The story rated only four paragraphs deep inside the morning paper. Israeli and Syrian forces had exchanged tank and mortar fire--the first such flare-up in four months. No big deal, on its face. Still, only a day or so before, a top Mideast hand in the administration had been laying out a hair-raising "scenario" that began with just the same sort of mindless shoot-out.

Except that in this hypothetical version, the shooting doesn't stop. Both sides suffer losses. As the fighting intensifies, Israeli warplanes rush in to attack Syrian tanks. Syrian surface-to-air missiles with Soviet crews open up from Syria on the Israeli aircraft. The Israelis swiftly cross into Syria to wipe out the missile installations, inflicting casualties among the Soviet crewmen. Can the Soviets sit still for this? Do they intervene in Syria's support in a way that challenges the United States to respond on Israel's behalf?

Scenario writers are paid to make the worst case. But this one is sufficiently plausible to be adding to the considerable pressure on the Reagan administration to find a way out of the impasse over removal of foreign forces from Lebanon. The pressure is all the more acute because: (1) there are armed U.S. Marines in the multinational peacekeeping force ashore in Lebanon; and (2) contrary to the popular assumption, there is little prospect for their early disengagement--or for the removal of the heavy U.S. military presence offshore-- even given a quick agreement on a withdrawal timetable.

The impression you get from the administration's public pronouncements is that withdrawal would follow swiftly upon an agreement on its terms. The main differences seem to center only on the extent of a residual Israeli presence (patrols, observation posts) in southern Lebanon. The Syrians are holding out for no Israeli conditions, but would probably accept a few.

But even if the parties could agree on such a happy ending, the question increasingly worrying policy- makers and members of Congress is how you get to it. And the answer, it's increasingly recognized, is that "cold turkey" won't work for Lebanon.

The government of President Amin Gemayel is barely able to exert its authority in Beirut. According to a "policy alert," prepared by experts in the Congressional Research Service of the Library of Congress and now circulating on Capitol Hill, "Most observers estimate that it will take two years to train and equip the minimum size (Lebanese army) force that could effectively police the country."

Thus, a precipitous withdrawal by the Israelis and Syrians would leave the Lebanese government where it has been for almost a decade: at the mercy of bloody, sectarian feuding between Christian and Moslem factions. "The United States could then face the alternatives of committing more troops for an extended stay under dangerous, if not hostile, circumstances, or witnessing renewed violence and bloodshed in Lebanon," the congressional study concludes. It argues for a "phased agreement, whereby Israeli and Syrian troops would progressively turn over control of Lebanese territory" to the Lebanese army.

Even this arrangement would probably require a larger U.S. contribution to an expanded multinational peacekeeping force. It would mean maintaining a precarious balance between Israeli and Syrian forces as they phase their way out of the country, with a continuing risk of outbreaks of shooting between them. It would put American armed forces even more deeply into what the War Powers Act refers to as a "hostile environment," unlike, say, Nicaragua or El Salvador, where the involvement of the CIA and/or American military advisers is at least once-removed from an actual combat role and likely to remain so.