TAS USUAL, the United Nations has selected as secretary general an experienced public figure from a small country that enjoys good relations with its neighbors. He is Javier Perez de Cuellar, a veteran Peruvian international lawyer and diplomat. He knows the U.N. scene, has dealt extensively with Soviets as well as with Americans, and enjoys a reputation for fairness and competence. You have never heard of Mr. Perez? It is hard to become head of the United Nations without being widely regarded as self- effacing and discreet.

Something is known, however, about the strategy that brought him his new post. At first, he hung back, content to see the incumbent, Austria's Kurt Waldheim, who was starting out with 94 Third World votes against him, take defeat after defeat until the total reached a mortal 16. It was evident, too, that while the United States publicly opposed the premier Third World candidate, Salim Salim of Tanzania, the Soviet Union privately opposed him as well--too unpredictable. That made the only solution, in the Peruvian reading, a "transaction candidate." Latin America, being a region with many founding members, felt it had a good chance to beat candidates from other regions.

Sorting out the Latin candidates was the next phase. Neither of the two Argentinian diplomats running, including Alejandro Orfila of the Organization of American States, could count on the support of his own government. Mexico's candidate, the foreign minister, was strongly identified with a position on El Salvador that the Reagan administration finds unpalatable. The Panamanian foreign minister suffered from the fact that Panama already has a Security Council seat--share the wealth. Even before the voting began in New York, Mr. Perez was back in Latin America, campaigning there.

We wish Mr. Perez well. We hope he has pondered his new duties. Americans are a bit nutty on the United Nations. They persist in believing that something better can come out of the institution than has been evident in recent years. We think that what is needed is a firm conviction on Mr. Perez' part that he is there for one purpose: to serve the charter's grand commitment to solve disputes peaceably and fairly. He cannot disburse his political capital casually. Nor can he be so intent on preserving it that he evades his principal calling. To have it said when he finally leaves office that he navigated skillfully among the powers, great and small, and left them all more or less equally content, or discontent, is the definition of failure.