IT'S A GOOD sign that Nigeria is trying to use diplomacy to ease the Angola-Zaire tension that led the United States to start shipping emergency military supplies to Zaire last week. As the biggest, richest and most influential black African state, Nigeria has an evident capacity to reduce the prospect of great-power involvement in an African quarrel and an evident self-interest in doing so. Its early support of the faction now ruling Angola gives it access in Luanda. Its determined opposition to tribal splintering underlies its credibility in Kinshasa. Fortunately, the scale of the fighting in southern Zaire, where Angola-based separatists have been conducting operations, has not become so great as to eliminate any reasonable chance of successful mediation.

Americans should understand that in no sense is Nigeria undertaking its diplomatic initiative in order to do the United States a favor. Nigeria is neither client nor ally of Washington. Its foreign policy is premised on the idea of African independence. At the same time, one should recognize that Nigeria, by acting in its own interest, can sometimes serve goals shared by the United States. The last administration's failure to realize this point helped produce a nasty and unnecessary spat between the two countries. Nigeria, needless to say, was not itself blameless; its petulant treatment of foreign correspondents is, to us, especially reprehensible.But the current administration has stressed the interests it has in common with Nigeria. Ambassador to the U.N. Andrew Young, wisely, made a personal point of not waiting for a crisis in order to open up lines of communication with Lagos.

However the Nigerian initiative works out, Zaire will remain in heavy trouble for reasons having nothing to do with tribal unrest or Angolan subversion in its southern regions. Like many other developing states, not excluding oil-exporting Nigeria, Zaire needs topflight leadership and favorable international circumstances - rare commodities both - to bring its ambitions and resources into rough balance. The United States has a role to play in helping Zaire and countries like it find this balance.

But in black African political disputes involving the use of force, the role open to Washington is much more limited. American involvement, even if token, is bound to stir African nationalism. It is very hard to think of any black African dispute in which the United States would not be better off if a local state, such as Nigeria, took primary responsibility for resolving it. Mr. Carter, at his news conference yesterday, could discern "no outstanding commitments" to Zaire. In truth, there are none. We assume that the only reason he failed to acknowledge and encourage Nigeria's diplomacy was that he forgot to.