Israel's symbiotic relationship with the Christian militias in southern Lebanon -- and its seemingly inexorable military entanglement there -- began, in a sense, with an anonymous note left in a chain-link fence on the northern frontier one night in 1976.

The next morning, an Sraeli Army patrol found the note. It set into motion first a meeting between the Israelis and an obscure Lebanese Christian sergeant-major and then a medical evacuation into Israel of soldiers and civilians wounded in the fighting that was raging among the Christians and Palestinians and Moslem leftists in Marjayoun.

Although Israel had periodically attacked Palestinian guerrillas inside Lebanon since 1968, it was on this humanitarian note that Israel began formally to cement its relationship with the Christian militias and thus thrust itself into a series of events that made it a central figure in the continuing tragedy of Lebanon.

By all appearances, the spirit of humanitarianism -- which Israel's political and military leaders invoke to this day as justification for waging sporadic war on sovereign Lebanese soil -- has led Israel into the same kind of colonial trap of which it relieved Britain when it obtained independence in 1948.

Israel's presence in southern Lebanon, both through its own troops and through the surrogate Christian Lebanese force led by Maj. Saad Haddad, has evolved into a cycle of violence that in turn has hardened the Israeli resolve to remain there and crush the resistance.

But as it was for the British in Palestine, the Israeli Army's capacity to assert its military superiorityh once and for all has been constrained by international political considerations.

Waving his hand casually at a huge map of southern Lebanon in the Army's northern command headquarters in Nazareth, a top-ranking Israeli general said self-assuredly: "From a military point of view, there is no problem getting rid of all the Palestinian terrorists in the area. But the politicians determine policy when altimate control of the military is the responsibility of civilian government."

The general, who, under Israeli regulations cannot be identified, also noted the presence of 6,000 U.N. peacekeeping troops [UNIFIL] positioned between the narrow, 60-mile-long Christian enclave and Palestine Liberation Organization guerrilla concentrations north of the Litani River.

"We know exactly where the PLO are. We have very, very good intelligence. We can get rid of them in a matter of hours. We have enough equipment and tools. The reason we are not doing it is that UNIFIL is in the area. UNIFIL would suffer casualties, and we don't want one finger of one UNIFIL soldier to get hurt," he said.

Despite the general's concern, the relationship between Israel and the U.N. forces has been strained ever since the Israeli Army completed its withdrawal from Lebanon three months after the March 1978 invasion and turned the area over the Haddad instead of U.N. troops in hopes of creating a kind of cordon sanitaire along the northern border in which it would have some control.

Since January 1979, Israel has officially considered the whole south -- and anywhere else in Lebanon where Palestinian guerrillas maintain an armed presence -- a free-fire zone for their preemptive strike policy.

No longer does Israel limit its strikes to purely military targets or act on a strict retaliatory basis. No longer does a Palestinian act of violence need to be directly linked to Lebanon to prompt a reprisal there.

A grenade in theory need not go on in an Israeli bus in Tel Aviv or Jerusalem for the 200,000 residents of war-swollen Sidon on the southern Lebanese coast to be punished by shelling from-range, often U.S.-supplied artillery. American-made warplanes fly missions to ensure electronic protection against hostile ground-to-air fire.

Former Israeli foreign minister Moshe Dayan has publicly taken issue with this policy.

The U.S. government except from August 1979 to April 1980 has not sought actively to dissuade Israel from using U.S.-supplied arms against Lebanon, despite an apparent bar to this use under the 1952 law regulating U.S. weaponry supplied israel.

Israel often encourages -- or by its account tolerates -- Haddad's use of his Israeli-provided artillery against both military and civilian targets in southern Lebanon. The goal apparently is either to drive away the Lebanese civilian population or turn them against the guerrillas.

Despite this activist military policy and the security belt Haddad maintains for Israel in defiance of U.N. Security Council resolutions, the Palestinian guerrillas still can -- and do -- hit and hurt northern Israeli settlements with long-range Soviet-supplied artillery and rockets. By their own admission, however, the guerrillas have been placed increasingly on the defensive by the new Israeli policy.

While Israeli military leaders are openly contemptuous of what they regard as the ineptness of the U.N. peacekeeping force in cutting off Palestinian guerrilla infiltration to the border, the seven U.N. battalions deployed across the length of the Lebanese Christian enclave's northern boundary do serve a function that is in Israel's interest.

Some Israeli military analysts estimate that despite the U.N. contingent's spotty record in stopping PLO infiltration, the presence of U.N. troops does make it harder and saves Israel the need of putting three additional brigades along its northern border.

Moreover, the U.N. presence has not hindered Israel's efforts to move large numbers of troops north of the U.N.-controlled zones. The Israelis can either si9mply fly over them or hike through a narrow gap west of the Christian town of Marjayoun, which was purposefully left free of peacekeeping forces. Or it can fire over the U.N. positions with artillery.

