These are the leanest of times for the Palestinians in Lebanon who have jettisoned once-romantic beliefs in the inevitability of their cause's early trimup over Israel in favor of an uncompromising policy of self-defense and survival.

Battered by Israeli military might, resented by their reluctant Lebanese hosts, often privately patronized in the Arab world and increasingly overlooked elsewhere as yesterday's cause, the Palestinians are taking no chances in Lebanon.

This is the only country where they can still operate as an independent entity -- their last redoubt -- and they intend maintaining maximum freedom of action against all comers.

Running throughout Palestinian thinking is the conviction that the Palestine Liberation Organization's primary duty now is to endure and to remind the world that no Middle East peace is possible without its participation. And South Lebanon is the key to its ability to carry out that mission in violent and diplomatic ways.

The Palestinians privately acknowledge that their own behavior has cost them dearly, especially in the eyes of those Lebanese who not so many years ago idealized their cause.

Effort at damage control with the Lebanese have failed partly becasue of an often unspoken contempt for all things Lebanese on the part of an uprooted Palestinian people who consider themselves at war.

But also responsible has been Israeli military punishment on such a scale that inevitably the Lebanese, and especially the southern Lebanese caught in an ever expanding war zone, were doomed to get hurt.

Before the Lebanese civil war broke out in 1975, the Palestinians stood accused by Lebanese critics of forming a state-within-a-state.

Now there is a conviction among many Lebanese that the Palestinians are attempting "implantation" now that the Lebanese state has collapsed. By implantation is meant clandestine Palestinian acquistion of southern Lebanese land as part of a supposed scheme, aide by the United States, to partition the country and settle the Palestinians there once and for all. This would presumably end the Palestinian quest to retake the land that has been Israel since 1948.

First circulated by the Palestinians' arch-enemies among the Christian militias, that charge has been taken up by the Shiite Moslems, the largest community in Lebanon, which is especially strong in the south.

There a decade or so ago, Shiite Moslem villagers welcomed and cooperated with Palestinian guerrillas and their leftist Lebanese allies in the first attacks against Isreal. But that shared idealism soon paled. The Palestinians misbehaved, ruined orchards and crops and the Israelis simply raised the pain threshold.

Like rings on a tree, villages next to the border and then farther and farther north, bear the marks of increasing Israeli retribution over the years. Gelignite and artillery gave way to air raids and gunboat fire.

In classic counterinsurgency fashion, the Shiites have formed a political and military organization called Amal, or hope which militantly opposes the Palestinians. So far clashes have kept largely to the south, but scarcely a month passes without shootouts in Beirut between Amal and Palestinians.

The southern Shiites have gained such a reputation for nationalism and anger at the Palestinians that one can hear frequent suggetions that they supply much of the detailed operational intelligence that Israel uses against the guerrillas.

Thus have some Lebanese and Palestinians come to question the wisdom of armed struggle against Israel waged from Lebanese territory. Originally the rally cry of Yasser Arafat and his associates who wrested control of the PLO from a less military-minded leadership, armed struggle has lost much of its meaning for an organization that, whatever its rhetoric, relies on diplomacy, the oil weapon and Arab states' influence to advance its cause.

Palestinian officials insist that all but a handful of their armed operations are conducted from within the occupied territories of the West Bank and the Gaza Strip or within Israel itself.

Such claims have not deterred the Israeli armed forces from expanding the area it controls in Lebanon as a part of its controversial preemptive strike policy justifying attacks anywhere.

Complicating appreciation of Palestinian motives has been a surge since November in aggressive actions by the PLO and its Lebanese National Movement allies against the U.N. peacekeeping force.

The clear increase in incidents in which U.N. soldiers have been killed, helicopters hit by gunfire, vehicles hijacked and troops fired on contrasts with formal promises by Arafat to cooperate with the U.N. force.

While published U.N. reports indicate a modest increase in such attacks in recent months, U.N. officials in the Middle East privately insist that these reports reflect only part of the deteriorating atmosphere that their silent diplomacy is trying to keep in check.

