In the sideshow that is U.S. foreign policy, there is a perennial rerun involving Latin America.

Conservatives announce that a military buildup in some small place is Soviet-inspired and poses a threat to the United States. Liberals respond that there is no threat and that the conservative agenda is really to squash all indigenous opposition in the U.S. marketing area.

The truth is always gauzy and the tendency is to believe that reality lies somewhere in between. Usually it does.

But occasionally, just often enough to galvanize the audience, the script gets played out: Cuba got Soviet missiles in 1961; the United States invaded Grenada last year.

That is why the world snapped sharply to attention on election night when President Reagan's victory was interrupted by the latest installment in this long-running serial, the non-arrival of Soviet MiG fighter planes in Nicaragua.

Now that the episode is over, there still is mystery over the sources of initial reports that the high-performance MiG21 jets were about to be delivered, over the reasoning behind the leaks, if any, and over the split in the administration that appeared to surface afterward.

There is continuing mystery over the future course of U.S. policy regarding Nicaragua, an issue on which major decisions will soon be unavoidable.

The MiG story began when CBS News broke into its election coverage with the announcement that unidentified Defense Department sources had said there might be MiG21 jet fighters on a Soviet freighter heading for the Nicaraguan harbor of Corinto.

NBC quickly followed with excited word from "intelligence sources" that 18 to 21 jets might be arriving and that "the administration views this as a direct challenge to its commitment not to tolerate Soviet MiG fighter jets in Central America."

On cue, Nicaragua promptly denied that any jets were imminent and charged that the leaks were designed "to prepare the climate for direct military attacks against our country."

Since U.S. officials frequently had warned the leftist Sandinista government of Nicaragua that acquiring high-performance jets would prompt some unnamed U.S. retaliatory action, the leaks were front-page news nationwide and were carried all day on the airwaves, along with Reagan's reelection landslide.

Nicaragua prepared for a U.S. invasion. Sandinista leaders insisted in Sherman-like statements that neither now nor at any time in the past, present or immediate future were MiGs arriving. Camera crews were invited to watch the Soviet freighter unload. Sandinista officials called a military alert, put the country's coffee bean harvest on hold, and tanks rolled in the streets of Managua.

The tension held for three days. U.S. Defense and State department sources, who usually had thorough background explanations of any current flap, maintained an ominous silence. Official spokesmen repeated endlessly that they did not know what was on the ship and that while invasion was not in the cards, other possible military responses had not been ruled out, if the jets appeared.

U.S. military exercises materialized in the Caribbean. High-flying jets zoomed over Managua, their sonic booms rattling windows and jangling nerves. One such boom occurred just as newly elected President Daniel Ortega was complaining about them to a group of U.S. journalists over breakfast, and the usually poker-faced Ortega allowed himself to smile.

On Nov. 9, U.S. officials said they had no evidence of MiGs, and let it be known that the Soviet Union had notified Washington that there was "nothing to worry about" on the freighter. The administration quickly shifted its focus to Nicaragua's military strength and current buildup, calling it unprecedented and unjustifiable. The military exercises continued.

Then, predictably, the sonic booms stopped and the daily pronouncements did, too. Nicaragua said its military alert had been only a drill.

Reports later said Secretary of State George P. Shultz had personally obtained the assurance from the Soviets, and on Nov. 10 he said that whoever first leaked the intelligence worries "engaged in a criminal act, in my opinion."

That was the first indication that the MiG episode might have reflected splits in the administration over what to do next in Nicaragua.

As if in confirmation, there were leaks to The New York Times warning of possible new and stronger U.S. pressure on Nicaragua, including recalling the U.S. ambassador or blockading Nicaraguan ports, but these reports were strongly denied by high-ranking State Department and White House officials.

The officials insisted that existing pressures would be maintained but that no new initiatives -- and especially no military initiatives -- were in the offing. Instead, they said, internal U.S. debate is focusing on what to do with negotiations in the region, both in bilateral talks with Nicaragua and in regional discussions over the so-called Contadora peace treaty.

Decisions will be required soon on the ultimate U.S. goal in the area, these officials say. They admit to internal division over whether a major shift toward U.S.-style democracy is absolutely required in the internal political procedures in Nicaragua to preserve U.S. and regional security. Those who say it is tend to also say that only military action can do the job, and they are in a distinct minority, according to these officials.

The majority of foreign policy decision-makers insist that negotiations, combined with military and diplomatic pressure, will enable U.S. goals to be met. But those goals have not been clearly spelled out to Nicaragua nor to the satisfaction of the decisionmakers, the officials said.

As a chapter in the long-playing U.S.-Latin American drama, the MiG episode has left political scars. Congressional critics of the Reagan administration and many Latin Americans are convinced that it was just a ploy, deliberately designed to scare the hemisphere and convert war nerves into U.S. and allies' support for a tougher line against Nicaragua.

If there was such an effort, it failed. Democrats are outraged and Latin diplomats say it looks as if the hemisphere's biggest bully is once again trying to push them around.

"I am outraged over the leaks by the White House of raw intelligence data creating the MiG myth," Rep. William V. (Bill) Alexander (D-Ark.), deputy majority whip in the House, said in an interview this week. "If I did something like that they'd send the FBI in to read me my rights."

Several other Democrats said congressional resistance to financing rebels against the Nicaraguan government has, if anything, been strengthened by the MiG episode.

"I become very, very sad when I see foreign planes overflying my country," said Arturo Cruz, the Nicaraguan opposition leader whose conditions for participation in Nicaragua's elections were strongly backed by the Reagan administration before they fell through. Reagan's hard line, he said at a midweek news conference, is "not helpful" to his cause.

The Defense Department is expected this week to announce new estimates of Nicaraguan military strength, building on what the administration regards as public awareness resulting from the MiG scare.

"It gives us too much credit to think we sprang this story to achieve this, but if it sensitizes the public to the fact there is a very close relationship between Nicaragua and the Soviet Union, so much the better," said one State Department official known as a moderate. "It's still reprehensible that this government can't keep its secrets.