In a report on Dec. 12 to the Security Council covering the last half of 1980, Secretary General Kurt Waldheim complained forcefully about the Israeli Army activity in southern Lebanon. He wrote:

"The Israeli forces have themselves stepped up their activities in and near the [Christian] enclave. . . . They have established encroachments along the international border, increased their presence within the enclave, repeatedly violated Lebanese airspace and territorial waters and, on many occasions, have launched attacks against targets in Lebanon outside the UNIFIL area."

Waldheim said the Israeli forces had established new artillery positions near Marjayoun and in the coastal area, increased troop and vehicle movements in the enclave -- and even in the U.N. zone -- and even in the U.N. zone -- and, on a number of occasions, "joined with [Haddad's] forces and fired at [Palestinian] positions" from Israeli gun emplacements in Lebanon.

The report cited "an increasing number of border encroachments by the Israelis, including the fencing in of strips of land on the Lebanese side, the laying of new minefields and the construction of roads in the enclave." U.N. sources said the number of times Israeli soldiers entered the U.N.-controlled zones jumped from more than 400 in May to more than 2,000 in each of the last three months of 1980.

Israel's originally stated rationale for supporting and -- to a point -- controlling a 2,000-man combined Christian Lebanese army and militia in foreign territory, not to mention its own widespread and unabashed military operations there, is simple enough.

Prime Minister Menachem Begin stated it on Aug. 8, 1977, when he acknowledged for the first time that Israel was supplying and providing artillery support for Haddad's militias. He called it Israel's "duty to prevent a genocide" of Christians by Palestinian forces in Lebanon.

That justification for being in southern Lebanon still stands for Israeli political leaders and diplomats. Foreign Ministry Director General David Kimche said in an interview: "If we stopped helping Haddad, there would be massacres in south Lebanon. UNIFIL would never be able to prevent them, [and] we do not want to give the orders."

But policy justifications in Israel often involve parallel purposes, and in this case, security for Israeli civilian settlements near the border is given equal weight to the fear that Haddad's militia and civilian supporters -- of whom 30 percent are Christian, 65 percent Shiite Moslem and the rest Druze and Sunni Moslem -- would die in a bloodbath of Palestinian vengeance.

Many Lebanese are cynical about Israel's professed reasons for its presence in the southern part of their country. Haim Fayad, governor of southern Lebanon, summed up a common view when he said: "It's the ultimate card in the hands of the Israelis, who are not interested in restoring peace, security or Lebanese authority in the area."

One effect of Israel's activities there has been to make it impossible for the weak Lebanese Army and the fragile central government to resume a role in the south.

But by now, both Israel and the PLO, in the view of many diplomats, have become victims of their own self-fulfilling prophecies. If the Israelis pulled back to their side of the border and withdrew support from Haddad, the Palestinians almost certainly would move into the vacuum.

Once Israel pointed with satisfaction to Lebanon's flourishing democratic institutions and prosperity when lecturing other Arabs about the benefits of future coexistence in the Middle East. Now Israel stands accused by critics of aiding in Lebanon's disintegration.

Beginning with the Lebanese civil war in 1975-76, Israel has preferred to play on Christian-Moslem differences to the point of arming not just Haddad but the much more significant Christian militias in northern Lebanon.

Such a policy bogged down both the Syrian and Palestinian forces but it has frustrated attempts to strengthen the Lebanese state in what even observers usually sympathetic to Israel see as short-sighted advantage masquerading as long-term policy.

Since it was founded nearly 33 years ago as a Jewish refuge from the Nazi horrors, Israel has experienced a painful history of terrorist attacks across its borders with Lebanon, Syria, Jordan and Egypt and its worry for the safety of its civilians has become deeply ingrained.

"There are 17,000 PLO soldiers in Lebanon. They aren't fighting the Israeli Army. They are fighting civilians inside Israel," said the northern command general. "Why should we wait for them to come to us?"

Or, as an Army officer stationed on the border, who has a family living on a nearby kibbutz, said, "What concerns me is not the threat to Israel. The PLO is never going to destroy Israel. What concerns me is the threat to my family, and to me my family is everything in my life."

According to the Israeli military, the number of guerrilla attacks on Israel from Lebanon last year dropped by more than half from the previous year, to 18. Of these, eight were border infiltrations or attempted infiltrations through the sensor-guarded border fence patrolled by the Army. Nine were long-range shellings and one was a seaborne operation.

On all Israeli's borders and in the occupied West Bank and Gaza Strip last year, there were 262 guerrilla attacks or bombings. Thus 7.7 percent of the total were in Lebanese sector.

Israeli officials maintain that the reason the incidents along the Lebanese border dropped sharply is the policy of preemptive strikes against Palestinian bases in Lebanon, and that the Israeli Army's continued presence in southern Lebanon and its support of Haddad has headed off many attacks.

To some Arab critics of Israel's policies in southern Lebanon, the intertining of the humanitarian and security-based justifications has only served to obfuscate the issue.

