The four-engine U.S. Air Force jet was flying southeast toward Australia on Saturday morning, June 20, but Secretary of State George P. Shultz's discussion with his senior aide on Asia, Assistant Secretary Gaston J. Sigur, was about the crisis in South Korea, thousands of miles to the north.

Sigur had been up late the night before pondering U.S. Embassy reports from Seoul that President Chun Doo Hwan had been seriously considering the use of massive force to stop nationwide demonstrations. Earlier that day, Chun had received a letter from President Reagan counseling restraint, but his course of action was uncertain.

In the front cabin, which was fitted out as an airborne meeting room, Sigur proposed that he leave Shultz's 12-day tour in Australia and fly to Seoul to deliver U.S. views in person and to obtain on-the-scene impressions. After telephone calls to Seoul and Washington from the plane, it was agreed that Sigur would make the trip.

The Sigur mission was the most visible feature of the Reagan administration's interaction with South Korea as the government, politicians and people of that country made a dramatic turn toward direct presidential elections and popular rule. Washington officials are still savoring a result that seems to match Korea's economic miracle of recent years with a political miracle: "the damnedest thing I ever saw," in the words of a senior U.S. official.

As in the Philippines and Haiti early last year, the United States played a significant role when a friendly country was gripped with a crisis of authority. This time no U.S. plane whisked an authoritarian leader away to exile. The evolutionary change, rather than revolutionary change, is being hailed by American officials as a sign of South Korean political maturity -- and of a more mature relationship between Washington and Seoul.

"We were listened to in part, but we played at the margins," said a U.S. policy-maker of the carefully timed messages of advice, which did not require any special National Security Council meetings because the broad lines of U.S. policy already had been established. "We were nudging {the Koreans}; we can't force them," the policy-maker said.

This sensitivity toward a militarily and economically advanced Korean regime seems a far cry from the patron-client relationship of 1960, when Koreans cheered U.S. Ambassador Walter P. McCon- aughy's automobile, with its U.S. flags flying, as he proceeded slowly down a main avenue in Seoul to tell then-President Syngman Rhee that further repression could not salvage his position and that it was time to leave. Rhee did.

Later U.S. efforts in times of political crisis were less successful. In 1961 strong statements from the U.S. Embassy did not dissuade Gen. Park Chung Hee from carrying through a military coup against the elected civilian government, though in 1963 U.S. pressure persuaded Park to go through with promised presidential elections rather than extend military rule.

The Nixon administration said little and did nothing as Park declared martial law and disbanded civil government in 1972 to keep himself in power beyond a third term. After Park was assassinated in 1979, the Carter administration was unable to prevent the seizure of power by Chun, then commanding general in the Seoul area, or his use of Korean army troops to suppress brutally an uprising in the southern city of Kwangju.

The recent crisis developed in large part because of an opportunity: Chun's longstanding promise to step down in February 1988 and hand over power to an elected successor. The unanswered and controversial question was how the successor was to be picked.

In February this year, in recognition of the importance of the year in prospect, Sigur laid out U.S. policies and preferences for peaceful change in unusual detail in a speech to the U.S.-Korea Society.

"The United States will continue to encourage all sides in Korea to work together to create a new political framework," Sigur said. "We will provide positive support, not interference. We do not and shall not support any particular proposal by any Korean political party, but we shall continue to urge accommodation, compromise and consensus."

The South Korean government and opposition were deadlocked over the succession, and the opposition party was splintering when, on April 13, Chun suddenly barred further debate and declared that his successor would be named indirectly by a council of electors, which he could easily control. Chun's move generated immediate and expectable protests on campuses and elsewhere, followed by a police crackdown.

Two days before the April 13 announcement, Chun dispatched then-Prime Minister Lho Shin Yong to notify the U.S. Embassy of his plans. Shultz was en route to Moscow to discuss arms control when the news came. State Department sources said that after conferences in Washington, U.S. Ambassador James Lilley was sent back to see Lho with the message that "we didn't think this is a good idea in view of our position on dialogue and compromise." But in a decision that has drawn criticism from some Korea-watchers in the United States, Lilley was not told to say flatly that Chun should not cut off the debate, and the State Department made only bland public statements in the aftermath of Chun's bombshell.