For the first time, U.N. sources report mixed patrols made up of mainstream Fatah and radical Rejection Front Palestinians infiltrating the U.N. zone.

Recently, Palestinian guerrillas have begun driving into the U.N. area and firing their jeep-mounted Katyusha rockets at Israel and then dashing back north again. U.N. officials see these acts as designed to invite retribution against both U.N. forces and Lebanese villagers in the U.N. area.

Stepped-up nighttime patrolling and more forceful U.N. behavior has resulted in checking increased Palestinian infiltration efforts, especially attempts to break out of the so-called Iron Triangle.

U.N. officials are at a loss to explain the hardening Palestinian attitudes, which are variously read as a basic change of mind at the top of the PLO or a breakdown in the chain of command.

Some analysts say Arafat's diplomatic offensive has run out of stream and he is resorting to action in southern Lebanon to disarm radical critics in and out of the PLO.

Others maintain that the PLO is simply replying to Israel's stepped-up preemptive strikes, which have coincided with the U.S. election campaign and the early Reagan administration.

Still others argue that the surge is due to presures on various Palestinian factions generated by the Iranian-Iraqi war, or that the PLO is seeking to regain the international limelight.

But such explanations ignore the fact that since the Israeli invasion of Lebanon in March 1978 the PLO has been on the defensive in its heartland, which extends north of Trye to Zahrani and now, because of last year's Syrian Army pullback, north along the Mediterranean coast to Beirut and east to Nabatiyah.

"Infiltrations south of the Litani River basically do not affect the situation once north of Tyre is pretty much under our control," a Beirut-based Palestinian analyst said.

That situation has not changed since 1976. Then American-aided diplomatic exchanges between Damascus and Jerusalem accepted that the Syrian Army would enter Lebanon to end the civil war, but would stop at a line running roughly from the Mediterranean refinery town of Zahrani east to Jezzine.

In 1977 Israel vetoed U.S. efforts to persuade Syria to send troops down to the Litani River in an effort to bring the PLO under Syrian political and military control.

The Israeli motives, according to Western military analysts, intelligence specialists and diplomats, were military: they did not want Syrian armor that close to the border. But many of these same sources acknowledge that the political result was that Israel was seen as preferring to keep the PLO alive and well on its own turf for Israeli armed forces to play with. Thus could Israel maintain insecurity in southern Lebanon and, by extension throughout the Middle East.

In 1978 at the last minute Israel also vetoed deploying already dispatched Lebanese Army troops in the U.N. zone as a first step toward reestablishing central government authority there.

PLO military commanders make no secret of Israel's success in destablishing Palestinian control north of the Litani River.

"Until 1978 we had the initiative in the south," a PLO commander said in his Sidon office, "but in successive stages Israel has turned the tables through stepped-up air and artillery attacks, coastal raids and attacks on civilians."

"After first neutralizing us, Israel tried to seize the initiative by heavy, sustained attacks on our bases," he added, "which forced us to spread out."

"Then in a second stage they sought to put us on the defense and they have practically succeeded. We were obsessed about how to defend our bases and our refugee camps. We concentrated our forces around isolated bases."

Then last year with PLO bases withdrawn, because of Israeli pressure, to well north of the Litani River, he said, Israel launched attacks in August, September, October and again in December. "We are expecting another soon," he said, "and we are recognizing our forces."

Relieving that bleak assessment are independent military observers' convictions that the Palistinians are well enough armed, motivated and in the final analysis protected by the 22,000-man Syrian Army in Lebanon to withstand almost any Israeli onslaught short of all-out war.

But such readings do not obscure the fact that the PLO has little offensive strength. The low level of PLO operations initiated from Lebanese territory has been a constant since well before Israel inaugurated its preemptive strike policy against targets in Lebanon.

If anything, observers say, the PLO military tactics seem dictated by fear. Last fall, about 5,000 Palestinian university students were recalled for military training in Lebanon. When the Israeli-backed Lebanese militia forces recently announced plans to provide compulsory military training to about 17,000 secondary school students under its jurisdiction, the PLO let it be known it was studying a similar program for all young Palestinians living in the Arab world.