A YEAR AGO, Syria stood by while the Israeli army rolled up the PLO in southern Lebanon, and now Syria is helping one PLO faction roll up another in eastern Lebanon. The faction under duress is the one that looks to Yasser Arafat, longtime PLO chairman, as its leader. He claims he is being betrayed by the Syrians, whose goal appears to be to make the PLO entirely beholden to themselves. On Friday, they underlined the point by throwing him out of the country. A veteran survivor of defeats and betrayals, Mr. Arafat cannot yet be counted out, but he looks more like a loser every day.

Does it make a difference if Yasser Arafat drifts into political limbo? He has represented the Palestinian people in their desperation and their diversity--no small feat. But he has not had the courage-- or, perhaps it is the conception of his role--to make the hard political choices necessary to lead the Palestinians toward a homeland. He has often seemed distracted by the excitement and motion of being attended by heads of state and the international press. He deserves to be regarded not so much as "moderate" as ineffective. No one will miss him who believes that realism is the Palestinian national movement's vital need now.

It is suggested that any new PLO leadership sponsored by Syria will be more militant, readier to fight than talk, perhaps bent on terror. How quickly memories fade of the policies conducted or indulged over the years by Mr. Arafat. In fact, the government of Hafez Assad has a record of keeping "its" Palestinians on a tight leash--preventing them from doing anything not subordinate to its interests. It would be uncharacteristic for the Syrians to authorize the PLO to act against Israel in a way that would stir the Israelis to retaliate, as predictably they would, against them.

The Syrians regard their sponsorship of Palestinian nationalism as their ticket to Arab leadership. In derailing King Hussein while he was on his way to an American-sponsored negotiation earlier this year, they perhaps had foremost in mind not yielding him the Palestinian flag. This does not mean Syria has no interest in a negotiation. It almost certainly has a deep interest--in a negotiation on its terms.

In its troop-disengagement accord with Israel in 1974, Syria said: "This agreement . . . is a step toward a just and durable peace." President Assad faithfully put into effect all the specific obligations he undertook in that agreement. There is no reason to conclude prematurely that he has changed his mind about the larger commitment.

With the Syrians still inexcusably stonewalling on their pledge to get out of Lebanon, no early movement can be expected on the Syrian-Palestinian front. But there need be no excessive mourning about Syria's humiliation of Yasser Arafat. It is a time to start pondering what new possibilities-- peaceful ones as well as dangerous ones--may open up as President Assad tightens his grip on the fighting core of the PLO.