The chain of events that led to the overthrow of the 20-year-old regime of President Ferdinand Marcos began on a sweltering August day three years ago on the tarmac of Manila International Airport. It ended at dawn today on the tarmac of Clark Air Base, a U.S. military installation 50 miles north of Manila, when Marcos, his family and an entourage of more than 60 people left the Philippines for Guam and Hawaii.

On that day in 1983, a military man, probably a member of the Philippine Constabulary, fired a single shot into the back of the head of former senator Benigno Aquino Jr. as he was being escorted down a stairway from an arriving jetliner, according to a subsequent fact-finding board's report. The 50-year-old Aquino, jailed for more than seven years by Marcos, was returning to the Philippines from self-exile in the United States.

His assassination eliminated Marcos' political arch-rival, the one man seen as capable of uniting a divided opposition and challenging Marcos for the presidency. But those who planned and executed the murder undoubtedly never foresaw that it would launch a period of political upheaval culminating in the "yellow revolution" led by his widow, Corazon Aquino.

The phenomenon seemed to have bewildered Marcos to the end, a swiftly accelerating downfall from what he had hoped would be a position of strength with a "new mandate" to remain president for another six years.

The downfall began with a stunning military mutiny Saturday led by Marcos' defense minister, Juan Ponce Enrile, and his acting armed forces chief of staff, Lt. Gen. Fidel Ramos. The rebels quickly endorsed Aquino, who they said had been cheated out of the presidency by Marcos in a Feb. 7 election, and gained the support of the powerful Roman Catholic Church and, apparently, the vast majority of the Manila area's 8 million residents.

The end came after an emotional meeting between Marcos and Enrile, who had served him for 20 years but had turned against him three days earlier and played a key role in engineering his overthrow. Shortly before Marcos was taken from Manila in a U.S. helicopter, he had a long talk with his renegade defense minister in a park across the Pasig River from his Malacanang presidential palace. The meeting ended, according to witnesses, with words of conciliation and a long embrace between the two men.

Then Marcos and his family took off for Clark Air Base, where he spent the night in the "distinguished visitors' quarters," a U.S. Air Force spokesman said. Shortly after 5 a.m. today, the ailing 68-year-old president, walking stiffly but under his own power, boarded a U.S. C9 Nightingale hospital aircraft and took off for Guam with his family and his longtime confidant, Gen. Fabian Ver. A second aircraft, a C141 Starlifter transport, left a few minutes later with his entourage and the party's baggage.

What transpired between the two tarmac scenes was a steady erosion of Marcos' control over a country once described as consisting of one SOB and 55 million cowards. The authoritarian domination that he built up during nearly nine years of martial law from 1972 to 1981 began to evaporate following the Aquino assassination, when huge crowds poured into the streets to pay tribute to the opposition leader. The events of the last three days finally disproved the epithet.

The outpouring of sentiment in 1983 spawned the "confetti revolution," in which protest marches through the capital were showered with confetti -- usually yellow, in memory of Aquino. The color was chosen because of the song, "Tie a Yellow Ribbon," about awaiting the return of a prisoner. Crowds awaiting Aquino's return on the day he was killed sang the song and waved yellow ribbons to show their support. The killing alienated the largely docile middle class, the business community and elements of the military ashamed of their tarnished image.

All three groups became instrumental in the overthrow of Marcos through a massive display of "people's power," as the huge anti-Marcos turnouts of citizens from all walks of life came to be known in the last few days. Encouraged by the powerful Roman Catholic Church here, they were outgrowths of the "yellow revolution" launched by Aquino in her campaign for the presidency this month. So it was that when Marcos was falling, it was yellow flags -- not the red of the pro-Communist and other leftist groups opposing Marcos -- that waved over the hundreds of thousands of people who turned out to physically prevent the tanks, troops and armored vehicles of forces loyal to Marcos from attacking military mutineers.

The left was relegated to the revolutionary wilderness as "people's power" became a largely middle-class affair dominated by moderates.

While as many as 20 persons were reported killed during the mutiny, the revolution was seen here as largely peaceful considering that so many potentially bloody confrontations ended without violence when loyalist forces refused to fire on the unarmed demonstrators serving as "human barricades" for the rebels.

The stage for all this was set by the Aquino assassination in the sense that it mobilized large numbers of political moderates in street protests for the first time under Marcos' rule. That process coincided with a steady decline in Marcos' credibility because of the handling of the murder, which he blamed implausibly on a hired Communist assassin.

