I wrote this almost two years ago but have updated for various reasons. But I think it is a solid background to what is happening in. Lebanon today.
The main point this; the Shi’a Hezbollah ,heavily dependent on Iran, has the carried the war for the Iranian regime against the United States since 1979. They fought for the Syrian regime, forming the major part of the Iranian regime to keep Assad in power.
Today they constitute a state within a state, controlling every important aspect of this small but unique country. The Christian population continues to leave and the Sunni Muslim population subdued. The Hezbollah is totally corrupt, ruled by mega rich Shi’a Imams who manipulate the population by threat of violence, cash and usual Arab largesse to relatives and friends.
With that read this brief look at Lebanese history

Michael Aoun the Chameleon
The recently elected president might be considered the model Arab Politician. He has survived assassination attempts, multiple coups, exile, and has betrayed every political ally he had. He was formerly in the pay of Saddam’s Iraq, a commander of the Lebanese army that turned on his fellow Christian allies, the Phalange, and despite being driven from his Presidential palace by Syrian troops, has found common cause with them and their Hezbollah allies to once again occupy the Presidential palace. No doubt his supporters will say that he has done these acrobatic acts of political loyalties to protect the diminishing Christian population, which no longer holds the position of power in Lebanon. This is true. The Christian population, the Maronites to be specific, need Hezbollah support, and that of the Assad Government of Syria to survive. This is ironic because for many years the Christians and Sunnis allied themselves to confront the growing Shi’a power. For decades, centuries actually, the Shi’a were powerless and considered a rather low class peasant society in the rigid caste system of Lebanon. The Maronites (and still do) considered themselves as Lebanese not Arab, and spoke French. They abjured the Arab world in which they lived. With the advent of the Shi’a revival and the radical Sunni Islamic resurgence they could no longer do this. To write anything about Lebanon one needs a score card.
First Some terms you have to know
Phalange or Kata’ib a Maronite Christian militia patterned after the Fascist Black shirts of Mussolini’s Italy. They represented themselves as the saviors of the Christian population of Lebanon. Their core supporters do not consider themselves as Arab and frequently refer to themselves as Phoenicians or simply as Lebanese.However, generally speaking ,the Christian Greek Orthodox do consider themselves Arab and rarely allied with the Maronites. The same could be said of the Armenian Christians who simply tried to stay out of the cross fire.
PLO Palestinian liberation organization made up of about 12 different organizations, including the Palestinian Front for the Liberation of Palestine ( PFLP) the Democratic Front for the Liberation of Palestine ( PDFLP) and Fatah, the largest and under the PLO leader Yasir Arafat. They were all nominally under Arafat but the PFLP and PDFLP, both led by Christian Palestinians, often did their own thing. Other organizations of the PLO were under State control sich as al Sa’iqa i under the Syrians, and the Arab Liberation Front (ALF) under Iraqi control.
Lebanese Forces. A amalgamation of Christian forces, including the Phalange, which turned against Aoun in the civil war and was largely destroyed by him using regular army troops under his command. It was Maronite on Maronite
Amal the original Shi’a militia which fought against the Palestinians, the Christians, and later Hezbollah. They were particularly brutal in their war against the Palestinians, who had totally alienated the Shi’a population of the south, acting as occupiers after being initially welcomed by them.
Hezbollah.The Shi’a liberation/Terrorist organization which fought against their rivals the Amal and destroyed them, becoming the Shi’a power in Lebanon. Later they became the primary antagonists against the Israelis. Initially, ironically, the Israelis, like the Palestinians, were welcomed in 1982 as they rolled into what had become known as “fatahland”. They destroyed the Palestinian infrastructure in southern Lebanon but unfortunately the Israeli heavy handed occupation forces like the PLO before alienated the Shi’a people and resulted in continual warfare and their eventual ouster from Shi’a land.
South Lebanese Army (SLA). Was a Christian officered Maronite-Shi’a organization which split from the regular Lebanese Army and was equipped by the Israelis as they withdrew from Lebanon. It was later overcome by Hezbollah and no longer exists.
Maribitoun A Sunni Nasserist militia allied with the Palestinians. They were mostly destroyed by the Shi’a Amal in the late 80’s
UNIFIL. United Nations Interim Force in Lebanon. A multi national force charged with keeping the Peace in Lebanon, a mission in which they have completely failed. When wars or confrontations begin, they generally just get out of the way. It seems their only use is to obtain funds for the countries furnishing troops. It has never been popular with the Israelis who have accused some of the forces involved of cooperating with the PLO and Hezbollah. Example.They said the Irish contingent was selling arms to the PLO.
DPSP. The Druze Progressive Socialist Party. A Druze militia that was neither progressive or socialist but a militia under the Druze warlord, Walid Jumblatt. They humbled the Christian, mostly Christian Lebanese army when President Jemayel attempted to subdue the Druze of the Shouf region of Lebanon. The many Christians living in the region were driven from their homes and have not returned. Many years later the Druze took on Hezbollah and were in turn humbled in 2008 when Hezbollah took over West Beirut and parts of central Lebanon.
The U.S Marines After the massacre of Palestinians by the Shi’a militia, President Reagan ordered US troops back to Lebanon. Ironically it was the US troops who protected the PLO leadership from further attacks by the anti Palestinian forces. Arafat sailed away to Tunisia . Unfortunately for the US, the US forces became involved in the multi-sectarian war in Lebanon and ended up being attacked by Shi’a and Druze forces. The tragedy of the Marine barracks destruction and the French paratroop barracks in 1983 prompted the US president to withdraw from Lebanon. The attacks were Hezbollah executed attacks nd supported by the Iranians. Added to this score card one can add other militias with ideological programs or warlords like the Maronite Suleiman Frangieh of north Labanon who was a personal friend of Hafez Assad (When Frangieh was kicked out of Lebanon he was provided protection by Assad) and in fact invited the Syrians to intervene in 1976.
The Syrian Forces. Invited in By President Frangieh, who also had his own militia, and fought against the primary Maronite forces under Bashir Gemayel. Initially the Syrians destroyed the Christian Maronite forces and then when challenged by the PLO, destroyed them as well. Under great pressure the Syrians forces withdrew in 2005. Of course with their allies, the Hezbollah, the Syrians still have great influence in Syria supported by the ambitious Iranians.
The French carved Lebanon out of Syria creating a majority Christian Maronite enclave with the idea that the Christian Lebanon would be happy to remain part of the French Empire. However there was over reach on the part of the French and Maronites in including the Bi’qa, the territory east of the mountains into Lebanon. The Bi’qa was and is heavily Shi’a Muslim in composition

