The U.S. commitment in the Persian Gulf announced by President Carter last night is the product of a year's deliberation within the government in reaction to turbulent events in one of the world's most important strategic zones.

The regular pumping of the oil wells of the Persian Gulf is the heartbeat of western civilization, and the globe-girdling line of oil-laden tankers an exposed artery, in view of the continued high dependence of this country and its allies on imported petroleum.

Two developments of the recent past have remade the map of this vital region -- the fall of the shah's imperial Iran, a major oil producer and a pillar of regional security; and a Soviet buildup by fits and starts, culminating in an open invasion by Soviet ground forces of Afghanistan, a former buffer state extending like a mountainous wedge toward the waters of the Indian Ocean.

Washington's response to internal challenge and external threat in the region was uncertain and episodic at first, but it gathered force last summer and took on urgency late last year in the twin crises of hostage-taking in Iran and Soviet intervention in Afghanistan.

As early as last Feb. 28, nearly 11 months ago, national security adviser Zbigniew Brzezinski proposed to Carter in a memorandum that a new "security framework" be established in the region as the basis for a reassertion of American influence and power.

This was the genesis of Carter's plan for greater U.S. presence and operating access in the area under arrangements that are less than a formal alliance but more than the status quo.

A new policy in the area, Carter was told last February, would be as important as the Truman Doctrine, which drew the post-World War II line against the Soviet Union in Europe, or the establishment of the NATO alliance. Brzezinski suggested last February that Carter announce his plans in a major speech to the nation, a proposal which came to fruition last night.

It remains to be seen whether the "Carter Doctrine," as many are calling it, will prove to be as successful or as lasting as that of the Truman era.

There is doubt within the government, especially among Middle East experts, whether Carter's plans are relevant to the most serious problems and challenges in the region, and whether the essential political underpinnings of a reassertion of American influence have been prepared, either at home or abroad.

There is little doubt, though, that Carter is enunciating a policy shift of great significance. For the first time since the high point of involvement in the Vietnam war a decade ago, the United States is increasing its military forces and security commitment in a far-away region rather than reducing them.

The ebb tide of U.S. military deployments abroad is being reversed in a region of unsurpassed importance, bringing with the onrushing waves inevitable costs and entanglements, military and economic.

Carter's plan is a sweeping reversal of a number of early policies of his 3-year-old administration. In March 1977, Carter pledged to pursue "complete demilitarization" of the Indian Ocean (later amended to a military freeze) in negotiated cooperation with the Soviet Union. Now he is proposing to remilitarize the Indian Ocean in competition with the Soviets.

In the fall of 1977 the Carter administration was unwilling or unable to come up with $300,000 to finance a continued British presence and operation of a communications intelligence station on Masira Island, off the coast of Oman, and it turned a deaf ear to pleas by the sultan of Oman for a close alliance and substantial aid to fill the shoes of the departing British. c

Now the discussion has been resumed, this time by high-level officials flying from Washington on missions of urgency, asking for extensive and doubtless expensive rights to use Masira Island and for a close relationship with the sultan's unsteady regime.

The area in question is as remote from the United States and its traditional concerns as any on earth. Although the common childhood fable is that a hole dug through th earth from the United States would come out in China, in fact it would come out in the Indian Ocean, which is just halfway around the globe.

U.S. lack of experience and precision about the region is so great that there is continuing uncertainty about what to call it -- the Middle East, greater Middle East, Indian Ocean, Persian Gulf of Southwest Asia. All have been tried.

The requirements and difficulties imposed by geography are major factors in the present situation. The "arc of instability," as Brzezinski called it more than a year ago in a much-quoted interview, is far from the United States, but close to the Soviet Union. The essence of Carter's plan, when stripped of its less-developed political and diplomatic elements, is to improve the U.S. ability to project and sustain military power in his faraway, yet vital, area.

For decades the British fleet patrolled the region, and American strategists considered it essentially a British zone of influence. After the withdrawal of the British east of Suez after 1968, the United States sought to build up Iran as a regional military power with surrogate responsibility for thwarting the Soviets and keeping the peace. Iran and oil-rich Saudi Arabia were the two renowned "pillars" of U.S. policy in the 1970s.

