Lebanon is a nation composed of many Arab minorities. The cultural diversity, the central location all contributed to its prosperity as long as the gentle balance was maintained between the various religious and ethnic sects - Shi'ite and Sunni muslims, Palestinian Refugees and Christian Arabs.
The situation changed sharply after 1970`, as King Hussein kicked Yasser Arafat and the PLO out of Jordan in a series of events called the Black September. The PLO needed a new place to stay, desirably a country bordering Israel, to continue its infiltration attempts and harassment. Syria jumped on the opportunity, and as soon as Lebanon was starting to become unstable, moved by the change of balance due to the PLO's "immigration", it moved in to "help impose order".
Soon afterwards, the terror started.
The former infiltration attempts from the Jordanian border were mostly thwarted by the IDF, due to a wide, unsettled buffer zone (The Jordan valley) that was effectively used to buy time for the IDF in each infiltration attempt. It would take a band of terrorists a few days at the least to reach any populated target, a time during which the Israeli Airforce was putting to use its then-new helicopters and Bedouin scouts in getting on their tracks, encircling them and eventually capturing them.
The situation on the Lebanese border was different. The Galilee was fully of border-line settlements that became the target of constant shelling, rocketing and infiltration attempts from across the border, the most notorious of which are the massacre in Ma'alot, in `76, and The Coastal bus takeover in mid-`78. The status on the only border in Israel known as "the good fence" was quickly changing.
Israeli retaliation became increasingly more aggressive, until in `78 Israeli P.M Menachem Begin ordered the launch of Operation Litani, a first large-scale incursion into Lebanon. The IDF swept across the hills and cleaned a buffer zone of about 10 kilometers inside Lebanon prior to withdrawing a few days later, to be replaced by UNIFIL troops.
This did not help much, however, and Israel increasingly started supporting the Lebanese Christian Militia in the south of the country.
In July 1981 Ronald Reagan's administration sent the Lebanese-American Philip Habib to mediate a cease-fire between the sides, an effort resulting in an 11-month relative cease-fire, interrupted by more PLO attacks and Israeli reprisals. Israel was waiting for an opportunity.
On June 3, 1982, a splinter group led by Abu Nidal seriously wounded Shlomo Argov, Israel's ambassador to the UK in an assassination attempt. Israel responded with a series of bombings of PLO targets, including the Beirut Stadium underneath which was a big PLO weapons cache. The PLO, which was increasingly growing in strength answered with a prolonged shelling of the Galilee, forcing residents to either flee or stay in shelters.
The invasion, operation Peace for Galilee started on June 6. It was a massive IDF armor drive towards Beirut. Heavy fighting erupted and Syria's forces stationed inside Lebanon interfered. Israel took a preemptive strike at Syrian SAM stations throughout Lebanon in southern Syria, as a lesson from the Yom Kippur war. It was highly effective and Israel gained its aerial superiority during one day of intense dogfights during which an amazing 100 MIGs were shot down, 24-30 stationary and mobile SAM sites destroyed and not a single IAF jet hit, making the battle pretty much lost for Syria.
During the subsequent days the IDF drove further north into Lebanon, encircling Beirut. Up to that point, the war was being very popular in Israel.
The biggest turning point, however was not a military one. The IDF gave the Maronite Christian Phalanges freedom of action, and a militia force, commanded by Elie Hobeika (who was recently mysteriously killed in a carbombing) entered the Sabra and Shatilla camps south of Beirut, and committed a well-televised massacre, as a revenge for the killing of the Maronites' leader days earlier, put up by Israel as the new Lebanese P.M.
After the massacre and the first losses, the war became widely unpopular amongst Israeli public, and constant protests were being held against P.M Begin and Security Minister Ariel Sharon.
The IDF took most of Beirut and started closing in on the entrenched PLO. Urban warfare was a new science to the relatively-new Israeli army, and doctrines were formed overnight to tackle the problem of high-rise buildings and sewage warfare. But due to international pressure both from abroad and domestic unrest, the time frame was not big enough to finish what was started. The PLO negotiated a deal to leave Beirut, under the crosshairs of Israeli snipers.
The same year the IDF withdrew from Beirut with US marines taking its place. By 1986 the IDF presence in Lebanon was reduced to a self-proclaimed Security Buffer going along the old border, about 30 kilometers wide.
The Americans withdrew as well after a series of deadly bombings by the new rising force, the Iranian-backed Hizbullah. Syria quickly took control.
A static, constant geurilla war was to be held for the following 15 years along the buffer zone between Israel, the South Lebanese Army allied with it and the various Islamic opposition organizations lead by the Hizubllah and Amal.
The biggest winner in these events is undoubtedly Syria, who is in charge of matters in Lebanon up to this very day. Israel had its first attrition war, and its own Vietnam Syndromes with it. The war undoubtedly had such a long-term effect on Israeli public that almost all of the ensuing public debate about the current state of warfare in the territories captured in 67' (Jeudea, Samaria and Gaza Strip) are viewed through the Lebanese "lense", ignoring the facts when the comparison is out of place.
Lebanon remained divided, in ruins and in a state of civil war until the final IDF retreat under Ehud Barak. Israel is hated there, as a contrast to the joy with which it was accepted during 1982 by many.
liveforever says re 1982 Lebanon Invasion: Trivia, but worth a mention: The Israeli plan to capture West Beirut was named Operation "Moah Barzel", which translates as "Iron Brain".