil war and the start of what is called the second Lebanese republic, which is divided into two eras. In the first, from 1990 to 2005, Syria dominated the country and its ruling class. The Muslim or Druze war chiefs, Jumblatt, Berri, along with the Hezbollah leaders, but also the less powerful Christian leaders who had pledged allegiance to the Syrians, all took over the controls. The other Christian leaders, such as Geagea and Aoun, found themselves respectively either in prison or in exile. The allocation of posts along religious lines was reinstated during this period, but with a notable difference: the dominant positions were given to Muslims and no longer to Christians.