The killing haunted Marcos' ruling party, the New Society Movement, during May 1984 legislative elections in which the opposition made a much better showing than expected, winning about a third of the seats in the National Assembly. The biggest victory came in Metropolitan Manila, where Marcos foes won 16 of 21 seats. Especially infuriated was the president's wife Imelda who had managed the ruling party's campaign in the capital.

That election, however, did not solve the opposition's main problem: finding a single leader who could challenge Marcos in a presidential election scheduled for June 1987, or in the event of a rumored "snap" election that Marcos might call to catch the opposition off guard.

While this search was under way, the Aquino case was dragging on inconclusively. His widow, Corazon, refused to participate in a process that she was sure would end in a whitewash, and she was proved right in the end. After a fact-finding board appointed by Marcos surprisingly implicated Ver, 24 other military men and one civilian in October 1984, a special court took up the case in a protracted trial that culminated in the acquittal of all 26 defendants. The verdict, widely viewed as a travesty of justice, plunged the Marcos government's credibility to a new low and further alienated the public.

Aquino during this time served as head of the Benigno Aquino Foundation, dedicated to her husband's memory, and as a member of an opposition "convenors' group" formed to help choose a single presidential candidate against Marcos. She appeared often at opposition functions but steadfastly refused to become a presidential contender herself. She was finally persuaded to when two conditions were met: supporters gathered more than 1 million signatures on a petition for her candidacy and Marcos called a "snap" election. She formally announced her candidacy on Dec. 3, the day after the court trying her husband's murder case acquitted all the defendants.

Marcos, meanwhile, was under increasing pressure from the United States to implement sweeping political, economic and military reforms that Washington saw as essential to combat the country's number one problem: a Communist insurgency that analysts said could take the Philippines out of its alliance with the United States within five years. It was largely in response to the pressure that Marcos announced the early election on U.S. television in November.

It seemed a brilliant move. Eventually set for Feb. 7, it caught his foes still divided and raised the prospect of more than one opposition presidential candidate -- a situation that would guarantee Marcos victory. But the wily master politician clearly underestimated the 53-year-old widow's popular appeal.

At the eleventh hour before a deadline for filing candidacies, Aquino succeeded in forming a united opposition ticket with her main rival, Salvador Laurel, who agreed to run as her vice president. From then on, the opposition election campaign steadily gathered steam.

In sharp contrast to Marcos' campaign, Aquino and Laurel set a grueling pace of campaign appearances all over the country that aroused an enthusiastic following. By comparison, the vaunted Marcos political machine could produce only stage-managed affairs that relied on rent-a-crowd tactics and a propaganda blitz over the government-controlled airwaves.

It was taken for granted that the Marcos machine would cheat in the election, but the extent and brazenness of the fraud and accompanying violence shocked everyone, especially American congressmen and other international observers invited by Marcos to monitor "democracy" in his country.

A principal, and novel, means of cheating was the massive disenfranchisement of voters that effectively held down the turnout in heavily pro-Aquino areas, notably the capital. An estimated 3.3 million Filipinos nationwide were unable to cast their ballots (although voting is compulsory here), and more than half a million were disenfranchised in Metro Manila alone.

The election resulted in a Marcos "victory" by 1.5 million votes in an official count by the assembly, which he controlled. But an independent count by a citizens' poll-watching group ended with Aquino ahead by nearly 800,000 votes. The independent count was accepted as the more credible, and the election was denounced by foreign observers, the U.S. Congress, the Reagan administration and a broad cross section of Philippine society that included the Catholic Church and reformist elements of the military.

The election also left vast numbers of Filipinos determined to be rid of Marcos, and a postelection campaign of civil disobedience, strikes, demonstrations and boycotts led by Aquino got off to a rousing start, mobilizing even bigger crowds than she drew during the election campaign.

Then, when a few hundred reformist elements in the military mutinied to prevent the arrest, so they said, of Enrile and a crackdown on both their own ranks and on the opposition, the "people's power" phenomenon was born. Many of the participants were those angry Manila voters who felt cheated in the election and now voted with their feet to resist Marcos.

A fiesta atmosphere prevalent in so many opposition street demonstrations belied a determination this time to see the "revolution" through. And, in a strange way that risked bloodshed and chaos but never stepped over the line, Filipinos finally got what they had voted for.