When President Carter summoned his top national security advisers to the Cabinet Room Friday, April 11, it was to act upon a daring rescue plan with which they were already familiar.

The plan had been drafted in secrecy in the Pentagon just after the U.S. Embassy in Iran had been seized and Americans taken hostage. It had been refined and simplified several times since then. And at every step, the president and his advisers had been fully briefed by the chairman of the Joint Chiefs of Staff, Gen. David C. Jones.

On April 11, the president wanted to review the details once more, and to review the changing situation he felt might make it advisable to put the secret rescue plan into effect.

Attending were Vice President Mondale, Secretary of State Cyrus R. Vance, Defense Secretary Harold Brown, CIA Director Stansfield Turner and national security affairs adviser Zbigniew Brzezinski.

Yesterday, with the once-secret rescue effort now aborted and very public, administration officials outlined the factors that had led Carter to act.

All diplomatic efforts to free the hostages had failed, administration officials said, and there was no reason to believe that the hostages would be released in the foreseeable future.

Carter and his advisers had concluded that unrest and divisions within Iran, plus unrest between Iran and Iraq, were endangering the safety of the hostages.

The day before Carter's April 11 meeting, a spokesman for the militants at the Embassy had warned on NBC's "Today" show that they would "destroy" hostages if Iraq attacked Iran. Iraq was a vehemently Anti-american country with which the United States had severed diplomatic ties.

The statement apparently weighted heavily upon the president. In a comment just three days ago, Carter told a group of editors and broadcasters, when asked if he would be willing to "jeopardize" the lives of the hostages to end the crisis, that the hostages already were jeopardized.

He cited some "disturbing statements made by the terrorists. For instance:

if Iraq invades Iran, this would be a puppet of the United States, and the hostages would be executed."

And he added: "There was not any immediate counterstatement made by either Khomeini or the government officials."

On April 11, there were other concerns that administration officials now cite as reasons for opting for the rescue effort, as opposed to some other, less riskly military step. The United States always had the option of trying to halt Iran's international commerce by mining its harbors or instituting a naval blockade.

But American officials say they were as concerned about the drawbacks of mining or blockading as were some of the vocal critics outside the government. Either step could add greatly to tensions in the Persian Gulf and unite the Islamic world against the United States.

And the officials felt the approach of summer was a compelling factor in attempting the rescue effort now, for two reasons:

First, temperatures would rise with the coming of summer, making the helicopter and C130 transport takeoffs more difficult and more costly of fuel. h

Second, the operation had to be conducted under cover of night (two nights, actually, with the intervening daylight hours to be spent largely in lying low and trying to avoid detection.) The hours of night were growing shorter.

That Friday, April 11, meeting adjourned with the president giving tentative approval to begin the operation. The following Monday, the same group of officials reviewed the details, and Carter gave his final approval for the rescue plan.

It would take a couple of weeks to put the plan into operation. It called for creating a refueling area in the desert about 200 miles from Tehran -- they called it Desert One. The helicopters than would fly to a second staging area on the outskirts of the city.

From there, the rescue party would make its way to the Embassy.There were already certain elements in place in Tehran that were necessary for the eventual success of the plan.

Every morning, in the days after the final approval was given April 14, America's top national security officials convened at 9 a.m. in Brzezinski's office to go over the details.

While the plan was to be carried out under cover of darkness, it also was being set up under cover of official presidential pronouncements, warnings and threats.

The most crucial element in planning the mission was totaly secrecy, and as the date for the operation neared, the U.S. government embarked on what was in part an elaborate attempt to deceive not just Iran but the entire world.

In words and actions, the administration focused world attention on the period around mid-May as a sort of deadline, after which the prospects for some kind of military action -- presumed to be a naval blockade or mining of Iranian waters -- would be much more likely.

This process of increasing economic and diplomatic pressure on Iran by the United States and its allies between now and mid-May was "a separate and genuine track" that will continue despite the failure of the rescue mission, an administration official said yesterday.