Although a rapid buildup of Soviet and Cuban forces in Ethiopia and South Yemen and a pro-Soviet revolution in Afghanistan jolted the Carter administration, there is common agreement that the tottering and eventual fall of the shah of Iran, more than anything else, triggered the policy shift in Washington.

The demise of imperial Iran 12 months ago not only eliminated a pillar of the previous U.S. policy, but the manner of its fall created new opportunities for the Soviet Union in revolutionary Iran and generated doubts about the U.S. role in Saudi Arabia and other friendly states and fears about their future.

There was agreement in Washington that something should be done, but there was debate about the nature and form of the response. Internal military power had proved ineffective in the hands of the shah, and external U.S. military power irrelevant as the Iranian revolution proceeded. Nonetheless, American military assets were closer at hand than any new political or economic remedies.

The decision to send Secretary of Defense Harold Brown as the crisis-time emissary to Saudi Arabia, Jordan, Israel and Egypt last Feb. 9-19 signaled the military nature of the Washington response. Brown brought the message that the United States intended to be more deeply involved in the troubled region. With no successor in sight for Iran's surrogate role, this necessarily meant a more direct American role.

"The Nixon Doctrine (of relying on regional surrogates) died in January and February 1979, and Harold Brown was sent out to bury it," said a senior Pentagon official. But a replacement "doctrine" -- involving regional arrangements to facilitate the direct application of American power -- was still being formed in the high counsels of the Carter administration.

The justification and guidelines for an expanded U.S. role were developed just before and after the Brown trip, mainly in the National Security Council. Brzezinski and his NSC staff have played the central role in new policy toward the Persian Gulf-Indian Ocean, partly because it is a function of crisis in which the NSC has special clout and responsibility.

The crucial problem, as outlined by Brzezinski in his Feb. 28 policy paper for Carter, was the continuing dependence of the United States and its allies on oil from this vulnerable and volatile region.

Any one of several potential misfortunes, including collapse of Carter's Camp David program for Egyptian-Israeli peace and internal collapse in Saudi Arabia, could gravely damage the position of the West. There was no mention of a Soviet invasion of Afghanistan, but the possibility was foreseen that the projection of Soviet force in unspecified fashion closer to the center of the region could bring about a new crisis.

The "security framework" to shore up friendly nations and augment U.S. power was conceived as a loosely constructed yet clearly cooperative arrangement among moderate states, including Egypt, Israel and Jordan. Saudi Arabia would be closely associated but treated as a special case in view of its sensibilities regarding any association with Israel. Peripheral states in the area, including Morocco, Sudan and Turkey, would play a cooperative role and benefit from U.S. aid and arms.

U.S. forces in the area and facilities for their support would have to be increased. Oman would be brought into the alliance, and given protection into the alliance, and given protection and assistance. The overall cost of military assistance would be $10 billion to $15 billion over five years, in addition to existing outlays. The greater part of the buildup, though, would be in such U.S. elements as the "rapid deployment force," based at home but ready to move to trouble spots abroad.

This outline of a more muscular U.S. role was hardly out of the typewriter when a sudden challenge from the region tested Washington's new thinking. Border war flared between South Yemen, which had experienced a buildup of Soviet and Cuban force, and North Yemen, which had closer ties to Saudi Arabia.

On Feb. 28, the Saudis pushed the panic button, as an American official would later describe it, canceling all military leaves in anticipation of a Soviet-backed invasion of its territory. The Saudi officials informed the United States privately that the moment of truth for American loyalty was at hand.

After an emergency National Security Council meeting, Carter ordered the aircraft carrier Constellation to steam to the Arabian Sea from the Philippines, dispatched two U.S. Air Force air warfare control planes to Saudi Arabia, offered to send land-based tactical Air Force jets if the Saudis wished, and ordered up an additional $400 million in U.S. weaponry for North Yemen on an emergency basis.

The brief but intense display of U.S. forcefulness sent tremors through the area. Two traditional rivals, Iraq and Syria, joined in mediation between the Yemens before hostilities could get out of hand. The political maneuvering has continued since, with the two Yemens shifting toward an alliance or unification, North Yemen receiving new Soviet aid on top of the U.S. aid. The Saudis are still apprehensive, though no longer in high alarm.

After the Yemen episode early last March, U.S. plans for a new regional security arrangement were laid aside while Carter negotiated the final terms of the Egyptian-Israeli peace treaty, which was signed March 26.