The critics, who also include anti-Zionist Israelis such as Israel Shahak, chairman of the Israel League for Civil and Human Rights, recall that the early Zionist disciples of Theodore Herzl drew maps showing the proposed Jewish state reaching northward beyond Beirut. They also note that early Zionist plans called for Israel to control the Litani River as a source of much-needed water.

As a result, many Arabs are convinced that Israel intends at least to occupy southern Lebanon indefinitely, and, at worst, plans someday to annex it.

These fears were not soothed by the 1979 publication of the diaries of former prime minister Moshe Sharett, who recounted a 1954 meeting of Israeli foreign and defense officials at which plans were discussed that bear a striking resemblance to current events.

Sharett wrote that according to Dayan, then chief-of-staff, "the only thing that's necessary is to find an officer, even just a major. We should either win his heart or buy him with money, to make him agree to declare himself the savior of the Maronite [Christian] population. Then the Israeli Army will enter Lebanon, will occupy the necessary territory, and will create a Christian regime which will ally itself with Israel. The territory from the Litani southward will be totally annexed by Israel and everything will be all right."

Mindful of such Israeli thinking, the Lebanese government has complained to the United Nations about five "encroachments" where it says Israel, through fencing, minefields or bulldozed earthworks, has substantially changed the lay of the land to its tactical advantage.

While formally respecting the international border, these actions have effectively given Israel disposition of key high ground on Lebanese soil.

Another motive ascribed by some to Israel's policy on southern Lebanon is a desire by the Israeli military to maintain a ready-made training area for live maneuvers.

Officials in the Army command, however, while acknowledging that the combat experience in Lebanon is invaluable, dismiss as preposterous the notion that this is the basis for Israel's being there.

But the focus of much of the criticism from abroad is Maj. Haddad and his contingent of 500 to 600 regular troops and 1,500 militia.

Haddad is viewed, especially by the U.N. contingent in southern Lebanon, as a loose cannon on the deck of a violently pitching ship, who, at any time, could trigger a general conflagration in the area. He is also widely perceived as a puppet of the Israelis, acting at the beck and call of the Israeli Army.

"We could make Haddad into a puppet of the [Israeli military] very simply, so that when he wanted to go to the bathroom he would have to ask our permission. Our policy is not to do that [so] that anyone could think we did it to make a de-facto extension of southern Lebanon. We try to give as few orders as possible," Kimche said.

The northern command general says: "Haddad is independent. He looks for targets, and his people are shooting. Why should we control him? He's doing it beautifully."

Then, invoking a cynical phrase that repeatedly cropped up in interviews with military officials in Israel, he added, "We control Haddad to the degree that the United States controls us."

Israel's standing in the international community has been damaged by massive preemptive air strikes as far north as Beirut, which have resulted in hundreds of civilian casualties and which have been condemned repeatedly by the United States, which supplies Israel with F4 Phamtoms, A5 Skyhawks and F15 Eagles, all of which have been used in the attacks.

The air strikes grew out of a policy set forth by then-defense minister Ezer Weizman in January 1979, after a series of PLO rocket attacks in Galilee.

Weizman warned that Israel would strike in Lebanon not only in retaliation to Palestinian attacks, but "at any time and at any place that Israel deems desirable."

Because of the emotional reactions in Israel to Palestinian raids on civilians, it is doubtful that any Israeli government would survive the public pressure from not making an aggressive military response to terrorism inside Israel.

But the practice of sending fighter-bombers deep into Lebanon to bomb PLO targets -- often located in or near croweded refugee camps -- has been suspended from time to time as the result of intense U.S. pressure.

The result has been a cycle of sorties by Israel followed by lulls after occasional strong reactions from the United States.As a result, Isreal is now perceived in much of the world as having a free rein in Lebanon as long as Washington is not forcing the issue.

Israeli officials said they see no end to the cycle of violence as long as the Palestinians are determined to destroy the Jewish state.

When asked if he had any regrets over Israel's long entanglement in Lebanon, the Israeli government replied, "Yes, the mistake that we have not done enough."

Some Israeli leaders, however, have had serious doubts about Israeli military operations in London.

When he was foreign minister, Dayan opposed the Litani invasion on the groud it would draw Israel into a Vietnam-like quagmire. Lt. Gen. Shlomo Gazit, former chief of military intelligence, was said to be against the invasion and the deepening of relations with Haddad. Weizman, as defense minister, closely scrutinized military operations into southern Lebanon and often demanded proof of their necessity, although he supported an aggressive policy in Lebanon.

But when Weizman resigned and Begin took on the job of defense minister as well as prime minister, the resulting power vacuum in the ministry meant that Army Chief-of-Staff Rafael Eitan and his general staff were given virtually a free hand in planning and obtaining approval for military operations in Lebanon.

Since Begin had no immediate knowledge of the military situation and was burdened with domestic matters, the generals found it possible to get ministerial approval of incursions with little civilian scrutiny, according to military observers here.