RONALD REAGAN did something splendid in intercepting the plane with the four Palestinian terrorists. He not only found a precise and bloodless way -- Jimmy Carter approved -- to assure the prosecution of a gang that had brutalized a ship and killed an American passenger. He also provided a vital tonic for Americans and others who were in despair over whether the United States could cope effectively with the scourge of terrorism.

The sickening undeniable horror of the murder of a 69-year-old invalid in a wheelchair gave the United States the impulse to act in this episode. Nicholas Veliotes, the American ambassador in Egypt, expressed this impulse unforgettably when, on learning of the crime, he instructed his embassy to demand that Cairo prosecute the killers and, only then, to tell Washington what he had done.

Terrorists customarily demand safe passage for themselves if their larger purpose is frustrated. A chain of circumstances that put this gang in a plane wandering over the Mediterranean without a place to land provided Mr. Reagan a rare opportunity to thwart safe passage this time. Israel contributed to this chain by refusing to consider handing over its Palestinian prisoners. Italy and Egypt helped talk the terrorists off the ship. Tunisia and Algeria denied sanctuary. Whether Egypt contributed further by joining a plan that let it make good on its felt obligations to its fellow Arabs, while at the same time allowing capture of the terrorists, is a question not likely to be suppressed by official denials.

The American operation was a success, but conclusions from it must be sparingly drawn. Its two key elements -- international cooperation and national resourcefulness -- are hard to combine in any one incident. The United States had a clear moral right -- a necessity -- to violate the safe passage that others had offered the four killers; this could affect the play in future incidents.

Bringing terrorists to justice is a moral imperative that also serves domestic morale. For the Israelis, who are at war and are therefore prepared to absorb the costs, retaliation works similarly. But it is less certain whether either of these responses in itself deters terrorism. Preventing such acts by vigilance and good intelligence remains essential.

It is a truism that the most effective way to undo the terrorism is to tackle it at the political source -- to resolve the Israeli-Palestinian dispute. But no effective progress can be made while Palestinians are shooting a paralyzed old Jewish man in the face and dumping his body into the sea. The PLO disavowed the four who committed this atrocity, but evidence mounts that the disavowal, like similar previous disavowals, is a lie. Very few Americans are going to ask Israelis to accommodate an organization, the PLO, that murders and lies in this fashion.