From the slopes of Mount Hermon in the east to the heights overlooking Tyre in the west, the Israeli invasion of southern Lebanon has left a broad path of death and wide-scale destruction unprecedented in the region south of the Litani River.

Except for the Christian villages and enclaves of the Lebanese Christians, who have long been allies of the Israelis, hardly a town is left undamaged. Some have been all but totally flattened by air strikes and explosive shells.

Nothing that had gone before - the desultory shelling of the past two years involving Christians and Israelis on one side and Palestinians and Moslems on the other - prepares one for the devastation that has been visited on the ancient stone towns in this rolling, rock-strewn farming country.

Much of southern Lebanon between the Litani River and the Israeli border now looks like the devastated sections of Beirut, and like the ruined and blasted Lebanese towns to the north, such as Damour.

The scope and sweep of the damage here makes a mockery of Israeli claims to have staged surgical strikes against Palestinian bases and camps. But then, such military surgery was never possible, as the Palestinians were deeply entrenched and living among the civilian population.

Nonetheless, it is clear that the Israelis have used the same tactic that the Americans used in Vietnam: concentrated and heavy firepower and air strikes to blow away all before them - be they enemies or civilians - in order to hold down their own casualties.

Here in Bint Jbail, once a Palestinian stronghold, just a few miles from the Israeli border, most of the population has now joined the estimated 200,000 refugees that fled northward before the advancing Israelis. Almost all the rest are dead.

The town hall has been gutted. Bits of paper, birth certificates and other records, lie in the rain-soaked streets, dogs wander about sniffing the refuse confused and masterless. A few old Arab women huddle in ruined doorways, seeking refuge from the cold wind and hail that comes in off the mountains.

The corrugated iron shutters of abandoned shops, torn and shattered by shrapnel, bend in the wind. Shredded awnings flutter, and a few white flags can be seen on abandoned houses.

But here, as throughout most of southern Lebanon, the overwhelming impression is of silence and desolation, broken only by the movement of Israeli trucks and armor and now the vehicles of the United Nations making their way into the territory.

One old woman recoiled in fear as we approached and pointed in the direction of the sound of some distant firing - as if sensing that the war is what we foreigners had come to see.

But the war is here. I had last visited Bint Jbail in August when it was in the hands of the Palestinians. Then, they were under a state of semi-siege, as were their Christian neighbors in the next village not 15 minute walk up the road. The Palestinians in August would take you to see the shell holes in the roofs and walls of the town. Those same buildings are now hard to find. Many of them no longer exist.

I looked for the village clinic, where a tired doctor had once showed me his wretched facilities and the dirty cots in which his patients lay. The clinic was housed in a beautiful French-style villa, and I thought that the doctor might have stayed on despite the Israeli advance.

The clinic, however, is now a complete ruin. It took a direct hit in an Israeli airstrike. Among the flattened walls and heaps of rubble, only a stone staircase leading nowhere gives any hint of what was before. The pilot could not have known it was a clinic as there was no marking on the roof to identify it.

It was clear that the Palestinians had put up stiff resistance here, and an Israeli military spokesman said that the battle for Bint Jbail had been among the most difficult of the campaign.

It was Holy Thursday when we set out by car to tour the area of southern Lebanon which the Israelis have occupied. At the border, Christian children had been gathered in and fed by the Israelis and were there to demonstrate for the television cameras against the United Nations troops who are trying to move into the area.

The Christians preferred that the Israelis remain, fearing the Palestinians would return seeking vengence.

Israelis sources told us that the Christian militiamen had been looting what was left of the Moslem villages. In the Palestine Liberation Organziation stronghold of El Khaim, reliable sources accused the Christians of having slaughtered about 70 Moslems huddled in a mosque. The Israelis had just begun to set up roadblocks to prevent looting.

There is little doubt that if the Palestinians had gotten their way, they would have killed the Christians. Indeed, communal slaughter did occur in the 1920s and before.

As we drove into Lebanon with the gathering rain clouds darkening the sky, there were reminders everywhere of the long history of death, war and intolerance that have so plagued this region. Crusader castles dotted the hills and in one section of the road, dug up by shells and passing tanks, one could see the stub of a Roman column.

Randurya, the town above the narrow sweep of the Litani River and the Akia Bridge, Iranian troops of the U.N. force, looking miserable, were stretching plastic coverings over their field gear in the rising wind and rain.

Most of the houses had been destroyed and, judging by the huge craters by the side of the road, Israel's F14 Phantoms had their way with this town with 500-pound bombs. A child's bicycle lay flattened and pressed into the pavement by passing tanks, and some furniture had been pulled out of the ruins by the roadside.

Farther down the road, we saw our first burned-out Palestinian tank - taken from the Lebanese army during the civil war in the north. There are destroyed Israeli vehicles, too - most of them the victims of land mines. Everywhere, there are automobiles, wrecked, burned and full of bullet holes.

I wandered into an abandoned house. Suitcases half-packed, lay about with clothes pulled out of drawers, and food was still in the kitchen table. I glanced into a bedroom and quickly backed out. A body of someone dead several days lay in the room. I did not look long enough to see if it had been a man or a woman.

Finally, in Bint Jbail we had had enough. We passed from the silent, empty and ruined countryside with its dead and homeless into the Christian villages where people were out and about, preparing for Easter.The shops were open and full of goods from Israel.

Beyond lay Israel itself. Its forces now control about one-tenth of Lebanon, but even the Israelis themselves are beginning to wonder what their offensive in southern Lebanon has really gained them.

The PLO has been bruised, but not broken. The respite can only be temporary. And the prospects for real peace have seldom seemed so far away.