THE LATEST episode in the trials and tribulations of the United States as an alliance leader is unfolding in New Zealand. Its new Labor government had said it would not allow nuclear-powered or-armed ships to enter its ports or waters. The United States had responded, entirely properly, that such a prohibition is inconsistent with New Zealand's obligations under the ANZUS treaty binding the two countries and Australia. It is the time of year when these things are ordinarily arranged, and the United States is now testing the New Zealanders by requesting permission for U.S. Navy warships to make a routine port call next March. A countdown of sorts has begun.

What most strikes the American eye is New Zealand's evident zeal for this gathering friction between two allies. The impetus for it does not lie simply in the familiar European-style combination of anti-nuclear and leftist elements. The cause appears genuinely popular and nationalistic: a small country making its special contribution to the harnessing of the world's nuclear furies. New Zealand may not be troubled by threats to its security, but large parts of its electorate are agitated by the perceived challenge to its integrity as a sovereign state.

The United States has been quietly worrying the issue, trying to talk the new prime minister, David Lange, a Methodist preacher's son, into finding a way to continue the defense cooperation required for a working alliance. Mr. Lange's response is perhaps best indicated by his scheduled participation in a coming Oxford Union debate with the Rev. Jerry Falwell on the motion "That the Western nuclear alliance is morally indefensible." Mr. Lange is arguing the affirmative.

It was always possible for the United States simply to avert its gaze, to pretend that Labor's election was a bad dream, and to wait for a fresh turn of New Zealand's political wheel. For there can be no pleasure or profit for Washington in entering into what is bound to be a tense encounter with an ally -- an encounter, moreover, that can easily be painted in David-vs.-Goliath colors.

But an alliance that is an alliance only in the even- numbered years is not an alliance. An alliance leader -- or, more precisely, a leader of several alliances -- does not have the luxury of sitting it out. Its leadership responsibilities require it to make its best fair effort to engage its fellow democracies' participation in the agreed modes of cooperation. New Zealand retains its sovereign right to decide whether alliance with the United States still serves its national needs.