Syria 1920
When the Germans overran France in 1940, the French government in Syria and Lebanon were loyal to the Vichy French government, the pro German government authority in France. French troops loyal to General De Gaulle and the allied cause ejected to Vichy forces in 1941.

British troops enter Lebanon 1941

US Marines in Beirut
The awakened forces of Arab nationalism and communist agitation creates upheavals all over the Middle East as the pro western rulers are ejected and removes by revolutionary forces inimical to Western interests. Lebanon was no exception. Pro Arab, mostly Sunni Arabs clash with the Lebanese army controlled by the Maronite Christians. American marines land on the beaches near Beirut.

Palestinian refugees in Lebanon
About 200000 Palestinians are in Lebanon in a number of refugee camps. They have never been welcomed and never offered citizenship. Many of the Palestinians fled their homes in 1948 during the first Arab Israeli war and others in 1967. Many more arrived after the Jordanian army kicked the PLO out of Jordan, The PLO then begin forays into northern Israel, attacking settlements, bringing Israeli reprisals, a pattern which has continued till today when Hezbollah took over the harassment role.
The agreement signed was a disaster for Lebanon as the Palestinians did not adhere to their part of the agreement, and armed Palestinian fighters soon began to appear on the streets of Beirut. This was despite the fact that the first armed confrontation( in 1968) between the Palestinians and the Lebanese army was won by the Lebanese army. However under the pressure from the all powerful Egyptian president Abdul Nasser and his security apparatus in Lebanon, the Lebanese caved in to Egypt’s demands, and the path from southern Lebanon into Israel, known as the Arafat trail, became the primary conduit for the Fedayeen ( PLO) attacking Israeli settlements and towns.
The presence of so many Palestinian fighters in Lebanon was an impossible situation bound to boil over and it did. At first it was a primarily Maronite Lebanese – Palestinian war but as it dragged on it took on the appearances of a Christian- Muslim war. In reality, as usual in the Middle East, it was not that simple. The most bloody part of the war for Palestinians. was the “war of the Camps” in 1984 to 1990, in which Shi’a fighters of the Amal organization besieged and finally over ran the Shatila and Sabra refugee ramps killing many Palestinians. These camps were the same ones in which scores of Palestinian civilians were murdered by Phalangist militia in September 1982.…a reprisals for the murders of Maronite Christians in the city of Damour in January 1976. NOTE. In the Middle East the concept of revenge and reprisal makes placing blame irrelevant, like deciding which came first, the chicken or the egg, Revenge can exacted for events decades ago.

Israeli invasion 1978

Yasir Arafat leader of the PLO and previous tormenter of Lebanon
By 1993 Hezbollah has attained ascendancy over its rival Shi’a Amal and has begun launching rocket attacks on Israel, Israel again invades Lebanon to end the threat from remnants of the PLO and the more potent Hezbollah militants.
Although many in the Arab world took heart from the dedicated and strong defense by Hezbollah in 2006 against Israel, It brought havoc to the Lebanese. The Lebanese have suffered from the wars of the Palestinians and the Hezbollah against Israel but do not have the armed might to stop them..
Finally as Hezbollah and various corrupt politicians achieved total control of Lebanon, we just had an example of a country which does not have s control of its own destiny, we have the mammoth Beirut Port explosion. It is clear that it was an explosion of Ammoniun Nitrate, much the same as the cargo that devastated Texas city in 1947, killing 500 people.

Sayyed Nasrallah…. the Secretary General of Hezbollah and sometimes Cleric The new tormentor of Lebanon
It was an accident and a disaster perpetrated by the negligence and corruption of the Iranian Hezbollah overseers of the port operations. Supposedly it had been there since 2016. Why? Not for fertilizer!! Undoutedly A stock for future Hezbollah use against Israel or other targets desired by Iran. Iranian incompetence and carelessness of human life is exemplified by the shoot down of a Ukrainian passenger jet earlier this year., sinking one of their own ships with a missile and numerous other mysterious events, which they always blame on Israel.