When Secretary of State Cyrus R. Vance returned from a rare and long-planned four-day holiday in Florida on Monday afternoon, April 14, he learned for the first time that the National Security Council, in his absence, had taken up a U.S. military operation in Iran and that President Carter had given his approval.

Vance was appalled. As his deputy, Warren Christopher, described the unfolding of the hostage rescue plan at the White House meeting of the previous Friday, Vance understood immediately that the planned action was heavy with consequences for diplomacy and war, policy and politics.

Neither Vance nor Christopher had advance warning, according to informed sources, that Carter and the National Security Council were to consider the military plan. Christopher knew nothing of the evolution or details of the plan, since previous discussion had been limited to those at the very top. He therefore declined to take a position on the proposal in Vance's absence, but decided to inform the secretary of state the minute he returned to Washington.

After hearing Christopher's report, Vance turned to the proposed rescue mission as an urgent order of business. That night and the following morning, April 15, he thought it over very carefully in his office and at home, marshalling his thoughts in his typically lawyer-like fashion.

Vance's conclusion was that it would be a momentous mistake under these conditions to use U.S. military force, whether for a high-risk rescue mission or the much-discussed blockade or mining of Iranian ports. The secretary of state, according to sources familiar with his thinking, was convinced the military action was unlikely to accomplish its mission but was likely instead to create an explosive reaction in the vital Persian Gulf. w

Moreover, Vance feared that one action would lead to another in deepening military involvement similar to that he had witnessed as deputy secretary of defense in the middle 1960s, when the United States became bogged down in the Vietnam war. The more he thought about the proposal, the more certain he was that he was right.

Vance's unshakable conviction, coming after months of dismaying setbacks and tremendous pressures in his job, led to the announcement yesterday of his resignation as secretary of state.

In conduct characteristic of this extremely loyal, proud and private man, Vance resigned on a matter of principle -- yet refused to state publicly the policy objections that impelled him to his extraordinary act, in order to limit the consequences of this act.

Vance insisted on resigning despite presidential entreaties to the contrary.

Yet yesterday he appealed to all his principal subordinates at the State Department to say on. He so strongly opposed the military operation that he privately told Carter he would resign a week before its inception and submitted his handwritten resignation three days before. Yet he helped refine the plan as D-Day approached and, when disaster struck, he worked to explain it to members of Congress and to foreign governments.

He lived "within the system" by a strict code of organizational behavior, perhaps too strict for the shifting winds and bureaucratic infighting of official Washington. His departure was in keeping with the same internal code.

"He is very, very tired. He fought so many draining battles -- the Soviet brigade in Cuba, the Middle East vote, the invasion of Afghanistan," said a senior aide.

A midlevel State Department official pronounced Vance's administration of foreign policy "a shambles."

Yet his official spokesman, Hodding Carter, and others close to Vance insisted yesterday that despite disappointments and setbacks, he would have stayed at his post for the rest of his four-year hitch had it not been for the disagreement about the military action in Iran.

After his overnight consideration of Christopher's report, Vance asked to see the president privately on Tuesday, April 15, a day that had been consumed mostly in White House meetings with Israeli Prime Minister Menachem Begin. In the privacy of the Oval Office, Vance stated his objections in full to the plan for the hostage rescue mission.

Carter arranged for Vance to submit his arguments to the full National Security Council, although the president, as is always the case, retained the power of decision. And after hearing Vance out, Carter was unpersuaded. t

Until this point there was no suggestion that Vance would quit over the disagreement. But on Tuesday, April 17, Vance told the president that he thought he would have to resign if the military action proceeded as planned a week thence.

The die was cast the weekend of April 19-20 in what his friends recalled as a period of anguish about resigning. Vance still felt there was a chance to head off the action that was abhorrent to him, speaking of playing a long game against the odds within the high policy circles. But at the same time, he had finally decided that if the action became inevitable, he could not remain. He was very determined.

Vance's reasoning was that because of his opposition to the military plan, a stand he considered a matter of principle, his position would be untenable in the aftermath of the action no matter what its result. He would have the choice, as he saw it, of saying things in public he did not believe, which he refused to do, or of admitting his opposition and thereby sapping his strength and that of the administration.

On Monday morning, April 21, he sat down at his desk in the State Department and wrote out his letter of resignation in his clear, well-ordered script on two pages of official stationery. That afternoon he handed it to Carter at the White House.

According to those who recently have discussed the matter with him, Vance is not opposed on moral or other grounds to the use of American military power in all circumstances.

"He has said he is ready to use military power, that he has used it, that he is ready to use it again under the right circumstances," said an associate. But Vance believed the use of military power in these circumstances was not in keeping with American objectives or the American national interest, this official continued.

In this situation, however, Vance believed the slow, frustrating and uncertain path of diplomatic negotiation and nonmilitary pressure was the way to achieve the goals of safe return of the hostages and protection of the U.S. interests. He remarked that it took 11 months of patient negotiation to obtain release of the Pueblo crew in 1968 -- but they came back safely and without widening regional or worldwide consequences. Clearly Vance preferred the safer, if longer, way.

When the news leaked out over last weekend, Vance was home suffering from a debilitating gout which kept him in bed for many hours and forced him to lean on a cane as he left home for the State Department early yesterday morning.

As he prepared to leave his spacious seventh floor office just before 11:30 a.m. to face the cameras and the press, someone handed him the cane. Despite the physical difficulty, Vance gritted his teeth and put the prop aside. He walked briskly and deliberately through the corridors to state his support for Carter and the system he lived within and was leaving.