SEOUL, JULY 7 -- As thousands of chanting students streamed through the gates of Yonsei University two days ago, a young soldier in combat fatigues shouldered his way upstream through the crowd.

The demonstrators, decrying military dictatorship in South Korea, ignored the soldier as he walked by. About their age, he watched impassively for a few moments and then moved on.

Except for an occasional infantryman out for a stroll, South Korea's 600,000-strong military has remained out of sight during the past month of street battles and political turmoil. To a nation accustomed to martial law, that absence is as striking as the rumble of tanks over city streets.

But the Army remains an unpredictable element as South Korea attempts to move from the controlled regime of President (and former general) Chun Doo Hwan to a more open democracy. Admired by officers around the world for its professionalism, the South Korean military is also secretive, factional, attuned to every nuance of domestic politics -- and a mystery even to the American officers who work with it every day.

South Koreans and Americans generally agree that most South Korean officers are relieved to be out of the fray, free to focus on the North Korean Army just 30 miles north of Seoul. But they also worry that the Army might abandon its self-restraint.

These observers wonder, for example, whether the military would allow a new government to try some of its officers for torture or human rights abuses, as new democracies have attempted elsewhere in the world. And they wonder whether the Army would permit its longtime nemesis, opposition leader Kim Dae Jung, to assume office if he won a fair election.

"A majority of officers would stay neutral," Kim said in a recent interview. "They have come to realize that the military has lost people's support and love because of its involvement in politics.

"But I know a small number of officers still maintain their mission to maintain control. They have enjoyed privileges under a succession of dictators."

Sources closer to the Army than Kim agree that a small group of colonels and generals, tapped early for "guided careers," control key positions in the Army and expect choice government or industry jobs when they retire.

"It's an elite within an elite, and that idea appeals to them," one source said. "There's some understanding that times are changing, but there's also a feeling of, why should we be the first to suffer?

"If you took a vote among the officer corps as a whole, they would not want to get involved in politics. But the small group that has the power to move the Army as an institution may feel differently."

Twice before since South Korea's independence, in 1960 and 1980, the nation seemed on the verge of democracy. Both times the military intervened, with Gen. Park Chung Hee staging a coup in 1961 and Chun emerging as leader in 1980.

Chun subsequently was elected in 1981 and has promised to step down peacefully in 1988. But when street demonstrations with an anti-Chun cast began attracting middle class support last month, the unseen tanks were on everyone's mind. While U.S. officials warned repeatedly against martial law, Chun seriously considered calling in the Army to restore order, sources here said.

"At one point, it was a near thing," one knowledgeable source said.

But Chun rejected that option in favor of concessions to the opposition, including direct presidential elections in the fall and release of political prisoners. The troops remained in their barracks.

Government officials, including the ruling party's likely candidate to succeed Chun, Roh Tae Woo, say they expect that the military will continue to stand aside as democratization proceeds. South Korea's economic growth is the envy of the developing world, its people are virtually all literate and so the nation is ready for civilian democracy, they say.

Choi Chang Yoon, vice minister of culture and information, said that as South Korea has developed, the Army has changed, too. Choi graduated from and later taught at the Korean Military Academy, South Korea's West Point that has trained most of the Army's top officers.

"The Korean military under President Chun is different from that of the 1960s and 1970s," Choi said. "In the past six years it has stayed out of politics, and Korean society has become more mature and pluralistic. Unlike his predecessors, President Chun has not relied on martial law or emergency measures in coping with domestic political problems."

Indeed, the South Korean Army scrupulously avoids the kind of involvement in day-to-day decisions common in some Asian nations. Generals do not comment on farm policy or foreign affairs.

"There are no talkative military people," said one government official.

But the Army does keep close watch. Political officers brief Army units each time Chun gives a speech, for example, and they always have been assumed to have veto power over any major decisions.

In addition, retired officers hold key positions in the Cabinet, the National Assembly and in many businesses with close ties to government. Their weighty presence leads opposition figures to label the government a military dictatorship.

"It's a question of personal advantage, not ideology," said one official, who questioned whether the military would easily relinquish the favored position that it has enjoyed.

Still, one memory haunts the Army as it haunts the entire nation and serves as a brake on any military action. In 1980, soldiers ordered into the provincial capital of Kwangju to quell demonstrations there killed hundreds of young people under circumstances that the government has never fully explained.

Ever since, "Kwangju" has been a rallying cry for dissidents, a cross for the Chun government to bear and, for the Army, a watchword of caution.

"Kwangju is a looming presence," said Aryeh Neier, vice chairman of Asia Watch, who visited South Korea this week. "The fact that it happened once gives strength to the people who counsel restraint . . . . They would have to be prepared for a real bloodbath if they tried to prevent anyone from becoming president."

"They do not want to be despised," said Yang Soon Jik, vice president of the opposition Reunification Democratic Party. "They want to be respected."

But Kwangju is also a watchword of another kind, because many students, relatives of Kwangju victims and sympathizers already are demanding investigations and trials for those responsible.

"There are lots of people in powerful positions or close to people in powerful positions who were involved, and I don't think the military would like to see that {investigations and trials} happen," said one source close to the military.

Opposition leaders, including Kim Dae Jung and Kim Young Sam, repeatedly say they do not want "retaliation." What matters, Kim Dae Jung said, is for the government to "proclaim the restoration of Kwangju people's honor."

But a western diplomat said the pressure for reprisals will be strong if the opposition party comes to power.

"I don't think their intentions lie in that direction," the diplomat said. "But I'm not sure if I were one of the people with my neck on the line whether I'd be inclined to accept their assurances at face value."