Documentary : Crossing Perilous Borders : A visitor to Bosnian territory held by Serbs encounters a weird drone of tension--and nightmarish memories.

The jumping-off point is the Mreznica River, or at least it is one of them, one of dozens, really, where the line is drawn most visibly not by the clear water running below a bridge and represented by the thin blue line on the map, but by the wrecked and burned-out trucks shoved into the bridge’s right-hand lane, and then by the shattered houses just beyond.

This is the beginning of “Indian Country,” you could call it, in the spirit of old American Westerns, beyond which comes a certain terra incognita : open, silent spaces, buzzing summer heat and the possibility of the war party, stirred from their rest in the shade of the cottonwoods.

Actually, it is checkpoint country at first. Borderlands. The line between what might be called de facto Croatia, which is marked by the river behind, and territory ahead, which is a swath of Croatia taken by Serbian guns last year and which the Serbs prefer to call “the independent republic of Serbian Krajina.”

It is also known on the maps of the U.N. Protection Force as Sector North, or sometimes, more informally, even facetiously, as “the forbidden zone.” Up ahead, beyond a tangle of locally administered checkpoints, each marking a separate authority--Croatian police, Croatian army, then a space, then the Serbian Krajina police--the United Nations watches the road from sandbagged checkpoints. Nigerians, then Poles, then Nigerians again and a cluster of reassuringly efficient Danes.

And then, across another river, Bosnia.

Those cottonwoods, so to speak. The Chetniks--a term applied to World War II Serb partisans and worn proudly here still. Serbian-cleansed territory and dubious prison camps. Or, in another direction, the morning and evening whine and bang of artillery--and gathered refugees, leaning in the shade against grimy walls, waiting.

The Croatian officer back at that first bridge was fretful, a junior filling in on a Sunday morning. His men, in front of a bullet-chewed building, its gutters dangling crazily, lounged in the shade of umbrellas (advertising a local beer) liberated from some sidewalk cafe. The lieutenant worried over the exact ownership of the car (a hapless rental agency charging exorbitant rates), stalled, relented, just doing his job, and then helpfully suggested taking off the Croatian license plates. On to the next.

There is always a weird drone of tension that grows in the very idleness, the requisite passiveness, of these crossings, as though at some sonic frequency just above or below the level of hearing.

Borders seem to come with the job. I have crossed borders that I later had cause to regret crossing. I have crossed them, too, when I was not forced to regret it later but regretted it while I was doing it, at moments when that whine, or drone or whatever it is, was becoming too audible. These borders always suggest, beyond the tangible artifacts of geography, buildings and terrain, a certain mental boundary as well--beyond which lies a psychology, a febrile concentration, shimmering like the heat haze down the road.

There are many such crossings now in the fallen Humpty Dumpty of the former Yugoslav federation, and not all of them have been marked or arrive with adequate warning. In 14 months of fighting, about 30 journalists have been killed here and many more than that wounded. It is tough country and a nasty war, and many of the sides, particularly the Serbian side, do not like journalists very much and tend to take aim at them.

The Croatian town of Glina is on the road toward northern Bosnia, and a reminder. Now part of Sector North, Glina is in Serbian hands and is peaceful, although it looks underpopulated and depressed, like a town with a bad headache.

A little more than a year ago, the war was here, and one afternoon a German reporter was here too, looking for a way out of town. The bullet that killed him was like the straw a tornado drives into a tree trunk: It passed through the headlight of his car, along the fire wall of the engine, through the floorboard and into his inner thigh. It took 40 minutes for his driver to get to a hospital, and by that time he had bled to death.

That same day in Glina, a reporter for a Canadian newspaper was pinned down in a house for most of a day. His car, outside, had “PRESS” spelled out in black tape in letters two feet high, and he spent the afternoon watching the radio antenna on the car flick back and forth as the bullets struck it.

And also on that same day a year ago, I was passing through the towns of Bosanski Novi, in Bosnia, and Dvor, just across the Una River in Croatia. Crossings . Things were very hot up the road from Dvor, toward Kostajnica, where the Croatians were holding out briefly against a Serbian attack. Dvor was a staging area, a command post for a variety of Serbian militia groups, vying with one another for an appearance of ferocity. They carried an astonishing array of portable weaponry. Commanders arrived and departed inside a circle of underlings bristling with grenades, knives, Kalashnikovs, sniper rifles, .50-caliber machine guns (with tripods dangling) and sunglasses. They were “tooled up.”

