Jimmy Carter's decision to withdraw ground troops from South Korea goes back at least to January, 1975, and the earliest days of his campaign for President. His original idea was to pull out all U.S. forces - ground and air - and to negotiate assurances from China and the Soviet Union that North Korea would not invade the south.

The origins and evolution of Carter's ideas are of unusual importance because his campaign stand has been translated directly into U.S. policy with a minimum of official review. In order to avoid a battle within the government, a National Security Council study leading to the U.S. withdrawal plan did not question whether American ground troops should be removed but focused instead on how many should be removed.

As sent to the White House in mid-March, the council's Presidential Review Memorandum 13 acknowledged that there are differences of opinion about the troop withdrawal policy and that the impact of it is difficult to predict. At the explicit instruction of Secretary of State Cyrus R. Vance, however, the State-Defense-Central Intelligence Agency study accepted as its promise Carter's previous announced conclusion that the troops should be taken out.

Without making an overt recommendation, PRM-13 reported the carefully hedged view that the risk will be within acceptable limits provided that the withdrawal of ground troops is carefully managed, that essential U.S. support elements remain, that adequate compensation be supplied to South Korea for the reduction in its defense capacity and that other U.S. actions do not send the wrong signals to North Korea.

While after-the-fact justifications have been made public, there is no indication that the government review considered Carter's own reason for the pullout and some responsible officials have conceded that they do not know what they are.

Major Gen. John K. Singlaub, who was relieved last month as chief of staff of U.S. forces in Korea after publicly criticizing the withdrawal, told Congress that the Joint Chiefs of Staff never gave its Seoul command a reason for the planned withdrawal despite requests for an explanation.

About the closest thing to serious governmental scrutiny of Carter's decision was a special National Security Council meeting which was convened to hear the reservations of Central Intelligence Agency Director stansfield Turner. Informed sources said that after hearing Turner's misgivings, Carter issued orders in early May that the troop withdrawal plan proceed.

A CIA staff briefing, which presumably reflected Turner's views, caused heated discussion Friday in a closed meeting of the Senate Foreign Relations Committee. The committee decided to ask for the text of a CIA estimate which was prepared in connection with the withdrawal plan.

Carter told another White House aide that he begun to form his troop withdrawal ideas before 1975. He said the Nixon administration's action in the early 1970s to remove 20,000 U.S. troops from Korean had been a factor.

Carter, in response to an inquiry about the origin of his views, replied through press secretary Jody Powell that they arose from his "basic inclination to question the stationing of American troops overseas." Powell quoted him as saying that keeping troops abroad "is something you need a good reason to do" and that he "has yet to see a convincing argument keeping those troops in Korea in perpetuity."

Powell went on to point out that Carter has taken a different position over many months about U.S. troops in Europe, never saying that he believes they should be removed.

Carter told another White House aide that he began to form his troop withdrawal ideas before 1975. He said the Nixon administration's action in the early 1970s to remove 20,000 U.S. troops from Korea had been a factor.

In the background of Carter's views - and to some who know him well, fundamental factors - are his Annapolis Academy schooling and his early career as a naval officer. The stationing of ground forces in exposed and static positions abroad is counter to traditional naval thinking. Air and sea power operating from offshore, augmented by mobile landing forces if needed, are the preferred solutions of naval doctrine as well as the "Nixon doctrine" enunciated in 1969.

Several of those whom "Carter consulted in 1974 and early 1975 believe the development of U.S. ground troops in South Korea is especially dangerous because they are a "tripwire" intended to guarantee nearly automatic U.S. involvement in any new Korean war. Though Carter did spell out to these advisers the basis of his views, some of them believe the "tripwire" danger is central.

Retired Adm. Gene R. LaRocque, director of the privately funded Center for Defense Information, recalled a telephone call from Carter asking about U.S. troops in Korea while Carter was still Georgia governor. laRocque said he told Carter that either North korean President Kin II Sung or South Korean President Park Chung Hee or their successors "could get us involved in a land war in Asia and it would tear this country apart." The retired admiral added that, "We have to think of the Middle East and Europe. On a scale of importance to us, I'd put Korea about 1 and the Middle east and Europe about 10." Carter listened carefully but did not disclose his own views, LaRocque said.

