Life on the border between China and Vietnam is a barometer of the volatile political relations between the two staunchly communist countries. In late April and early May, according to official reports, China and Vietnam exchanged heavy artillery attacks and Vietnamese infiltrators slipped into Chinese territory. This attack registered China's latest dissatisfaction with Hanoi - particularly, the Vietnamese offensive against Chinese-backed guerrillas in western Kampuchea.
In years past, however, the border stations have been the scene of friendly, even enthusiastic, exchanges. The two countries were comrades in arms during Hanoi's war with the United States.
The border is now quiet. But a visitor to the tense frontier can find much evidence of the military confrontation that has typified Chinese-Vietnamese relations in recent years and considerable pessimism among local villagers about the prospects for peace.
The writer visited the Chinese side of the border last year.m
In the sleepy county seat of Pingxiang, deep in China's tropical south, there is a hotel that can accommodate up to 1,000 people handily. But when I visited there last August it was empty. The dust on the surfaces of things suggested it had not seen a visitor in years.
The Pingxiang Hotel was built to lodge the constant flow of guests from Vietnam in the days when Mao Tse-tung and Ho Chi Minh could clink glasses and speak of a Sino-Vietnamese alliance as close as ''lips and teeth.''
In the late 1960s and early '70s, Chinese leaders say they gave Vietnam $20 billion in aid to further the Vietnamese war against the United States. The vast majority of that aid came down the highway and rail line from Nanning, China's big southern city, to Hanoi. At the Friendship Pass where the actual border delineation between the two countries slices through a jagged set of mountains, Vietnamese representatives accepted consignments of goods from China. At Pingxiang, Vietnamese on their way to Peking for talks or on their way abroad would stop for the night.
Now those days seem like ancient history. According to the Chinese press, China mounted a ''mini-lesson'' in late April and early May, akin in purpose if not in degree to the three-week military strike deep into Vietnamese territory launched by the Chinese People's Liberation Army in 1979. That operation was designed, in the words of Chinese leader Deng Xiaoping, to ''teach Vietnam a lesson'' for its fickle decision to ally itself with the Soviets rather than China, and its aggressive military conquest of Kampuchea.
So, with China claiming over 4,000 instances of Vietnam stirring up border trouble since 1979, and with the two sides locked in a fierce war of words that frequently turns to a shooting war, the Pingxiang Hotel is no longer the friendly meeting ground of Chinese and Vietnamese it once was.
The approach to the tense frontier looks scarcely different than a thousand other rural locales in these lush green tropics. A narrow, one-lane ribbon of asphalt runs from Nanning to the Vietnamese border. Along it, a continuous procession of peasants goes about their appointed tasks. Some use water buffaloes and horse-drawn carts to haul bricks, timber, and coal. Some use bicycles and hand-carried poles to move baskets of cabbages and piglets to market.
At Kilometer 175 south of Nanning, however, everything changes. The first signs of the change are the big antiaircraft guns perched atop hills and buildings, poking their long necks south. Radar sky-scanners whirl rapidly from rooftops. The coming and going of People's Liberation Army (PLA) men becomes more obvious - troop-filled trucks roll by, bivouacked troops camp by the roadside, new barracks are being built.
The road, too, is being widened by numerous highway crews - and that means only one thing: Ever since the PLA ran into trouble in the 1979 war in getting heavy armor to the front, local road builders have been at work improving the highway, should the tanks need to roll down it again.
An electric tension builds in the air the closer one gets to the border. On the misty slopes of Mi Chi, a small village of Zhuang minority people whose terraced rice fields lie within 330 yards of Vietnamese howitzers, even the daily farming chores have become militarized.
According to Zhao Mingxin, the commander of a detachment of border guards assigned to the defense of the Friendship Pass area, the Vietnamese had fired rockets into Mi Chi's fields not long before and the frightened peasants had requested armed escorts before returning to the harvest. Now with sentries strung out across the mountaintops, green-uniformed soldiers with AK-47s strapped to their backs were tilling and threshing with the local people.
China claims that it is the victim of Vietnamese ''provocations,'' that Vietnam is intentionally nibbling at its border, shelling villages and killing civilians all to force China into a military response and thus make Peking appear to be Southeast Asia's aggressor. All the while Hanoi goes about its mopping-up campaigns against Kampuchean guerrillas, say the Chinese. Such logic may appear convoluted to Western judgment, but it is very much believed by the people who live in the affected area.
In Aiko, a small town three miles from the ''Friendship Pass,'' a rudimentary militia is made up of virtually all the town's young people, male and female alike. With old and rusty rifles, they patrol the streets and keep their eyes out for Vietnamese ''agents'' who might try to sneak across the border to provoke trouble.
Local militia leader Meng Pinyuan explained that the tactics used by the Vietnamese to destabilize Aiko and environs include everything from starting brush fires on the Vietnamese side when the wind is blowing toward China, to poisoning water buffalo, to smuggling opium to keep Chinese villagers in an unvigilant stupor. None of these techniques work, Meng declared proudly.
''If the enemy comes here, he will drown in a sea of the people,'' he said, paraphrasing a saying of Mao Tse-tung.
The scene inside the Friendship Pass building complex provides graphic visual evidence of how Sino-Vietnamese amity has turned to enmity. Coils of barbed wire ominously fill the roads that once bisected the border. The bilateral meeting hall and the railroad customs building are both heavily damaged. Some rooms have been reduced to rubble. In a wedding cake structure originally erected by Ming emperors, a set of photos of Marx, Engels, Lenin, and Stalin - the communist heroes revered on both sides of the frontier - is studded with bullet holes. Nervous border guards stare through their binoculars at activity in the Vietnamese pillboxes on the other side.
In earlier times, the Zhuang and other ethnic groups who lived on both sides crisscrossed the border freely. People frequently came to Aiko and Pingxiang from the Vietnamese side to shop or get medical care. But now there is little cross-border motion. Even tiny paths high in the mountains are mined and closely guarded. Among villagers on the Chinese side there is a deep bitterness at what is seen as Vietnam's ''betrayal.'' Pessimism prevails about prospects for peace.
Wearing a T-shirt that identifies him as a hero of the 1979 war with Vietnam, Shen Huabo, the deputy mayor of Pingxiang, wiped his brow of perspiration brought on by the excessive humidity and looked around at the emptiness of the Pingxiang Hotel. ''It is not likely that this place will ever be filled with Vietnamese again. The Vietnamese Communist Party seems bent on antagonizing relations. Their leaders grow more and more anti-China, not less so. They are insistent on dominating Kampuchea and all of Indochina.''
Would China try to teach a ''second lesson'' and make another major military move into Vietnam? Shen's answer last August seemed to predict what has happened in recent weeks: ''China has no desire to engage in conflict with Vietnam. However, if the Vietnamese don't change their ways, it is only natural that we will have to respond. Small-scale incidents happen daily. Medium-scale fighting could well take place soon. As for large-scale fighting, it is not so likely.''