TOKYO -- Even outside their own country, the North Korean students would always stick to themselves.

At 6 a.m. they would congregate for group calisthenics. Evenings and weekends they would gather in their dormitory or embassy to listen to North Korean radio and study the political thought of their communist leader, Kim Il Sung.

"Even in the dining hall, they would sit together and try never to talk to foreigners," recalled Takashi Uehara, who studied with North Koreans at a Chinese university and now follows Korean affairs for the Japan External Trade Organization. "If anyone was found to be on friendly terms with a foreigner, his political situation would be very, very deteriorated," he said.

Like its students abroad, North Korea has long been one of the world's most closed societies. Now it is more isolated than at any time in recent years, and in that isolation, many experts fear, lies growing danger for North Korea's enemies.

"This year is the most problematic year," said Kim Chang Soon, chief director of the Institute of North Korea Studies in Seoul, in a recent interview. "We know they are planning something, but we cannot say what. That is the problem."

According to interviews with analysts and officials here and in Seoul, North Korea's leaders must feel cornered as never before. Their nation has fallen far behind its capitalist rival to the south economically, and the gap widens month by month. Its military advantage is eroding and may be gone.

Last week the United Nations condemned North Korea, saying it had masterminded the sabotage of a civilian South Korean airliner last fall, in which all 115 aboard were killed.

Perhaps most painful, most of its communist allies have rebuffed North Korea's effort to stage a boycott of the Summer Olympic games in Seoul. Cuba, Ethiopia and Nicaragua have declined invitations to attend the Olympics. The Seychelles and Albania did not respond.

Now Pyongyang's leaders must watch as South Korea prepares to host those September games, celebrating its coming of age and threatening to tag North Korea as perpetual runner-up in the 40-year-old contest to be recognized as the real Korea.

Many in the West find it puzzling, almost beyond belief, that North Korea would think that sabotaging a civilian airliner could further its cause. Similar disbelief has been expressed in the past, when North Korean agents killed South Korean Cabinet members, tried to assassinate South Korean presidents, sank South Korean fishing boats or killed U.S. soldiers at the Demilitarized Zone.

But Pyon Jin Il, a Korean resident here, who worked for a pro-North newspaper for 10 years and now publishes the independent Korea Report, said he believes North Korea was responsible for the airliner's destruction. He said such acts should come as no surprise.

"North Korea views the Olympics as a grave threat to its national security," Pyon said. Many outsiders fail to understand, he added, that the Korean War of 1950-53 "has not ended."

U.S. officials said that they fear the destruction of Korean Air flight 858 last November may not be North Korea's last attempt to disrupt the games.

"We don't know, but you shouldn't assume that whatever they intended to accomplish was to be accomplished by this isolated act," said one official.

The difficulty is that western officials know remarkably little about North Korea and the longest ruling leader of the communist world, Kim Il Sung. They interview the occasional defector, they listen to reports from business executives and journalists who are invited on highly circumscribed visits, they study the propaganda and the hierarchy of officials in photographs -- and they guess to fill in the blanks.

Some things are known. It is clear that Kim Il Sung has established a personality cult unlike any other in the world. From his giant statues rising over Pyongyang to his cigarette butts preserved in the revolutionary museum, Kim and his ideology of self-reliance, or juche, in Korean, dominate the nation of 20 million people.

It is clear, too, that North Korea is a rigidly controlled society, perhaps the most tightly controlled in the world. Defectors report that no one can travel by train without a permit, no one chooses his or her own career, no one is free of the surveillance of neighbors and party cadres.

In human rights, a State Department official said last year, North Korea is "probably the worst offender in the world . . . the closest, I would say, to George Orwell's model of a 1984 state."

It is known, too, that North Korea's economy has stagnated. While South Korea exports cars and video tape recorders, North Korea defaults on its debt and desperately seeks foreign exchange by selling fish paste and minerals.

Most analysts say that until North Korea opens itself to the West as China has, it cannot acquire the technology it needs to modernize. But most also say that North Korea will resist the changes now shaking most of the communist world at least until Kim Il Sung, 76, dies.

What will happen then is a question to which Korea-watchers offer widely differing answers.

No one doubts that Kim is grooming his son to succeed him, a power transfer that would be unprecedented in the communist world. Kim Chong Il, known as "the Dear Leader," routinely appears in paintings and statues alongside his father, "the Great Leader."

The orthodox view in anticommunist South Korea holds that the two Kims now jointly control North Korea.

"In the North Korean regime, there is no hard-line or soft-line group," said Kim Chang Soon of the Institute of North Korea Studies. "North Korea is strictly controlled by Kim Il Sung and Kim Chong Il, and there is no other leading force."

The South Koreans maintain that Kim Chong Il personally orders major terrorist actions; the confessed saboteur in last fall's airline explosion said she was told her orders came from him. Kim Chong Il is portrayed as a plump playboy, fond of western movies and liquor, but a powerful presence nonetheless.

But Kim Chang Soon predicted that the younger Kim will not last more than three years. Both China and the Soviet Union dislike the notion of inherited power, he said, and young military officers also will chafe.

"They don't believe these legendary stories of Kim Il Sung's supernatural power to change sand to rice, to turn pine needles into gunpower," the analyst said. "These young officers who have studied modern scientific military strategies, they just keep quiet while Kim Il Sung is alive. But when Kim Chong Il is in power . . . those officers will start to make trouble."

Some analysts in Japan say that, even now, there is dissent in North Korea. They note that shortly before the airline bombing, some North Korean officials were speaking in conciliatory tones, just as some North Koreans were talking peace just before the 1983 bombing in Rangoon, Burma, that killed four South Korean Cabinet ministers.

These analysts speculate that such bombings may be weapons in an internal war, in which Kim Chong Il or other "hard-liners" show their power to quash any talk of accommodation with the West.

"I don't think there are factions, I know there are," said one high-ranking U.S. official in a recent interview.

But this official said the situation is more complicated than hard line versus soft line -- much of the Army, eager for advanced Soviet weapons and disgruntled at being assigned to civilian construction jobs, leans toward the Soviet Union and chafes under the personal cult of the Kims.

Some in the bureaucracy, the official continued, feel most sympathetic to the Chinese model and would like to experiment with economic incentives and more trade with the West.

Kim Chong Il, meanwhile, is beyond any faction's control, an unpredictable and dangerous force. No one dares attack him while his father lives, this official said, but he is likely to be assassinated or otherwise removed within months of Kim Il Sung's death.

In the meantime, all of the Korea-watchers are left guessing what will happen next, particularly between now and the time that Chinese and Soviet athletes are scheduled to arrive in Seoul for the Olympics.

A spokesman for North Korea, in a recent interview, said that South Korea is being unreasonable and hostile in refusing to cohost the games.

"Unless the South Korean side changes its position, the world public will blame the South Korean government for failing to realize co-hosting," said Pak Jae Ro, vice chairman of the pro-North association here that serves as its unofficial embassy. "They should take responsibility for the consequences."

Asked what those consequences might be, Pak replied: "I am not in a position to give you a clear image . . . nobody can foresee what will take place."

A worried U.S. official expressed a similar uncertainty, noting that U.S. and Japanese sanctions against North Korea for the airline bombing have increased its isolation and may heighten the danger of future acts.

"You have no choice but to condemn and isolate North Korea for such a barbaric act," the official said. "But you also in all honesty have to recognize that part of the problem is its isolation."