Being “tooled up” is the nice expression used these days by a Canadian officer on duty for the United Nations in the Bosnian Krajina.

“They like getting tooled up here,” he said. “All of them. The Bosnian Territorials, the Serbs, all of them. We come in, carrying clipboards, pens and paper. They’re not impressed. But they met some French U.N. officers the other day, and the French impressed them. The French know how to do it. They came tooled up.”

But the crossing between Dvor and Bosanski Novi is not heavily “tooled up” this Sunday. The Una River, marking the Bosnian border, is running wide and distinctly blue, and people are swimming in its shallow water.

Bosanski Novi already has been “cleansed.” Muslims were evacuated two weeks ago. The checkpoint has that tension, that ultrasonic whine. A Serb officer goes away with the papers. Another suggests the car be moved around the corner, into the shade. Another instructs us to make “PRESS” signs to place in the windows, an idea that seems less than reassuring. Time is passing, and the checkpoints behind us close at 4 p.m., and a choice must be made, somewhere down the road, whether to head back before the gate comes down (and the road is mined) or to spend the night in a place where our welcome is highly uncertain. We are headed to Prijedor, where the closest of the Serbian-run prison camps are located.

The pass we get will take us to Prijedor, and, if we want to go farther, another pass must be obtained there. Prijedor will have to do. There is another checkpoint two miles up the road, one where an officer painstakingly goes through everything in the car, studies the marks on our map. This does not seem promising, but he waves us past, and on we go, on a road that winds through green hills that look like Vermont.

Prijedor, also “cleansed,” broils sullenly. Sandbag emplacements stand at every major intersection and fortify the public buildings, each with a guard detail. At a shaded outdoor cafe, a lamb roasts on a spit and a soldier sits alone, drinking coffee.

We find, at the edge of town, the Keraterm ceramics works. This has been, since mid-May, one of the Serbian prison camps for Muslims in the area, and descriptions by prisoners who have been released from it paint it as one of the worst in the 100 or so that the Serbs are believed to run.

But it is empty. A lone soldier leans on its gate. Behind him, the low, one-story building, painted a dull red, stands with its doors open, a spatter of bullet scars across a section of its wall.

“There were Muslims here,” the guard says. “They were fighters. They were taken away two or three weeks ago, I think. I wasn’t here then. I don’t know where they went.”

The prison camp at Omarska, the former iron mine, which is also now being cleaned out, is farther down the road, beyond the reach of our pass.

In town, we find some people sitting under some trees beside an apartment building. There is a Serbian man, his Hungarian wife and a Muslim neighbor.

“There was trouble in May,” the Serb says, “when some Muslim fundamentalists wanted to set up their own state.” This is common wacko propaganda, and patently false, and the Muslim neighbor looks doubtful but says nothing.

Asked about the aftermath of the “trouble in May,” the Muslim acknowledges that, “Yes, many Muslims have been taken away.”

Will they come back someday?

“Yes,” the Serbian man says, “when the war is over.”

Simultaneously, however, the Muslim man’s eyebrows lift. He shrugs. “Hard to say,” he says. His expression suggests he is far from sure. He notes that his own brother, who was not a fighter, was picked up in a street roundup and locked in Keraterm for seven days. He was released and was home now.

“You know,” the Muslim says, “we all live here together--Muslims, Serbs, Croats, Hungarians. We never had any problems. It’s the leaders, above us.”

The Serb and his wife quickly add their agreement. “Yes, yes, that’s true.”

It seems the surest truth that Prijedor will yield today. Time is running out.

The crossings on the way back are easier. They’re always easier on the way out. The guards take a quick look, wave us past. The rivers seem clearer, the hills greener, the heat soporific. Back-seat passengers doze, once past the border, beyond the Danes in the U.N. zone. At some points where the road winds through narrow valleys, spring water spouts from pipes stuck in the hillsides and is finally irresistible. The water is clean and cold, splashed on the face and neck and sweaty arms.

Refreshed, we think we’ll get farther next time. They know us now on the crossings.

It’s what you always think, when you’re on the way out.