The final stages of one of the most heated bureaucratic struggles in the young Carter administration will begin Thursday morning at an unannounced meeting in the White House.

The subject under consideration - and dispute - is a document called "PRM-10," a "net assessment" of the Global balance of power in the broadest terms, and proposals for alternative American foreign policies to cope with economic, political, conventional and strategic nuclear issues.

Originally this Presidential Review memorandum was thought of as a grand design for American foreign and military policy in the Carter administration. By the time the senior officials of the country's national security departments discuss it Thursday, however, PRM-10 will have shrunk to more modest proportions. It is, according to sources who have read the latest draft, less bold and less ambitious than its principal author once hoped.

The story of this attempt at policy-making on a grand scale seems to tell a good deal about the inner and workings of the new administration, and about the government's best estimates of Soviet and American capabilities.

The principal author of PRM-10 is Prof. Samuel Huntington of Harvard, a friend of and onetime co-author with Zbigniew Brzezinski, the President's national security Adviser. Brzezinski hired Huntington as a consultant (after Defense Secretary Harold Brown had turned him down for a high post in the Pentagon) to oversee preparation of PRM-10.

Huntington has a reputation as a hard-liner, a fact that colored much of the initial reaction insidete the government when it was announced that he would coordinate PRM-10. Several of the people whom Huntington asked to help with the project said privately during its early stages that they hoped to create a document that would scare the Carter administration into greater respect for the Soviet menace.

For a time it looked as though this might happen. An earlier draft of Huntington's "net assessment" that circulated within the bureaucracy, sources said, included the contention that the Soviets enjoy military superiority in Central Europe and could conceivable believe that they might win a war in that region.

This view was sharply disputed within the government, however, and has now been substantially modified, the sources said. The draft that will be considered at Thursday's meeting reportedly says that the military balance in Central Europe is sufficiently uncertain that the Soviets could not confidently predict the outcome of an armed clash.

The latest draft is "surprisingly optimistic" about the capability of the United States to cope with the Soviet threat, according to one authoritative source. In a preliminary meeting at the assistant secretary level last week, a representative of the Joint Chiefs of Staff criticized the latest draft of PRM-10 as too optimistic, sources said.

Some officials who participated in the PRM-10 drafting process say the study has been ill-organized, shoddy and intellectually weak. Others defend it. One source predicted that the senior officials who begin to consider it this week will be able to turn the draft material into important substantive decisions once they turn seriously to the questions PRM-10 poses.

PRM-10 has been the subject of bureaucratic dispute since the idea for it first emerged from the National Security Council. The original terms of reference for the study called for a thorough investigation of alternative national strategies for the United States.

This upset the State Department, which saw an invasion of its diplomatic domain, and the Pentagon, which reportedly feared that an outside determination of "national strategy" would compee the armed forces to accept new and unwanted force structures.

Early manueavering resulted in a division of the study into two parts: Huntington's net assessment, which was to include possible "national strategies" in a general sense, and a"force pasture statement" on the specifies of the military balance and alternative military strategies to be prepared in the office of the assistant secretary of defense for international security affairs (ISA).

This meant that the Pentagon would coordinate specific inquiries into the country's military needs, thus disminishing the practical significance of Huntington's study, at least in the view of the military services.

Although the bureaucratic acotrs have not ignored Huntinton or his study, many of them have concentrated on the second study, the force posture statements as more important.

The force posture study became an extremely ambitious undertaking. Interagency task forces studied five broad areas:

1) A possible conflect in Central Europe involving NATO and the Warsaw Pact. 2) An East-West war outside of Europe. 3) Possible conflects in East Asia. 4) "National" wars (i.e., was that might be fought by choice - "I don't want to say more Vietnams, but that's what we're talking about," in the words of one source), and 5) All out nuclear war with the soviet Union.

Assessements of American needs to meet contingencies in these five categories led to more than 200 possible "strategic packages" that the Carter administration might opt for. After much boiling down, the document now proposes half a dozen alternative strategic postures.

The PRM-10 process has left a substantial number of government officials discouraged, though several said they would withold judgement until a final National Security Council decision on the topics it covers is made.

Carter came to office proclaiming that the cold war had ended, one participating official noted, but Huntington's latest draft says instead that we are now in "Era Two" of the cold war. The document treats the Soviet American relationship as the overriding issue of American diplomacy and military strategy, and makes few if any concessions to the vision of a new international order that Carter evoked in his presidential campaign and in rhetoric since Jan. 20.