But, conceding that a secondary purpose was to cover the planned rescue mission, the official added:

"To the extent that people were looking not at next week but at next month -- and that includes everybody -- the chances for success were increased and the chances of loss of life were decreased."

On April 12, the day after setting in motion the final stage of the operation, the president granted an interview to correspondents from British, French, West German and Italian television networks that was the key to diverting attention from anything dramatic happening this month.

Carter told the European correspondents he had given U.S. allies "a specific date" by which time he expected growing economic and diplomatic sanctions to succeed in ending the crisis. This was a de facto deadline, which administration officials subsequently said was in mid-May, after which the United States might resort to military measures.

At a news conference April 17, Carter announced yet another new series of sanctions against Iran. One was a total ban on travel by Americans, except journalists, to Iran. At the same time, the president appealed to U.S. news organizations "to minimize as severely as possible their presence and their activities in Iran."

Both the travel ban and the appeal to American reporters to get out of Iran were made chiefly because of the approaching rescue mission, officials said.

There was a final diversionary tactic just two days before the helicopters were to head for Iran. Last Tuesday, White House chief of staff Hamilton Jordan convened an unusual meeting of about 40 White House staff aides to discuss the Iran situation.

The next day, a story about the meeting was on the front page of the Los Angeles Times. In it, one participant quoted Jordan as saying that a rescue mission "had been studied and just wasn't feasible."

A White House official said the Times story was not part of a master plan of deception. But once word of the meeting leaked, as it was almost certain to do, the White House made sure that Jordan's comments about a rescue mission were included, the official conceded.

This week, with the rescue operation about to begin, two developments shook the highest officials in the U.S. government.

On Sunday, The Washington Star published a commentary in which Miles Copeland, a former CIA official familiar with Iran, presented a detailed plan to rescue the hostages. One administration official complained that the next day the story was "picked up and carried in Iranian newspapers as though it was the official American rescue plan."

Then, he complained, Israeli intelligence put out a story to the Israeli press saying that there had been an increase of U.S. air activity in the region, and they believed the United States was preparing to run a rescue effort.

On Thursday morning, just about the time Carter was meeting with Shimon Peres, chairman of the Israeli opposition Labor Party, eight huge RH53 helicopters took off from the U.S. aircraft carrier Nimitz.

"We were very conscious of the fact that we did not want to appear tense or anxious," said one of the few senior White House officials who knew of the plan. So the president tried to keep to his daily schedule and so did we."

Throughout the operation, the president was apprised of the progress of the mission by telephone calls from Jones, who was, in turn, in contact with the rescue party in Iran. Carter could have spoken directly with the field commanders. "But the president made a decision that he did not want people in the field to be distracted by having to serve as play-by-play and color announcers," one official said.

At midday, Carter was told that one helicopter "had gone down." The president and his top assistants tried to go about their business. Later, word, came that it had just made a forced landing, and that the mission was still proceeding.

Then word came that a second helicopter had turned back to the Nimitz with mechanical problems. Then, with the rescue party on the ground in Iran, in Desert One, the president was told that a third helicopter would not function.

The president sat at his desk in the unpretentious study adjacent to the Oval Office as he listened to Jones on the other end of the phone. He was told that the top field officers in Desert One had argued among themselves about whether to try to continue the mission with only five helicopters -- one fewer than the plan required. They recommended that the plan be scrubbed.

The president listened. At 4:45 a.m. he gave the order officially ending the rescue mission.

One assistant who was present recalled that Carter appeared clearly disappointed as he gave the order. "But at least there was no casualties, the president said after hanging up the phone. "And there was no detection. It could have been worse."

The president and his advisers had feared that the Soviets might have detected the operation and told the Iranians. But that had not happened. They still hoped that the mission could begin anew in the very near future.

An hour or so later, the president took another call from the Pentagon command point. As he listened, his face fell noticeably, according to one official who was there. He looked up from his telephone.

"There has been a serious problem . . . an accident . . ."

Eight Americans had been killed.