At the last outpost along this critical juncture of the Sino-Soviet border, seven young Chinese sentries and a German shepherd dog are all that would stand in the way of a marching Soviet Army if the Russians were to decide to move into this vast, frozen Inner Mongolian plain.

Here along what would be the main boulevard into northeast China, this tiny band of soldiers, Chinese-made rifles at their side, gamely stand guard in -60 degrees weather peering through binoculars at the busy, well-armed Soviets across the border.

Most of the 4,500-mile boundary separating the two communist giants is a fiercely contested, tense line of hostility bursting with the world's largest mass of troops and arms -- approximately 500,000 to 600,000 Soviet solidiers facing 1.5 million Chinese.

But here, in the remote yet strategically important corner of land where China abuts Mongolia on the west and the Soviet Union on the north, an eerie, surreal tranquility prevails.

In a rare-visit for American journalists last month, we found an isolated, virtually undefended Chinese border stretching 50 miles across the top of this pass.

Arching around the town of Manzhouli, the wide, unobstructed mouth of the pass sits like an open invitation for the hundreds of Soviet tanks, howitzers and tactical rocket launchers the Chinese say are gouped across the border.

For centuries the well-worn invasion route for warlike states from the north, Manzhouli Pass remains today the main access route to northeast China for roughly 100,000 Soviet troops reportedly massed along the nearby Mongolian and Soviet borders.

Like the border with Vietnam in the south, which we also visited, the northern frontier is considered by the Chinese a key to their security and future development. But while both borders are important, they reflect the tremendous diversity of China in geography, climate and local folks cultures.

Traveling from the town of Manzhaouli by military jeep to the westernmost border outpost 35 miles away, you quickly realize why this pass was the traditional invasion route for Tartars, Mongols and Russian Cossacks.

Without natural barriers such as mountains or water, the flat, open grassland provides easy access for invading armies. A railway built through the pass in the 1900s linking European Russia with the Far East makes the plain even more attractive to military planners.

With little hope of stopping a land invasion that most certainly would be supported by superior Soviet air power, the Chinese Army has withdrawn hundreds of miles southeast to the mountains rimming the nation's industrial and oil-producing heartland, according to Western military analysts.

What they left at Manzhouli Pass is a tiny unit of lightly armed soldiers -- no more than 11 at each of seven border posts -- the only apparent defense for the thinly populated Inner Mongolian frontier of herders coal miners.

This frontier town, just five miles from the border, boasts a civilian militia of 10,000. But the untrained group would offer little backup against Soviet tanks and planes.

Some stops along the border look as lonely as an abandoned beach, the sentries like idle lifeguards watching the waves. The distant sight of barbed wire tossing in the snowy wind or a Soviet patrol truck are the only reminders of a 20-year-old boundary dispute.

At a livelier border point, in the town of Manzhouli, you can watch the 4:40 p.m. train carrying goods such as trucks and cars from the Soviet Union into China or gaze through a high-powered telescope at a busting Soviet garrison town less than a mile away.

Planted in the snow-covered earth along the railway tracks, a broad white sign boldly procliams in large, red Cyrillic letters, "U.S.S.R."

From the top of a high observation tower, Chinese soldiers peek into the windows of the Soviet military command, recording every movement of the enemy troops with laboratory precision. Sometimes they even watch the beefy Soviet sentinels watching them.

Although they can recite almost every nut and bolt of The Soviet war machine facing them, they say they have not been told what to do if the Soviets started up their engines and crossed the border.

"Our only job is to observe and report immediately to our superiors," insisted Zhang Wembin, the imperturbable political commissar of the 280-man Manzhouli border defense.

Do you have contingency plans for a Russian attack? we asked him.

"That is not our job," he replied.

But surely you have a clue, don't you?

"Maybe the coordination work isn't too good," came the answer.

In this badly lopsided military equation, Zhang and his men can only hope to avoid the kind of tension that sparked the 1969 clash of Soviet and Chinese armies at the Ussuri River more than 1,000 miles east of here.

The Soviets occasionally like to display their military dominance, however, turning this normally stable border into a fiery stage for Army maneuvers, said Zhang, reminding the outnumbered Chinese that tranquilty at Manzhouli Pass lasts as long as the Soviets desire.

During the Sino-Vietnamese war in early 1979, for example, while China was fighting a Soviet ally in the south, Soviet troops here drove their tanks within 300 yards of the border and fired blanks, according to Chinese reports. wSoviet fighter planes hovered over the border for six days, said Zhang, and the town of Manzhouli was lit up by Soviet floodlights for nights.

Living in the shadow of the Soviet Army, Zhang took special precautions not to alarm the Soviets while American reporters were touring the front. Blending in with the border patrol, we were issued the long, green Army coats and black boots worn by the Chinese Army. We were allowed to photograph everything on the Chinese side but warned against taking pictures of anything Soviet.

Except for an occasional Mongolian village, there is little sign of life across a frontier that accumulates 40 inches of snow each winter. Churning through the icy pass, our jeep passed a horse-drawn cart hauling manure and a herd of ponies led by a single horseman wearing knee-length woolen boots and sheepskin hat and vest.

After a jostling, 90-minute ride, we arrived at the outpost named 841.9 Hill (designating the number of meters it rises above sea level) and received a formal military welcome -- all seven sentries lining up with their watchdog and applauding as we alighted from our vehicle, then saluting individually as we reviewed them.

Across from the small brick barracks and 80-foot observation tower that together make up the outpost, several large red Chinese characters implanted in a mound of dirt symbolize their lonely vigil: "We take the outpost as our family."

Inside the warm, neat barracks, we had a good look at the life of these border guards, all in their twenties, who get assigned here for three years and have only their Sunday furloughs in the town of Manzhouli 35 miles away to relieve the isolation.

Along a heated wall, six sets of bedding no more than a yard apart lie across a common straw mattress elevated onto a cement platform. A small, wood-frame bed for the commanding officer stands off to the side -- perhaps the only privilege at 841.9 Hill.

Apart from a small wooden stool placed at the foot of each set of bedding, there are no personal effects in this "family" room. The walls are strictly functinal: walkie-talkie and a pair of binoculars hanging on pegs and yellowing copies of the Army newspaper posted here and there.

Even though the closest Soviet encampment was beyond the scope of high-powered binoculars on the overcast day of our visit, the guards claim to have precise estimates of the Soviet strength across the border from them.

Shepherded into a small, unheated "staff" room with large photographs and blueprints of Soviet military equipment plastered on its walls, we received a long briefing from the commanding officer of this westernmost stretch of border.

Hovering over a meticulously designed sawdust model of the border, he wielded a wooden pointer to identify all five disputed border points within his command and then described the Soviet buildup on their side.

The Soviets, he said, have devised a system for detecting border violations: two walls of barbed wire enclose a wide strip of smooth, uncultivated soil that serves as a poor man's sensor, showing footprints or tire tracks. The second row of wire trips off an alarm if touched.

Judging by their own estimates and sightings during Soviet military maneuvers in the last few years, the seven residents of 841.9 Hill stand little chance of defending the border against what they say is a division of Soviet troops backed by T54 and T55 tanks, 122 mm self-propelled howitzers, Mig23 multipurpose fighters and Mig25 interceptor and reconnaissance planes.

"Our only job is to observe," reminded political commissar Zhang, who accompanied us throughout the visit.