By midday old women in blue aprons have begun winnowing sesame in the center of the red dirt road, joining the passing throng of peasants balancing heaps of turnips on bamboo poles or leading water buffalo to terraced rice fields.

For two American journalists, arriving in the south after a visit to the Sino-Soviet border in sparsely populated northeastern China, this narrow, meandering chickens and hogs looks more like the way to a country fair than China's main channel for troops and supplies to the troubled Sino-Vietnamese border.

Yet this dusty strip of countryside in Guangxi region is the traditional southern invasion route -- the "highway" taken by Han soldiers 2,000 years ago to suppress the unruly Vietnamese and by Communist Chinese troops two years ago to teach some "necessary lessons" to the same belligerent neighbor.

But Vietnam and China have not always been enemies. In the 1960s, this same road was the chief conduit for war materiel sent by Peking to assist the government in Hanoi, then thought to be a socialist ally, in its war to conquer the U.S.-supported South Vietnamese government in Saigon.

Today the colorful tableau of simple peasant life seems centuries removed from the modern Army and sophisticated tools of war set back a few miles from the tall eucalyptus three lining both sides of the road and shading it from the semitropical sun.

In a bone-crushing, 150-mile car trip last December from the provincial capital of Nanning to the frontier town of Pingxiang, we were reminded painfully of the great disparity between China's military might off the highway and the rural backwardness in its center.

The journey began at Nanning's modern airport, where signs are written in Vietnamese, English and Chinese and where the monotonous din of the lobby was broken every few minutes by the boom of a Mig fighter plane taking off a for training flight.

Driving southwest from Nanning, it takes just a few minutes to enter rural south China-- a land of lush, verdant sugar cane and rice fields cultivated by ox-drawn plows and gangs of peasants wearing sloping coolie hats and colorfully patched cotton trousers and shirts.

Most of the thin, fragile-looking people who work the fields surrounded by jagged, gray limestone mountains are of the Zhuang minority, a large ethnic group in Guangxi known for its love of dancing and singing and the brightly colored scarves worn by its women.

The road bears a tremendous pedestrian traffic, hundreds of Zhuang peasants every mile scurrying home from the fields under bulging baskets of vegetables strapped to their backs, children choming on stalks of sugar cane and clacking oxcarts loaded with limestone chunks.

Except for an occasional military jeep or truck, there are no motorized vehicles traveling down this ancient boulevard so narrow in some sections that only one car can pass and so rudimentary that rains cause instant flooding.

Whenever a truck broke down on the highway during the 1979 border war with Vietnam, the entire flow of materiel to the fornt stopped until the vehicle could be repaired or moved off the road, according to Western military analysts who have studied the logistical problems of this military route.

Along the way today there are signs that the problem is being remedied in the tortuous, quaintly archaic manner of underdeveloped nations.

For a short stretch of road, workers sit on top of limestone boulders placed along the side chipping away at the rocks' edges with hammers.

On an adjoining patch of highway, the stone fragments are dumped out of wheelbarrows and smoothed over the dirt surface by a crew equipped with rakes and shovels.

Finally the next section is paved with asphalt, the large, fiery vats of tar on the roadside polluting the air with clouds of black, sulfurous smoke.

For the first 120 miles, the road manages to obsecure its military significance, revealing instead a view of China that foreign visitors never see in the austere capital of Peking, or cosmopolitan Shanglhai or commerical centers like Canton.

Every few miles a village of thatched roof huts appeared, offering a fleeting glimpse into the daily life of these roadside dwellers. In one village, a man wrapped in white cloth sat in a barber chair getting an open air haircut in the community square. In another, young girls kneeled on the smooth rock of a clear stream, dunking and rinsing a week's wash.

Everywhere the bustling vegetable and livestock markets spilled into the highway, twitching roosters and myopic porkers obliviously ambling across this road to war like well-fed resorters out for a stroll on the promenade.

Rising along the side of the road on a small hill were several traditional burial mounds now forbidden in land conservation-minded China. The peculiar looking piles of dirt were simply marked by a tree branch and piece of white cloth.

The center of the road serves as an essential part of the production line for the old women winnowing sesame. They spread the small flat seeds from one side to the other waiting for carts and other vehicles to help crush the hard chaff.

The first sign of military tension emerged 30 miles from the border. More than a thousand yards off the road and hidden behind trees, anti-aircraft guns, tall floodlights and a radar screen indicated that an airstrip was nearby.

Five miles farther south, the first Army tents began appearing alongside the highway, and 10 miles later the only roadblock stopped traffic long enough for two soldiers to inspect papers and lift the white wooden rail.

The journey from rural China to militarized China dramatically ended in Pingxiang, a small town on the railway line to Vietnam located 10 miles from Friendship Pass on the Sino-Vietnamese border. Once a center for friendly meetings between Vietnamese and Chinese border officials, Pingxiang today lies within earshot of occasional mortar shelling.

In the town center stands the International Tourist Service Hotel, a colonial style inn of graceful balconies and flowered courtyard that once accommodated a steady flow of overnight guests traveling by train from China to Hanoi, 90 miles from Pingxiang.

Today the lovely hotel remains closed except for the rare visits by foreign journalists or government officials passing through Pingxiang for a quick look at the hostile border.

True to their reputation for hospitality, Pingxiang officials provided an effective wake-up call the first morning of our visit at the hotel.

At 6 o'clock just after the cock crowed, Pingziang's militia and regular Army unit staged their first joint military maneuver not far from the hotel room windows, perhaps hoping to underscore the danger of living so close to the front.

"We just hope the maneuver will raise the vigilance of the masses," an official explained later that day, his tongue planted squarely in cheek.