The United Nations General Assembly, so often denounced as a sinkhole of anti-Americanism, has given the United States an extraordinary triumph. After a debate of a businesslike sort that many thought had gone out of style in that forum, the assembly voted as one for an African development plan based substantially on principles proclaimed by the Reagan administration. "The United States firmly believes that our own development experience is a useful guide," Secretary of State George Shultz told the assembly's special session on Africa. The assembly largely agreed.

In their first generation after winning political independence, Africans tended to reject the free-market approaches identified with the West, to turn toward policies emphasizing state direction and the Soviet example, and to call upon the former colonial powers to provide something very like reparations. But calamity forced on Africa a deep policy review, which happened to coincide with the advent of an American administration preaching "the magic of the marketplace." Not so much perhaps under American pressure as under the power of American ideas -- an especially relevant view, by the way, in China -- Africans have moved the American way.

*The U. N. special session, however, did more than agree that policy reform and self-help, especially in agriculture, are the essentials of African progress. It detailed the external causes of Africa's despair -- recession, low commodity prices and so on -- and endorsed a development "partnership" between Africa and the industrial world. Reform is the intended African contribution; it is already coming, although not everywhere with equal vigor and effectiveness. The intended Western contribution is aid -- debt relief, food, development assistance, trade access.

*Black Africa is at a historic place, poised to embrace Western economic principles and, with them, a continuing, voluntary, ever-closer association with the West. It is a moment of rare promise in the struggle between communism and the noncommunist world -- a moment the Reagan administration, badly in need of successes in Africa, is entitled to savor.

*There's one hitch: Congress is in the throes of decimating foreign aid for Africa. This was reckless before the special session. It is doubly foolish now. American support is the key to contributions by others; the whole package is needed to give the new partnership a chance. The United States, if it does not find the funds somewhere, will be defaulting on its commitment at New York. It will be throwing away a rich opportunity for long-term American advantage. It will be robbing Africa of hope.