CHON THANH, Vietnam — "Should we move back to the ditch, captain?" the photographer asked nervously.

"The hell with the ditch," the young American adviser retorted urgently, "just move back!"

Something had just landed with a flat, lurching slam — the kind of personalized noise a mortar round makes when it comes testily close. There was the swirl of dust and grey fume. Before that, Capt. Ed G. Devos and two Pacific Stars and Stripes newsmen had just spent an unpleasant hour and a quarter lying under the snappish whine of small arms fire, first on their stomachs and then behind what would ordinarily be a poorly chosen place for shelter-a small mound of red ants. But it was blessedly available and there they lay.

Now there was a series of rapid, piercing snaps; someone with an AK47, secreted in the thick screen of rubber trees, was hosing down the roadside clearing.

The mortar rounds fell in terrifying rhythm. The ARVN position fell apart in a tangle of running men, pouring off the perimeter and out of the woods. One cracking thump followed another, like the beat of a timed sledge. The two newsmen puffed after Devos and joined him behind one of the armored personnel carriers that were pulling back, not having fired a shot to cover the sudden withdrawal.

The fire thickened, bracketing the clearing, as the captain coolly pulled his men back to the end of a long, dike-like mound that had been pushed up when American Rome plows made the roadside a wilderness a few years ago.

The fire abruptly lifted.

"Jesus," Devos said. "Had us bracketed and didn't even know it. Could have walked them right into us." Showing concern but no real stress, he held a radio handset and patiently waited for the airstrikes he had already called for. Artillery was punishing the Communists, VC or North Vietnamese regulars, who had pelted the clearing.

"If you're going to get out of here," Devos said, "you better do it now. Stay between the rise and the highway."

They did part of the way, but it was faster walking on asphalt than glue-like mud. Others were walking the highway, so they did, out of small arms range and hopefully beyond the teach of the mortars.

Down the highway, an air-conditioned Rambler was waiting to take them back to Saigon. "That," said an infantry officer who had taken them part of the way, "was as close as you'll ever come to being overrun ..."

It had promised to be little more than a brisk and interesting afternoon. Troops of the 21st ARVN Div., were to clear Highway 13 to a point just beyond Chon Thanh, which is 45 miles north of Saigon and only 15 miles below the besieged provincial capital of An Loc. A convoy was to follow along. Newsmen were allowed only as far as the fire base, then told to walk the rest of the way to the battle, a good distance down what Americans had once dubbed Thunder Road.

The fighting wasn't hard to find; it was marked by flaring napalm and thumping blasts as Republic of Vietnam Air Force A1E dive bombers maced the east and west side of the highway. Civilian vehicles were scattered all over the road; the Communists had spotted a column or refugees, impersonally spraying them with gunfire and killing three or four.

A small knot of ARVN soldiers was blown apart by a greyish puff. A French photographer sprawled away from the blast. He walked back out to the highway and said he didn't think anyone had been killed, but deplorably he was out of film and had missed some beautiful pictures. Did anyone have a spare roll? Thank you. He walked back to where he had been.

There was the crash of M79 grenade launchers, the swishing thump of the Communist RB40 rockets. One flayed shrapnel up the side of a personnel carrier and five men were carried back moaning and bleeding.

The two Pacific Stars and Stripes men linked up with Devos, who comes from Lake Worth, Fla., and is the adviser to the 2nd Bn., 33rd ARVN Regt. His troops were slowly pushing forward. On the long mound, they were suddenly scattered by small arms fire. They later moved forward to a ditch.

The newsmen began to realize they were riskily close; the stomach-jolting thumps from the next airstrike were sharper and there was a fluttering drone overhead. Still the captain had moved even farther ahead and they wouldn't know what was happening without him.

"Come on," the photographer said lightly to the reporter, "let's go get killed." It was a remark he would later apologize profusely for, because the swarm of iron hornets began to pass overhead as soon as they reached Devos.

One whined sharply and Devos said, "that one was kind of personal." The radio operator motioned to the anthill and they got to it in an awkward, spraddled duckwalk.

Some of the fire appeared to be coming from an ARVN force that was moving south from Chon Thanh and would link up with the northbound force to sandwich and erase the enemy in the woods.

A quietly infuriated Devos asked that it be lifted; then came the personalized enemy fire and the long freestyle run.

In the car back to Saigon, one of the newsman tuned in Armed Forces Vietnam on a portable radio, to hear the andante cantabile from Tchaikovsky's Fifth Symphony. It had never sounded so beautiful.