Having ventured into the political and religious quagmire of south Lebanon, Israel is now confronting a new political force on which it had not reckoned.
This force is the Shiite Muslims, adherents of the second-largest sect in Islam. It is a once passive but newly self-confident community that makes up 70 to 85 percent of the sensitive southern Lebanon sector, where Israel wants to set up a demilitarized zone.
Israel always banked on its Lebanese Christian allies to dominate all Lebanon once the Palestine Liberation Organization was driven out. Israeli leaders - and academic experts - had little knowledge of the Shiites before the invasion of Lebanon, although sect members comprise nearly one-third of the country's population.
But Israel, whose quick departure from southern Lebanon seems unlikely, may have to learn in a hurry. For the Shiites - having emerged over the past seven years of internal Lebanese strife from the most downtrodden segments of the population into a self-conscious, well-organized force - are pressing for greater power in Lebanon.
Resentment is already building in the south as Shiites, disarmed by Israel, watch armed Christian militias allied to Israel trying to move into areas the Shiites consider theirs.
The effective leader of the Shiites in the south is Mohammed Ghaddar, who lives in a comfortable two-story house inside a well-protected compound in Ghaziye, south of Sidon. Mr. Ghaddar is commander of military forces and head of social services in the south for Amal, the social and military movement of the Shiites. An example of Shiite upward mobility, he runs a large family construction-supply business, which was built from scratch by his father, who sent his four sons to receive college educations in the United States.
''In the 1960s, Lebanese joked that the shoe polishers and the porters were Shiites,'' says Ghaddar, sitting in a neat beige safari suit in his garden, where men of neighboring villages come in droves to call.
''Now we have doctors, engineers, real estate, sons studying in the West,'' he continues. ''The people are starting to feel Shiite pride.''
A key vehicle for developing that pride was Amal (hope), founded less than a decade ago by the Imam Musa Sadr, the Tyre-based spiritual leader of Lebanon's Shiites, who disappeared mysteriously while on a visit to Libya in 1978. Amal was created both to protect Shiites during the 1975-76 Lebanese civil war and to press for greater political and economic rights for the Shiites.
The Lebanese system, which awards political office by religious sect, left the Shiites underrepresented in parliament, the bureacracy, and the military hierarchy, and dependent economically on a handful of feudal landowning Shiite partriarchs.
Initially Amal found in the PLO and its Lebanese leftist allies a backer to help wrest power away from the dominant Maronite Christians. But that alliance soured as Shiite villagers in south Lebanon complained of arrogant, abusive treatment by PLO fighters. It worsened when PLO ally Libya was accused of kidnapping Imam Sadr and grew more bitter after Iraq invaded Iran, sparking fighting between PLO backers of Iraq and Amal which identifies religiously - though to a less fundamentalist degree - with Shiite Iran.
But the loss of the Imam and the rise of Shiite Ayatollah Ruhollah Khomeini - whose government in exile may have aided Amal - boosted Lebanese Shiite self-sconsciousness.
The PLO-Shiite relationship worsened as Shiite villagers near PLO bases became the innocent victims of Israeli retaliatory air and mortar attacks for PLO raids.
''We understand that the Palestinians have rights,'' says Mr.Ghaddar, ''but not at the expense of our people. They would kill one Israeli and the Israelis would hit 100 Shiites.''
Amal branches in every southern Shiite village collected money for arms and waged a series of bloody battles to keep the PLO out of Amal villages. These battles continued up to the time of the Israeli invasion.
''In June 1980 our house was attacked,'' says Ibrahim Ghaddar, Mohammed's San Francisco State-educated brother, who sports a baseball cap and brought his Baltimore-born bride back to Ghaziye.
''One of our men died and 20 were injured,'' Ibrahim says. He casually asks visiting neighbors to display their scars. In May 1981, Mr. Ghaddar's business was bombed with 25 kgs of TNT.
He survived when a section of concrete ceiling fell at an angle forming a protective wedge. Amal lost 380 men to the PLO in battles in the south.
The arrival of Israeli forces left Amal in a delicate position. Amal forces in Beirut, under the leadership of national Amal chief Nabih Berri, fought against Israeli troops around Beirut. But Mr. Ghaddar gave his men orders not to fire on Israelis nor give PLO men shelter.
In south Lebanon, the Shiites are both acutely aware of the need to come to terms with Israel's proximity and anxious not appear as agents of Israel.
''We are not for Israel, but not against her,'' says Mr. Ghaddar. ''No one else, no Arab government, could have driven out the PLO.''
But resentments are already brewing at suspicions that Israel wants to encourage Christian domination of the Shiite south, with Christian militias serving as a surrogate security force when Israeli troops withdraw.
Israel has disarmed Amal chapters throughout the south (though Ghaziye was allowed to keep, but not carry, some of its arms because of its well-known anti-PLO record). Israel arrested 180 Amal members in the south when PLO formers , bent on revenge, fingered them, Shiite villagers say.
At the same time, Maj. Saad Haddad - since 1978 the Christian leader of an Israeli-armed enclave just north of Israel's border - is trying to expand his influence throughout south Lebanon. Haddad's armed men are entering Shiite villages and offering weapons to locals who will follow him. Amal members charge that those who take the arms are riffraff, leftists, or local villagers out to settle old scores.
Amal members say the situation may explode if Major Haddad is not curbed. ''We are much better qualified to keep the PLO from returning to the south than is Saad Haddad,'' says one Ghaziye resident scornfully.
Israeli-Shiite relations will depend heavily on Israel's long-range intentions in Lebanon, about which Shiites are uncertain and uneasy. Despite some political and religious divisions in Amal, the Shiites have high hopes for boosting their influence and power in a new Lebanon. They have more to gain than any other sect from a united Lebanon, with a strong central Lebanese Army. Such an army they might ulimately dominate by sheer force of numbers. Last month Nabih Berri said the Shiites were entitled to Lebanon's presidency, now alloted to the Maronite Christians.
A coalition could one day emerge between Maronites and Shiites at the expense of the mainstream Sunni Muslims who long dominated Muslim politics in Lebanon. But the Shiites are unhappy that the Israelis are backing tough Maronite militia leader Bashir Gemayel to become president. Southern Shiites say Mr. Gemayel is too aggressive, his people too ''power hungry,'' and his public talk of ''one Lebanon'' countered by a private desire for Christian domination.
Several ugly incidents between armed men from Mr. Gemayel's Phalangist militia - newly arrived in scattered Christian villages in the South - and unarmed Amal members have also raised Shiite tempers.
''Nobody knows the intentions of Israel,'' complains Mohammed Ghaddar. ''They say they want a strong Lebanese central government, but then they disarm units of the Lebanese Army. They let the Christians carry arms, but they stop us. They are helping one group against another group.''
The importance of the Shiites as a political force is just beginning to penetrate the higher echelons in Israel. Some Amal men have recently been released from detention. But Israel views Bashir Gemayel as a key ally, and Saad Haddad is a known force whom they have long supported.
The Shiites provide a tricky challenge, possible pragmatic allies as enemies of a common PLO enemy, but potential trouble for the Israelis if they stay on in Lebanon.
''We are against all foreign forces in Lebanon,'' says Mr. Ghaddar, ''the PLO , the Syrians, and the Israelis.
''After Beirut is finished, whether by war or diplomacy, then the Israelis should leave Lebanon,'' he adds. ''We will fight the invasion if they stay. We Shia will be the real protections of Lebanon and true peace will protect both us and Israel.''