The continuing discontent exploded in much larger and more serious demonstrations starting June 10, when Chun's ruling party named former general Roh Tae Woo, a military academy classmate and close friend of Chun, as its presidential candidate and thus Chun's likely successor. It was no surprise that the demonstrations were led by students, the traditional vanguard of dissent in South Korea. What was a surprise both here and in Seoul was the unusually large size, boldness and persistence of the demonstrations and, especially, the strong indications of sustained middle-class approval and support. Previously, middle-class support for demonstrations had declined sharply after displays of violence or radical slogans.

"The common belief was that the Korean middle class, which had achieved so much in recent years, wouldn't risk much on political issues," said a Washington official. "It turned out the middle class didn't want to risk everything, but they were willing to risk a little. That jolted the {Korean} government."

Reports reached Washington of private motorists and taxi drivers honking their horns in support of the students and other older people applauding or cheering. When student protesters holed up in the main Roman Catholic cathedral near the center of town, there were reports that about $25,000 in small Korean bills was tossed onto the church grounds by passers-by in support of their cause.

After several days of the standoff with students in the cathedral, some factions in the Chun government were reported to be urging that police storm the sanctuary, arrest the students and take strong action against the political opposition. At that point the United States weighed in privately for the first time since June 10, with what is described as a strong plea for restraint and specifically against storming of the cathedral, delivered by Ambassador Lilley June 13 to Foreign Minister Choi Kwang Soo. Instead of storming the cathedral, the Korean government removed the riot police the following day, and the students went home.

As the disorders continued to grow, the hooded, padded riot police were stretched to the limit. Pressures grew in Seoul to use the army to suppress the demonstrations. U.S. officials now say that Chun was reported to be "thinking very hard" about massive suppression of the demonstrations on June 18 and 19. On June 19 the new prime minister, Lee Han Key, announced that the country was sinking toward "social chaos," which might require an unspecified "extraordinary decision."

The same day, Lilley delivered Reagan's letter to Chun. It called for restraint in handling the demonstrations, freeing of political prisoners, removing restrictions on the opposition and restarting the political dialogue with opponents. In other messages, private and public, the United States was warning that use of the army under any form of martial law would create a more serious crisis.

Of greater importance than messages from Washington, U.S. officials say, was the message to Chun from senior members of the South Korean military that they opposed use of the army to put down the demonstrations. Neither they nor Chun nor the Korean people have gotten over the deaths in 1980 of more than 200 people as army troops suppressed disorders in Kwangju touched off by Chun's takeover of power.

By June 22, as Sigur flew toward Seoul from the Shultz party in Australia, Washington was aware that the army leaders had rejected intervention. Nonetheless, the State Department issued an unusually direct statement that day urging "Korean military commanders" not to intervene. Officials say it was aimed not so much at top military leaders as middle-level officers who might have been tempted to consider seizing control of the situation, and of the government, in a coup. There was no hard evidence that a coup was brewing, but some officials noted worrisome gaps in the information available from operational levels of the army.

When Sigur walked into Chun's office in the Blue House, the Seoul equivalent of the White House, on June 24, he found a chastened leader who seemed convinced that sweeping political concessions would have to be made to solve the crisis. Sigur had heard much the same thing from Chun's friend and heir apparent, Roh, in a separate meeting and from other officials of the government and opposition.

Sigur would later say that he found "a sea change" in Seoul, with stunning shifts in the attitudes and positions of people he had seen there in the past. Another official pointed out that with demonstrations continuing, the riot police wearying, the army opposed to intervening and concern growing over the viability of the Seoul Summer Olympics next year, "Chun did not have a lot of options."

Sigur returned to Washington convinced that Roh and Chun would sponsor major conciliatory steps, in addition to the largely symbolic concessions already offered. After reporting to Reagan at the White House June 26, Sigur issued a statement calling for "further concrete positive moves" of more importance by the government, thus keeping up the pressure for what he believed would soon develop.

On Sunday night, June 28, Washington time, ruling party chairman Roh announced his support for direct presidential elections, which was the main demand of the opposition, as well as amnesty for opposition leader Kim Dae Jung, political and press freedoms and a variety of other measures. It was clear that Chun would have to back him.

U.S. officials learned of this startling development by monitoring Roh's speech on Korean television. Though there had been much U.S.-Korean discussion of the issues over the previous weeks, months and years, Roh had not told Washington what he was going to say.