Sitting in what is left of his home in the labyrinthine Palestinian refugee camp here, a Kalashnikov rifle cradled in his lap and a framed photograph of his dead daughter at his side, Aartia Mira Hassan talks about the inevitability of war in a voice heavy with resignation.

Hassan's wife, Hamdi, and his 16-year-old daughter, Mona, were cleaning up after breakfast one morning in July when a 175 mm shell fired from Israel, 10 miles south, shattered the roof, killing the girl instantly. A second shell nearby tore open the leg of his wife.

It was early in the 15-day Palestinian-Israeli war of attrition, and six weeks of a cease-fire mediated by the United Nations and the United States has given Hassan time to patch up his home with cement blocks and sheets of corrugated metal. But he says he expects the shells to fall again soon.

"It is only a matter of time. This time the Israelis will bomb us and then come into Lebanon like they did in 1978," Hassan said, referring to the March 1978 Israeli push to the Litani River following a massacre of Israelis by Palestinian guerrillas on a coastal road near Tel Aviv. "I can feel their presence now," he added.

The specter of a massive Israeli invasion into southern Lebanon sometime in the months ahead is pervasive in Lebanon. It inevitably comes up in conversation with Palestinians and Lebanese, even though the Lebanese-Israeli frontier has been quiet for more than a week and cease-fire violations before then were desultory.

Whether or not it happens, the widespread fear of an invasion offers an insight into the view here of Israel's intentions in Lebanon, a perspective honed by the moral, material and human damage sustained in a decade of almost continuous conflict following the Palestinian guerrillas' flight here after being expelled from Jordan in 1970.

In the Beirut headquarters of the Popular Front for the Liberation of Palestine, near where Israeli bombs in July collapsed several Palestinian guerrilla headquarters buildings and civilian apartment blocks, killing more than 200 persons, Omar Kadush predicted an Israeli invasion in a month or two. He said it would come after U.S. special Middle East envoy Philip C. Habib returns with an American peace proposal that the Palestine Liberation Organization cannot accept.

"The invasion will not come until after we have rejected the American demands," Kadush said. "But it will come. You will see it. We are sure of that."

A few blocks away, Shafiq Hout, head of the Beirut PLO office, said, "They are coming. It doesn't make much difference if you get killed by an airplane bomb or an artillery shell. But they are coming."

In a coffee house in Nabatiyeh, Yosef Abu Khadoud, 44, a Lebanese whose brother was killed in a 1979 Israeli bombing, looked up to the cloudless summer sky and said, "Why do you think their reconnaissance airplanes are over here every day without fail? They are getting ready to come."

PLO Chief Yasser Arafat, in a speech here last week, said he had information that the Israelis are preparing for a "large-scale" military offensive in southern Lebanon.

"There are now Israeli military concentrations, air and infantry maneuvers in northern Palestine and the Golan Heights. There are continuous preparations to blow up the situation once more," Arafat said at a solidarity conference of Palestinians and Lebanese. He said Israeli Prime Minister Menachem Begin discussed details of the intended operation with President Reagan and received a "green light."

And in an apartment in Christian-held East Beirut, Phalangist Party official Karim Pakradouni sketches a scenario for the weeks ahead: "The United States proposes a compromise. The Palestinians and Syria reject it because it is an American plan. Israel tells the United States its plan is not going anywhere, and they take the military option. I give it two months."

The signs of war fever are everywhere, even if sometimes misleading. The French-language newspaper L'Orient-Le Jour carried a photograph Saturday of amphibious armored vehicles of the 17th Battalion of Fatah, the main military wing of the PLO, being moved to the south from positions near the coastal Beirut suburbs of Ouzai and Baine Militaire. Usually, such movements are kept secret.

Fatah officials were quoted extensively on the battalion's redeployment, an unusual openness given Israel's penchant for seizing on such buildups as justification of military strikes into southern Lebanon.

However, a Palestinian source said the 17th Battalion had been in the south last spring when Syrian-Phalangist fighting erupted in Beirut and Zahle, and was moved here in anticipation of a possible Israeli-Phalangist pincer attack in central Lebanon. The amphibious unit's presence became an irritant to local Druze of the Ouzai area, resulting in some clashes, and the battalion was returned south on Arafat's orders, the source said.

"There's a lot more serious stuff than that battalion down south. We don't need it, and it's not exactly going to to change the balance of power," he added, explaining that the armored vehicles are used mainly for security and that the unit is not regarded as combat-ready.

"But I suppose it is grist for the Israeli mill," the source said. "It is this kind of thing that they will use to justify their invasion."

The deliberate effort to publicize the deployment, coupled with mention by Fatah that a heavy artillery battalion was moved south last month, suggested that the PLO might have sought to send a signal to the Israelis and the Reagan administration that it regards the buildup as above board.

Ever since the cease-fire went into effect, the Israeli Army chief of staff, Lt. Gen. Rafael Eitan, and other military leaders have been pointing to a PLO buildup in southern Lebanon as a cease-fire violation and an indication of Palestinian guerrilla intentions. The PLO insists, however, that its agreement with U.N. peacekeeping officials to refrain from cross-border attacks does not mention redeployment of military units.

Palestinian officials said that in retrospect, they have detected a pattern of such Israeli alarms in the weeks before every infantry incursion into Lebanon or series of pre-emptive air strikes, and that Israel is launching another propaganda campaign to prepare international opinion for a major cross-border military operation designed to drive the guerrillas far enough north to put them out of artillery range against Israeli towns and kibbutzim.

When the Israeli Army withdrew after the 1978 Litani invasion, the guerrillas returned, so most PLO officials interviewed said they believe that if Israel attacks again, it will push all the way to the Zaharani River. Also, they said they expect the Israeli Air Force to attack the Syrian surface-to-air missiles that were deployed in the Bekaa Valley on April 29 after Israeli jets shot down two Syrian helicopters during Syrian-Christian fighting.

A Fatah commander in Nabatiyeh said he expects a combined airborne and infantry attack, with armored units moving through a gap in the U.N. peackeeping force near the town of Marjayoun, where Israeli-supported Christian militia leader Saad Haddad has headquarters.

"They may occupy many parts of the land, but of course they will eventually lose, because occupying that land is not what determines the winner," the Fatah commander said. "During the July war, the Israelis lost their nerve and bombed everything. But they were forced to seek a cease-fire. We are not afraid of the Israelis."

Virtually every Palestinian official interviewed in Beirut and the south said they expected President Reagan to give his tacit approval to an Israeli attack once Habib's diplomatic mission seeking an end to the missile crisis fails.

"We have not surrendered in this cease-fire," Hout said. "We made no promises that we will not strengthen our military. But Israel will use that as their pretext for attacking us, and the United States will go along with it.

"But we've crossed the point of ever being annihiliated. The PLO is growing bigger and bigger all the time. We will be put to a continual series of tests, but we will always be ready to fight them."