Three months after Israel completed the withdrawal of most of its troops from southern Lebanon, the Israeli-backed South Lebanon Army has defied skeptics and thus far largely held its own in the "security zone" along the Israeli-Lebanese border.

At the time of the Israeli pullout, there were deep divisions in the military command in Tel Aviv over the concept of leaving the security zone in southern Lebanon in the hands of a predominantly Christian Lebanese militia headed by retired brigadier general Antoine Lahad.

Critics of the plan predicted that the SLA would buckle within a month or two under pressure by Shiite Moslem guerrillas and that defections by Shiites would turn the militia into a skeleton force through which the Shiite Moslem Amal militia or Palestinian guerrillas could slip en route to attack Jewish settlements in northern Israel.

There have been defections -- exactly how many is in dispute between the SLA and Amal -- but the South Lebanon Army, for the time being at least, appears to be holding together more than the pessimists in the Israeli command anticipated.

This is due in a major way to the remaining Israeli troops, who are maintaining the same kind of balancing act between conflicting sectarian and political interests in the south as they did beginning with their first withdrawal from Lebanon seven years earlier.

In June 1978, the Israelis pulled back after their spring invasion of the south and turned a newly created cordon sanitaire over to a precursor of the SLA, led by a former Lebanese Army major, Saad Haddad.

Israeli Army officials say that about 60 Shiite members of the SLA have defected to Amal since June, most from a Shiite-dominated battalion in the irregularly shaped, four-to-10-mile-wide belt that runs from the Mediterranean Sea to the foothills of Mount Hermon. Lahad said that 50 more have resigned but continue to live in the zone.

But Israeli military officials said that the reorganized militia, which in June had dropped to between 1,200 and 1,500 men, was now 2,000 strong as a result of a recruiting drive, mainly among Christians and Druze. The SLA is about 80 percent Christian.

At least 11 SLA soldiers and two Israeli soldiers have been killed in the zone, most as victims of eight suicide car bombings. But Lahad said in an interview in his headquarters here that the defections were less a result of fear generated by the attacks than of relentless psychological warfare and financial inducements offered by Amal to its Shiite coreligionists.

An Israeli northern command general, who spoke on the condition that he not be identified, said that although attacks by Shiite militiamen against the security zone were daily occurrences, most involved ineffective artillery or rocket-propelled grenade firing, or guerrilla attacks that have, for the most part, been repulsed.

"I'm not sure there won't be more defections, but I'm not worried about it. The fight is for the minds. It has nothing to do with security," said the Israeli commander. He said the Shiite militias to the north regularly warn that Israel is preparing to withdraw completely from the security zone and that the time to change sides is now.

Lahad said the most serious guerrilla operations against the SLA have been by groups controlled by Syria, including the Syrian National Socialist Party and the Baath Arab Socialist Party, but he said that the Iranian-linked fundamentalist Hezbollah (Party of God) has been intensifying its activity along the northern edges of the security zone in a struggle for influence with the mainstream Amal.

Paradoxically, Amal, which replaced the Palestine Liberation Organization in dominating southern Lebanon as a result of the 1982 Israeli invasion, is viewed by Israeli military strategists as potentially the most important restraining influence against attacks on Israel -- provided it can control other Shiite factions in the area.

In June, Daoud Daoud, Amal leader in the south, made it clear that his forces would not attack the security zone from the outside but said that the SLA could expect to be the target of Shiite attacks from within.

But the Israeli northern command officer said that despite Amal's harsh restraint of Palestinian guerrillas, Israel could not afford to disband the SLA and trust Amal with security in the border strip because Amal leaders would not be able to withstand pressure from more radical splinter groups bent on infiltrating Israel.

As an alternative, the United Nations Interim Force in Lebanon (UNIFIL), whose mandate comes up for renewal by the U.N. Security Council next month, is attempting to convince Israel to let U.N. peace-keeping forces redeploy farther south into the security zone, as required by the mandate. UNIFIL and Israel have a long history of bad relations over southern Lebanon.

A diplomatic source close to the U.N. strategy said, "Our perception is that Amal is fully prepared to cooperate with UNIFIL -- certainly much more than the PLO was. By not allowing a better deployment of UNIFIL, the pressures put on Amal by Hezbollah and the Syrians are greater."

The source said that "local arrangements" recently made between UNIFIL and the SLA and Israeli Army in the security zone had permitted the UNIFIL Ghanaian battalion to move slightly farther into the zone.

But at the moment, Israeli military strategists say they see no choice other than to maintain the SLA as a buffer.

The Israeli Army maintains a liaison unit in Lahad's headquarters here and several observation posts in southern Lebanon -- and regularly mounts armored patrols inside the security zone, occasionally sending raiding parties north of the zone.

Recently the liaison office interceded after a local SLA military commander defected to Amal with eight of his men and an armored halftrack. An enraged senior SLA officer retaliated by confiscating a large herd of sheep owned by the commander's family and, noting that the defector had been paid 250,000 Lebanese pounds to go over, announced that he would sell the sheep to help pay for a new halftrack.

The liaison office acted to stop the sale, but it let the SLA officer blow up the defector's house to vent his anger and as a warning to other Shiite members of the SLA.

"It's a kind of balancing act," said a senior Israeli commander. "Otherwise you'll find them firing like they do in Beirut. Better to let them blow up a house than to destroy a whole area."

The Israelis also equip and pay the salaries of Lahad's forces -- support of which both Lahad and the Israeli commanders appeared acutely aware.

The senior northern command officer said he exercises control over SLA artillery batteries by threatening to take away ammunition.

Lahad, asked if his militia could survive without the current level of Israeli aid, replied, "We are definitely in need of the logistical backing of Israel." But he added, "Israel is helping us not because they like the pretty faces of the SLA soldiers. They have an interest, too."