"The so-called Third World -- a misleading term, if there ever was one," Alexander Haig observed at his confirmation hearing the other day. With that phrase, the incoming secretary of state launched a major new departure in American foreign policy.

In the past, the United States dealt with the Third World, whether favorably or unfavorably, as a whole. The new approach breaks the Third World into its component, smaller worlds.

Roughly 100 countries compose what is generally called the Third World. They undoubtedly share many characteristics. Most are located in the continents of the southern hemisphere -- Africa, Asia, South America. Most have low per-capita incomes. Most are heavily dependent upon agriculture or minerals. Most have recently experienced some form of colonial tutelage, often mixed with race prejudice.

Assertive underdeveloped countries have used these common characteristics to forge, in various garbs, an international pressure group. As the Non-aligned Conference, the Third World works to assert its diplomatic interests against the two superpowers and their allies. As UNCTAD (the United Nations Conference on Trade and Development), the Third World lobby works for the redistribution of wealth from industrialized to non-industrialized countries.

American diplomacy, taking the Third World at its word, has usually dealt with it as a bloc. The Carter administration endorsed nearly all the grievances of the Third World, particularly at the United Nations. The Nixon administration contested vigorously most Third World claims.

To no avail. The Third World has neither moderated its sense of grievance nor scaled down its claims. Over the years, moreover, the United States has steadily lost the effective working cooperation of the underdeveloped countries. They regularly tolerate egregious power grabs by countries friendly to the Soviet Union, most recently the Libyan success in Chad. They regularly condemn regimes friendly to this country -- especially Israel. Whether colored by sympathy or hostility, in other words, the enbloc approach has failed.

Not altogether surprisingly. Diversity distinguishes the Third World countries more surely than sameness. In size they range from subcontinents with teeming millions to tiny island states with populations numbered in the hundred thousands. Per-capita income ranges from $250 annually for perhaps a third of the countries to four times that amount for another third. Growth rates, availability of natural resources, population policies and internal distribution of income within countries vary even more dramatically. As to politics, the Third World is a Noah's Ark of autocracies, military despotisms, theocracies and, even, the occasional democracy.

The Reagan administration proposes to tailor policy to diversity. A rough regional division of labor will be encouraged. The United States has special responsibility in Latin America, as does Japan in Asia, and France in Africa. The Middle East and the Persian Gulf engage this country and many of its allies, China and India represent global problems.

Within that framework, other distinctions assert themselves. Democratic regimes under fire from despotic forces -- for example, the new government in Jamaica -- will probably receive special help. Similarly, with countries driving effectively to improve their economic growth. The oil-rich nations will be expected to contribute to the common good, not hide behind the Third World international. Toward plain despotic regimes, whether of left or right, the attitude will be wary.

As to international troublemakers, the new administration is already baring its teeth. During his testimony Haig assailed Libya for supporting terrorism and threatening neighboring countries in Africa. He said it was "high time that the West as a whole assessed with clarity that situation."

The problems intrinsic to the Third World are so acute that no prescription can guarantee success. But placing bets on some tables and leaving others to other players improve the prospect for a return on investment.

More important, disaggregation of the Third World problem minimizes the damage. It works against the illusion, dangerous to everybody, that by holding fast and talking tough, the Third World can change the face of the economic universe. It works to break up in advance the threat of a universal anti-Americanism that looms wherever the Third World consolidates. It is a policy of dividing and not losing.