LAST SEPTEMBER, as the United States and Nicaragua were secretly discussing how they might improve their relations, Nicaragua complained of forthcoming American military exercises in Central America. One of these involved a lone tugboat, three patrol boats and two observation planes: the American side saw nothing unusual or provocative about it, and said so. Yet to the Nicaraguans the maneuvers "seriously affected" the negotiations then under way.

There was no claim in reporter Don Oberdorfer's account of these talks the other day that this little episode itself tipped the balance. The episode appears to us, however, to illuminate the hazards that must be overcome if the Reagan administration and the Sandinistas are to arrest their now-resumed drift toward confrontation.

Ask why the Nicaraguans got so uptight about innocent routine naval maneuvers. It is always possible that they used it as a pretext for a cooling whose real causes lay, say, in divisions within their leadership. But it is also possible that Managua saw in the exercise the suggestion of gunboat diplomacy. Did the administration, in going ahead with this exercise, give full consideration to how it might play?

In the talks overall, the United States offered to accept the Nicaraguan revolution, ending all hints of using force against it and opening up normal relations and aid. In return, it asked that Nicaragua stop intervening elsewhere in Central America and limit the size and armaments of its military. Are these terms fair? We wonder, for instance, why Washington should prescribe the dimensions of another country's army. But all that should have been arguable or, better, negotiable. We do not see that there was any irreducible obstacle to the warming of relations on the basis of mutual respect for the familiar international rules.

Nor is there reason why Washington and Managua should stop trying now. What might make the effort work this time? Perhaps Nicaragua is in fact too full of revolution for compromise. But it is not enough for a great power to go halfway in dealing with a country with a hundredth or a thousandth of its power. The imbalance, reflected here by a century's worth of history, requires the great power to go a bit more than halfway. The purpose of a negotiation is not to be able to win a debate later concerning which side was responsible for a failure. The purpose should be to see whether legitimate American interests can be satisfied in a success. Secretary of State Alexander Haig says the United States is still in the game. Good.