On Jan. 16, 1975, a month after declaring his condidacy for President and two weeks after leaving the governorship of Georgia, Carter told a meeting of the editorial page board of The Washington Post that he favored taking U.S. troops out of Korea and would be prepared to begin as soon as he became President.

Editorial page editor Philip L. Gevelin recalled that Carter said he would remove 5,000 troops as a start - which is remarkably close to the 6,000 which reportedly will be withdrawn during the first year of the new U.S. plan. In the discussion with the Post editors. Carter spoke of the troop withdrawal from Korea largely as a money-saving measure in the context of better defense management, Geyelin said.

Seven officials of the Brookings Institution discussed Korea and other diplomatic and military issues with Carter in a four-hour meeting on Jan. 28, 1975, at the organization's Washington headquarters. Stuart Eizenstat, the aide who accompanied the fledgling candidate, said recently the Brookings session was "a significant development" in Carter's thinking.

While in its official publications Brookings cautiously balanced the gains and risks of a withdrawal of U.S. troops from Korea, some of its staff members were much more emphatic in their personal views.

Senior fellow Barry M. Blechman recalled, "I told Carter we should take out the nukes (nuclear weapons) right off and phase out the ground troops over four or five years. I said the most important reason was avoid getting the U.S. involved with ground forces almost automatically in a new war which is, of course, why the South Koreans want them there." Carter listened carefully but did not say what he thought, Blechman recalled.

As Saigon fell in the spring of 1975, Carter planned a quick trip to Asia as a member of the Trilateral Commission, a private research and policy group which furnished him many of his contacts and appointees. In preparation for a campaign statement on foreign policy to be issued during his stay in Tokyo, Carter's staff solicited a draft speech from Harvard Law School Prof. Jerome Cohen, an Asia expert, and arranged for a Carter-Cohen meeting in Cambridge, Mass., on May 13, 1975.

Cohen's draft cautiously proposed that "in the immediate future we should maintain our forces and commitment in Korea" while proceeding with a modernization plan which would permit gradual reduction of the U.S. military force.To Cohen's surprise Carter - whom he had never met was much bolder. "I was struck by the urgency of his desire to get out of South Korea," Cohen recalled. The professor, who is close to South Korean dissidents and had previously traveled to North Korea, found himself cautioning that any withdrawal must be careful and gradual. "You don't want to pull the cork out of the bottle in a way that would upset Japan and South Korea," Cohen remembers saying.

At the time, Carter's determination to withdraw troops from Korea contrasted sharply with his caution about removing U.S. military protection from Taiwan, a stumbling block to normalization of relations with China. Cohen speculated to friends about a Carter "islands strategy" favoring off-shore positions but shunning mainland development.

Enroute to Tokyo late in May, 1975, Carter discussed Korea with the Peter Bourne, an aide who accompanied him on the trip. "What I think we should do is to strengthen the air force of South Korea and withdraw U.S. troops on a rapid schedule. With a dequate air cover they can defend themselves," Bourne recalls that Carter told him.

In a press conference with Tokyo correspondents on May 23, 1975, Carter said U.S. troops should be withdrawn over a period of about five years, that the United States should strengthen the South Korean air force and that Washington should seek guarantees from China and the Soviet Union that North Korea would not be permitted to invade the south.

Although Carter intended originally to pull out all U.S. forces, ground and air, Bourne recalled that even before leaving Tokyo he began to change his mind. Warned by some U.S. correspondents and Trilateral Commission members that a strong South Korean air force might make a preemptive strike against the north, Carter began to feel it was less dangerous to maintain U.S. airpower in South Korea. This is the present intention of the Carter administration.

Sometime later, Carter dropped the idea of seeking advance assurance from Peking and Moscow that they would restrain North Korea, evidently because such an effort seemed unlikely to succeed. Recently the Carter administration informed the two Communist giants of its withdrawal plans, but according to U.S. officials thee was no effort to negotiate a deal.

After all that Carter said as a candidate and as President-elect, the Communist governments had good reason to suppose by Jan. 20 that he would remove U.S. ground troops from South Korea no matter what their view about it.