It seems odd so much blood has been shed along the regged, hill-strewn border where neither Chinese nor Vietnamese ever really wanted to live.

Rising gradually higher and higher over about 500 miles from the Gulf of Tonkin in the east to the mountains of the Laotian border in the west, the Sino-Vietnamese frontier becomes in spring a place where the morning fog chills the bones of thinly clothed peasants and hides the movement of soldiers running low to the ground.

The hills and the mountains enjoy a subtropical midday warmth and heavy monsoons. They have been green and beautiful for centuries. But those ambitious farming people, the Chinese and the Vietnamese, have found them difficult places to grow sufficient quantities of their favorite rice. So relative to the rich Red River delta, the hills have remained parsely populated.

The area had become a convenient barrier, usually keeping two very energetic and expansionist naions physically apart. The harsh geological features that made it a good border in the past are now becoming apparent to the thousands of Chinese troops to the thousands of Chinese troops wh began to trudge over the hills and through the canyons into Vietnam that foggy morning of Feb. 17.

The Chinese were coming to punish the Vietnamese, allegedly for violating the border too often. But their action appeared to have moe to do with events in the Soviet Union and Cambodia and other places far away from those serence mountains on the edge of the southeast Chinese plateau.

A Western traveler who happened to visit the far upland end of the Red River Valley, near some of the bittrest of the border fighting last week, described hills all around "covered with six to eight feet of scrub brush. The hills appear to go up 900 to 1,200 feet, not rolling like the hills of Maryland, but with sharper features. It's a good country for defending."

Chinese dispatches speak of enemy troops hiding in caves and firing down from dug-in positions on hilltops. The slow pace of the Chinese advance has been blamed by some observers on the time needed to neutralize so many small pockets of resistance, easily hidden in the underbrush and small canyons. That a batallion of Vietnamese troops could cross into China repeatedly last week, and harass Chinese villagers in an area where Chinese troops are supposedly heavily concentrated, suggests how difficult it is even to locate the Vietnamese defenders.

The Chinese and Vietnamese have found rice cultivation so difficult in the area that they have left mucb of it to the ethnic minorities that often live nomadic lives on both sides of the border. "What a miserable life we villargers have!" China's news agency quoted one elderly woman living in a hilly area on the Vietnam side as saying "We get only enough rice for two months of the year, and have to live on casava and taro the other 10 months.

Ironically, the Chinese and Vietnamese often appear to trust the minorities in their respective border areas not much more than they trust each other. They Chinese province that borders Vietnam on the northeast is called officially the Kwangsi Chuang autonomous region, a gesture to the principal minority, the Chuangs, whom the Chinese still see as distinct from their own race. When I visited Kwangsi's capital, Nanning, about 100 miles from the border, last summer, guides hastened to point out the Chuangs and other minorities we ran across in offices and special schools. They seemed very sensitive to changes of discrimination.

In an important Vietnam Worker's Party congress in Hanoi in December 1976, perhaps the beginning of the latest outbreak of Chinese-Vietnamese hostility, some officials with links to China were removed from office, as were three army generals who had come from minorities living near the border. They apparently could not be trusted in a fight with China. Analysts thought they saw in months afterward further action taking responsibility on the borde away from minority officials and giving it to Vietnamese.

When the Chinese invasion began, it converged on the two main valleys that have historically let invaders through the ragged country -- the Red River valley at Laocai, a border town now in Chinese hands, and the Friendship Pass leading to the Vietnamese town of Langson, now the apparent focus of a major movement of Chinese forces.

These were the two places where the Chinese appear to have been able to bring heavy equipment, such as tanks and artillery, into Vietnam with relative ease. Years ago, the rail lines coming through these passes were the targets of bombing by U.S. airplanes during Hanoi's war to conquer the south. Washington shought a way to stop the flow of supplies from China to what was then Peking's socialist ally.

At perhaps two dozen other places along the border, Chinese troops also reportedly moved across on Feb. 17. But most were on foot or on horseback, taking the hills much the same way the troops of the Han emperor did when they came south to suppress the unruly Vietnamese 2,000 years ago.

Much of the battle n orth of Langson has reportedly involved Chinese moving up hills on foot to dislodge Vietnamese who have pinned them down with withering fire. South of Laocai, Chinese troops are reportedly now moving slowly south along the Red Rive, but staying out of the exposed valley and moving across hilltops instead.

West of Laocia across the Fansipan range, rising to just over 10,000 feet, the valley of the Red River tributary, the Black River, is even steeper and narrower. Observers outside the country say they find it hard to determine if the Chinese have taken the key town of Laichau and if they have moved farther south.

In the far east, at the coastal town of Mongcai, the fighting appears equally confused. The Chinese there seem to be cut off from the rest of the fighting, again by a small group of mountains, the first of many barriers that cut off all the the small border towns from each other.

"I can see why the Chinese ahd some trouble with their communications," said one outside ovserver. "I wonder after this experience, if they are going to ever want to try it again."