The scenes quickly became part of the Middle East legend: more than 8,000 Palestine Liberation Organization fighters, hands waving a ''V'' for victory, guns blazing, parading in crammed truckloads through the streets of Beirut to the port for their evacuation by ship to eight distant Arab lands.
Although trounced by the Israeli military machine, PLO leaders claimed they had achieved a political victory: International interest in the guerrilla organization had never been higher. A superpower had intervened and prevented the Israelis from invading west Beirut, then arranged for a peaceful PLO exodus.
But the world's most notorious liberation movement will mark the first anniversary of the evacuation on Sept. 1 as a year of lost opportunities rather than a period of promised growth and progress. The hope turned sour through a number of events:
* In April, Yasser Arafat balked at supporting President Reagan's Middle East peace plan, refusing to give King Hussein of Jordan a mandate to negotiate for the PLO.
* In June, Syria, the only one of four countries abutting Israel which allows the PLO bases of operation, expelled Mr. Arafat. The simmering distrust between Mr. Arafat and Syrian President Hafez Assad broke into an open feud.
* Arafat's own Fatah faction, which dominates the PLO's eight factions, has been in disarray for almost four months due to a mutiny by dissidents demanding major reforms and a change in leadership posts.
* And those 8,300 fighters are still effectively trapped in eight camps, spending most of their time practicing on weapons with no ammunition, monitoring radios for news, isolated from their families. The new PLO headquarters in Tunis is a long way from the operations center in Tripoli, Lebanon - and the majority of the dispersed guerrillas.
Palestinians are among the first to concede that this period is the most troubled in the PLO's 19-year history.
At the heart of the current problems is an ideological difference on strategy: the old guard of moderates who favor a political settlement vs. the new breed of militants who want to fight their way to Jerusalem.
A series of mediation efforts, most recently by South Yemen and a new committee from all eight PLO factions, have failed to close the gap represented by the Fatah revolt. Meanwhile, interventions by the Soviet Union, Algeria, Saudi Arabia, and the Arab League have also failed to heal the differences between the two key symbols of Arab policy, Mr. Arafat and Mr. Assad.
Even the PLO chief was quoted recently as admitting: ''The (Palestinian) movement is getting weaker with every day that passes.''
There is currently a lull in the fighting between loyalists and supporters of Arafat in Lebanon's eastern Bekaa Valley, the theater where the mutiny has been played out, after fierce clashes last month. And a truce in the war of words between Arafat and Assad has been arranged while the PLO committee shuttles between Tunis and Damascus.
But Abu Jihad, senior military aide to Arafat, predicted last week that the latest ceasefire would not hold much longer. And Assad has so far refused to talk to PLO mediators, delegating it to his foreign minister.
Time has been costly for Arafat and other moderates, for it has shown the depth of the rebellion. Although rebel leader Abu Musa clearly could not have made major gains without Syrian military and political support, the endurance of the mutiny has its roots in genuine and strong opposition.
And no amount of pressure appears capable of cracking a compromise out of either Syria or the rebels.
Abu Iyad, Fatah's deputy commander, said in an angry speech recently: ''What happened was not a call for reform. It is an attempt at a military coup.
''Syria can stop the fighting. But what is it that they want? Are we required to go there (to Damascus) raising white banners?''
Palestinians and Arab diplomats now suggest that Syria is content to let the rebellion simmer on, diverting the attention of Assad's main rival for the Middle East limelight. Throughout a summer of key negotiations about the region's future, Arafat's name has surfaced only in connection with the rebellion.
Meanwhile, the rebels have managed to eke out a few concessions that set the precedent for other demands. The two guerrilla commanders whose controversial appointments ostensibly sparked the revolt were removed in late July, and a Fatah council agreed to the principle of greater collective leadership instead of the singular rule of Arafat.
Abu Jihad declared recently, ''We in the PLO reject outright the Reagan plan. This is our clear position without maneuvering or any ambiguity.'' Rejection of the Reagan plan was one of the rebels' demands. However, Mr. Arafat has not yet publicly condemned it, despite persistent queries, leading Western diplomats to suggest that he may be trying to hold on to the only viable means of a political solution to the Arab-Israeli conflict.
The passage of time has also forced the other PLO factions to take a stand. All have endorsed the principle of reform, while most have at the same time supported the ''recognized'' Palestinian leadership. Two small, pro-Syrian factions have effectively joined the mutiny, occasionally fighting alongside the rebels against Arafat loyalists.
One year ago, the Palestinians were optimistic that the time for a settlement of the Middle East quagmire was ripe. Today, they are unable to settle their own squabbling.
On the final day of the 12-day evacation from Beirut, Abu Iyad boasted: ''We are merely moving from one fighting position to another. This revolution will never end.''
Little did he realize how new fighting and a new revolution might divert, and maybe even destroy, the PLO's new-found leverage. ONE YEAR UNDER THE REAGAN PLAN PLO evacuates Beirut, August-September 1982 Palestinian civilians massacred in Beirut refugee camp by Lebanese militiamen, September 1982 PLO refuses to let Jordan's King Hussein enter US-sponsored peace talks, April 1983 Fighting breaks out between factions of Arafat's Fatah guerrilla group in eastern Lebanon, May 1983 Israel approves 10 new West Bank settlements on Sept. 5, 1982 Israel's Prime Minister Begin rejects Reagan plan, Sept. 2, 1982 US Marines arrive in Beirut Aug. 25, 1982; leave Sept. 10, 1982; return in force Sept. 29, 1982