The controversial treaty split the Arab world, with Egypt on one side and Saudi Arabia, Jordan and most others on the other side and bitterly critical. The prospect of regional arrangements involving Egypt and Israel as well as the Saudis and Jordan was reduced to the vanishing point.

Senior policymakers focused their attention on the area again last summer, when long gasoline lines in many parts of the United States emphasized anew the vital role of imported oil.In Cabinet-level session in the White House situation room last June 21 and 22, then secretary energy James R. Schlesinger Jr., declared flatly that if the oil of the Persian Gulf were interrupted, the West was finished.

Schlesinger suggested an increase in U.S. military power in the region to balance that of the Soviets, but Brzezinski thought this was a flawed conception. The area is vital to the United States and its allies, but not to the Soviets, and therefore Uncle Sam should aim for clear superority, according to the national security adviser.

In view of the far-reaching nature of the White House discussions, the operational decisions seemed modest. Working from a four-option Pentagon paper, the president's National Security Council committee recommended the most gradual increase offered in visible American power in the region.

Specifically, the advisers proposed -- and Carter later ordered -- that the number of regular U.S. naval task force annual deployments in the Indian Ocean be increased from three to four.

Some of the participants also recommended, though full agreement on this was less than clear, that the United States move toward a sustained presence in the area with larger naval forces as soon as feasible. On July 9, after the White House decision, a five-ship task force of combat vessels entered the Indian Ocean for what the Navy called "routine operations."

At the same meetings, new military sales totaling $1.2 billion were approved for Saudi Arabia, and more Air Force "demonstration" visits to Arab countries were approved. But the most important result, in retrospect, was acceptance of the principle that greater U.S. military strength in the area was needed.

The next major episode, and probably the most important, began with the seizure of the U.S. Embassy in Tehran Nov. 4, and the inner circle discussions of the options for military action. An Entebbe-type raid is reported to have been immediately ruled out as not feasible, but as weeks passed with no sign of resolution the military options came under careful study.

The practical problem of applying U.S. military might in this region "was very, very sobering, causing some very, very serious rethinking of our ability to project power to that area to protect our interests," said a military official who was involved. In a meeting with Defense Secretary Brown and the uniformed Joint Chiefs of Staff at Camp David Nov. 24, Carter ordered a full report on what could be done quickly to provide emergency U.S. operating access to the region -- overflights of friendly nations, transit and operating facilities available to U.S. forces.

The answers were anything but reassuring. An aircraft carrier task force including the USS Kitty Hawk entered the Indian Ocean Nov. 24 to join the carrier USS Midway and its task force, which was already in the area. Just sustaining these ships, to say nothing of bringing in the sort of major ground combat force which would be required for assault opperations, was difficult.

At a National Security Council meeting on Dec. 4, it was decided to initiate exploration of improved U.S. access to military facilities -- the word "bases," is anathema in the Third World -- and to do so quickly. Success in this quest would place the United States in position to sustain larger American forces in the region on a routine, regular basis.

The State Department began studies, and some objections were heard about the probable reactions and political complications that would arise. A meeting of Brzezinski, Brown and Secretary of State Cyrus R. Vance decided that exploration for facilities should proceed, and on Friday, Dec. 14, Carter decreed that a team should go -- at once. After a frantic. weekend, the group was in the air Monday, headed for Saudi Arabi, Onan, Somalia and Kenya.

The study team was just back from the middle East when the Soviet Union began its large-scale airlift into Afghanistan Dec. 24. The coup in Kabul and the overland invasion by Soviet ground forces starting Dec. 27 left senior U.S. officials more concerned than ever. Suddenly the real danger of a Soviet push to the Persian Gulf added to internal instability, jitters and uncertainty in the area about the U.S. role.

The strategy of long-term U.S. involvement in the region, outlined in last night's State of the Union address, was adopted by the high policy officials after his address to the nation Jan. 4 on the Soviet invasion.

It was decided that Carter would speak to the nation to announce the new strategic importance being accorded to the vast region 'stretching through Southwest Asia." Carter liked the speech written for this purpose better than those which had been submitted for the State of the Union. So he decided to start his fourth year in office with declarations about a region halfway around the world.