In the early days of April, President Carter was clearly frustrated by the dire turn of the crisis in Iran.

He was going about the business of declaring new peaceful pressures to force the return of the hostages, but those close to him say the president felt deep down that the pressures would not work.

The state of the president's concerns, his personal frustrations and his shifting inclinations, are the largest unknown factor in this period in which the president began moving inexorably toward the military rescue plan that ultimately would result in a new international humiliation for the country, and would require a new secretary of state for his administration.

The circumstances that led up to Cyrus R. Vance's lone dissent and subsequent resignation occurred against a backdrop of strained fellings involving the Carter White House and the secretary of state.

A number of those advisers closest to the president -- advisers who were experts in politics more than a policy -- had come to be most unhappy with Vance. They had bitterly complained that he was chiefly responsible for Carter's landslide defeat in New York presidential primary because of his handling of that disavowed U.N. vote supporting an anti-Israeli resolution, and his subsequent congressional testimony that the vote had reflected U.S. policy after all.

It was in this setting that the president was eventually to override the strong objections of his secretary of state in the decision to launch the illfated rescue attempt in Iran. Carter's decision came amid mounting public concerns and frustrations that were in part of his own making.

The president had put himself painfully out in front on April 11 -- Wisconsin primary day -- when he called an extraordinary 7 a.m. news conference to herald what he called a new "positive step" in the negotiations to have the Iranian government take control of the American hostages from the militants.

The "positive step" proved a false step, however, as the Iranian Revolutionary Council and the Ayatollah Ruhollah Khomeini pulled the rug out from under an agreement that the president had been informed was already firm.

"That was one of the most frustrating moments for the president in the last six months [since the hostages were seized]," said one White House official. "First there was exhiliration and then suddenly he was at the point where he ceased to believe there could be a diplomatic solution in the forseeable future."

Carter began weighing anew the prospects of a military rescue attempt -- a plan had been drafted in November in utmost secrecy and had been revised and refined and tested in the field frequently since then.

Policy concerns and personal frustration both came into play in these anxious days in the White House inner circle. On April 7, the United States announced new economic sanctions against Iran.

On April 10, Vance left Washington for a brief Florida vacation. And the next day, at the president's 7:30 a.m. breakfast with his national security advisers, Carter announced that he would like to have what one source called "a fuller discussion of the Iranian situation" at a National Security Council meeting at 11:30 a.m.

Deputy Secretary of State Warren M. Christopher had been attending the breakfast in his chief's absence. Others-attending included Vice President Mondale, Defense Secretary Harold Brown, national security affairs adviser Zbiginew Brzezinski, and White House assistants Hamilton Jordan and Hedley Donovan.

The secretary of state had not been aware when he left for Florida that the president would be convening the National Security Council meeting, according to administration officials. Nor had Christopher, they said.

It was not until the session began in the Cabinet Room that Carter formally told the entire group that he wanted to discuss the advisability of undertaking the military rescue plan. The plan was such a closely held secret that Christopher had not even heard of its existence before then, sources said.

When it came his turn to speak, Christopher said he had not known of the plan in advance and could not offer a State Department position on its advisability. The president, according to several sources, assured Christopher that Vance already knew of the plan, and that Carter knew of Vance's views concerning it.

According to one White House official, the president also said in that National Security Council meeting that he had discussed the various military options generally with Vance, and that although Vance had concerns about all of the options, the secretary of state felt the rescue plan was preferable to the options for mining or blockading Iran's harbors.

At the meeting, Carter gave a tentative approval -- and later a more formal approval -- to proceed with the plan.

Christopher, perhaps because he had been reassured by the president that his boss had already made his views known, did not promptly contact Vance to inform him about the plan and the decision, according to informed sources. The secretary learned of Carter's decision upon returning from Florida on Monday.

A number of administration officials in the State Department and outside it now wonder whether the National Security Council session had been carefully arranged so that Vance -- whose reservations were apparently known in advance, at least to some extent -- would not be present when Carter approved the military rescue plan.

Presidential press secretary Jody Powell has strongly denied that this was the case, adding that Vance also says that he has no quarrel with his role in the decision-making process.

Vance had attended a meeting at Camp David on March 22, where the military plan was discussed, officials said. Upon his return from Florida, Vance raised his strong objections privately with Carter -- and the president then gave the secretary an opportunity to make his case -- opposing any military operation -- before the rest of the National Security Council. The president then declined to reverse his decision.

The storied policy balance between Vance and the more hardline approach of Brzezinski appeared to have tilted decidely toward Brzezinski after the Soviet invasion of Afghanistan.

Recently, Vance had asked Carter's permission to renew diplomatic dialogue with Soviet Foreign Minister Andrei A. Gromyko in Vienna next month at the ceremonies for the anniversary of the four-power agreement.

But the president withheld permission, according to informed sources, saying he had not yet made a decision.

Vance, meanwhile, had a history of frustration in his efforts at intra-administration diplomacy. In the summer of 1978, infuriated by Brzezinski's hard-line public pronouncements attacking Soviet-backed Cuban military efforts in Africa, Vance in effect threatened to resign.

He was reported to have told Carter then that he could not function effectively as secretary of state with Brzezinski making public policy pronouncements. Vance won that contest, as Carter told Brzezinski to carve himself a lower public profile.

Last summer, Vance is said to have told Carter that he would not stay as secretary if Andrew Young were allowed to remain as ambassador to the United Nations. Young left; Vance stayed.

Vance also clashed with then-Middle East negotiator Robert S. Strauss, who wanted to have a free hand in his negotiating, and who later left that job to become Carter's campaign chairman.

Recent public life was not all a downhill slide in influence for Vance. As recently as April 17 -- After Carter had already approved the rescue plan that proved the secretary's undoing -- Vance made one last lone appeal.He went to Carter and urged the president to reverse his decision and remove food and medicine from the list of items embargoed for trade with Iran.

That, it turned out, was Vance's last victory as secretary of state.