AQIA BRIDGE, Occupied Lebanon – On the southern approach to this bridge over the Litani, in the hollow of eight green hills, the Iranians are preparing for evening, stacking sandbags around their sentry posts.

"There, behind, another row," directs an officer. And over there, too."

Soon it becomes apparent that the sandbags are being stacked to protect against fire that could come from across the bridge, where the Palestinian Fatah control the hills; or from the no-man's land on either side, or from the rear, where the Israelis have staked out their lines.

The officer is asked where he expects the trouble to start.

"Everywhere," he says. "We have no friends."

It is a comment heard at a dozen United Nations positions in the belt just south of the Litani River, the new buffer zone manned by peacekeeping troops.

The Palestinian commandos are just to the north, some of them apparently uncontrolled by the Palestine Liberation Organization (PLO), which has endorsed the cease-fire in south Lebanon.

To the south stand the massive forces of the Israelis, a disciplined army also committed to the cease-fire but not openly confident of the U.N.'s ability to fight off a Palestinian advance and not necessarily likely to let U.N. forces deal with major trouble in their own way.

And in some spots, between the U.N. troops and the Israelis, stand the right-wing Phalangist Lebanese, who greeted the first groups of U.N. soldiers with bursts of machine-gun fire. The Phalangist Christian militiamen are still hostile to the U.N. presence here and as trigger-happy a group as any in the Mideast.

At this bridge over the Litani, the small river behind which the Palestinians retreated from last month's Israeli assault, there has been no shooting.

From the Israeli lines, the road winds down over hills almost perfectly round, from tops of sandpaper gray through ancient and cunning terrace work to fields of green velvet and orchards below.

At sunset, there is the song of birds on the hills, mingled with the rush of white water in the river and the sad, soft strains of a flute from the little Iranian camp. The failing light fades the sky from brilliant blue to a misty grey, and the hillsides, splashed since dawn with wildflower pastels, darken first to mauve, then to blue, and then to solid black.

Only up close does the ugliness crowd in: a twisted, burned Jeep and shrapnel in the road; the deep wound of a bomb that has carved a gash of red brown dirt, the color of dried blood, in a bright green field; six cows, and a bull lying dead on the road, stinking and bloated and buzzing with a million flies.

With the ugliness comes the fear.

"Yes, it is beautiful," the young Iranian says. "Beautiful and dangerous. That is what Lebanon is for me."

He is a lieutenant, lately promoted, serving a compulsory two-year army hitch. He doesn't want to give his name. At 25, with two sons at home in the north of Iran, he can think of a few places he'd rather be.

Most of the U.N. troops from France, Sweden and Norway are career army men who volunteered for the change of pace and the extra pay that U.N. duty affords.

Not the Iranian contingent.

"Yes, of course," says the Iranian with a wry smile. "We had the choice, too. I chose because my commander told me to choose."

He says he is uncertain why Iran or his unit was picked to stand in the buffer between or amid Israelis and Phalangists, Palestinians and a smattering of Lebanese national forces, and the Syrian troops who make up the bulk of the Arab League peace-keeping forces farther north of the Litani.