US piles pressure on Nicaragua; Sandinistas on alert

This has been the week of the MIGs that weren't. Both the alarm in Washington over the possibility of Soviet MIG fighter jets being delivered to Nicaragua and the wild rumors sweeping Nicaragua of an imminent invasion by the US 82nd Airborne proved false. But the war of nerves between the two sides is very real.

Above all there remains the question of what President Reagan is going to do about the Sandinistas, now that he has had his electoral triumph.

Many observers here think the Reagan administration has not made up its mind what to do about the Sandinistas. They speculate that the United States is biding its time, using the MIG scare to add a new form of psychological pressure to the political, military, and economic pressures it is already exerting on Managua.

Managuans who wanted to ignore these pressures have found it difficult over the past few days because of what seemed to be their physical incarnation over the skies of this city - a big black bird that goes boom. This is how most Nicaraguans view the SR-71 US spy plane that has zoomed over Managua each of the last four mornings, producing a sonic boom. This sound is now part of Managua's morning routine, provoking nervous jokes. When the boom occurred about 8:30 two days in a row, some joked that Nicaragua had a new time-telling national monument, a modern version of London's Big Ben.

Concern here was also heightened by naval maneuvers carried out by some 25 US ships not far off the Caribbean coast of Nicaragua and Honduras. In a separate move much publicized in the Nicaraguan press, 30 US members of an ecumenical group called Permanent Christian Action for Peace embarked in what one Nicaraguan paper decribed as a ''fragile fishing boat'' heading toward a US frigate off Nicaragua's Pacific coast to give US sailors their ''message of peace.''

In the meantime, Nicaraguan leaders have not only denied that any MIGs are coming but have also been telling their people and the world that a US invasion is imminent. They have called on thousands of Nicaraguans to leave the coffee fields to prepare for national defense.

Over the weekend, Nicaraguan President-elect Daniel Ortega Saavedra said, ''The situation which Nicaragua is now living through can be described as the most critical one the revolution has been through since its triumph in July 1979 .''

US intervention, Mr. Ortega said, was ''nearer than in other times.'' His warnings of imminent US invasions are growing increasingly harsh, as had earlier US accusations about the arrival of MIGs. (The US backpedaled from that accusation when no MIGs were seen offloaded from Soviet ships.) Both sides are loosing credibility with their comments.

The US case for the arrival of the MIGs just now is hard to make. Many diplomats and Nicaraguans on all sides of the ideological spectrum say that now, when the Sandinistas are petrified by Reagan's reelection, would be the most unlikely time they would choose to bring MIGs into Nicaragua. The US has made it clear that it might destroy any MIGs by air attack. So it is hard, observers say, to see how the Sandinistas would be so foolish as to bring in MIGs.

Observers also doubt the Soviets would feel they had anything to gain just now by an act that could only lead to sudden worsening of US-Soviet relations.

In fact, many observers would completely discount US charges about MIG shipments except for the fact that East-bloc countries have been training Nicaraguan pilots to fly MIGs for several years and that the Nicaraguans, with East-bloc help, have just finished building an airport near Nicaragua's Punta Huete. The airport, with its long runway and other features, seems to have been built to accommodate MIGs or similar planes.

One theory held by some here to explain this centers on the several years required to build a sophisticated airport and train MIG pilots. When both projects were started some three years ago, the US was in a recession and the Sandinistas thought the Democrats might win the US vote. These observers speculate the Soviets and Nicaraguans thought they could get away with it if the Democrats won.

Although Nicaragua does not have a pressing military need for MIGs, these planes would, in the Sandinista view, be nice to have. Nicaragua has the largest armed forces in the area, but Honduras has air superiority, in terms of sophisticated equipment, with the Mirage aircraft it bought from France in recent years.

Most observers here expect the next few months will see more psychological pressure on the Saninistas.