By the time President Carter convened his National Security Council on the morning of April 11, he had already made the most crucial and personally agonizing decision of all.

The crisis that had so paralyzed the country and his presidency had to be ended, he had concluded, even though he knew that it was quite possible that some of the 53 Americans held hostage in Tehran would be killed in the effort to free them.

The president decided that this human risk was now preferable to letting the crisis continue indefinitely, according to one of the president's most senior advisers.

A military rescue plan had been studied and refined for months. It offered no guarantees. Indeed, Carter had been informed that it was quite possible that some of the hostages might die in the course of the rescue operation.

Carter's willingness to risk the lives of some of the hostages to finally end the crisis marked a dramatic and personally wrenching shift in his thinking. For months, ever since the U.S. Embassy was seized on Nov. 4, 1979, the president had publicly defined America's primary objective in terms of protecting the lives of every hostage. a

"I cannot and will not rest until every single American is home, safe and free," Carter said in February. That was the heart of his policy. He kept his Christmas tree unlighted and tied it with yellow ribbons. He told Americans he was praying several times a day for the safe return of each hostage.

But four months later, after the rescue failed, the president would refer obliquely to his changed viewpoint in an answer during his news conference last Tuesday.

The president said:

"Had the operation been successful or even if it had been concluded without complete success, it would have ended a continuing crisis that is destabilizing for the people of Iran . . ."

That since phrase -- ". . . without complete success" -- was Carter's way of talking about the deaths of some of those he was trying to save.

A senior White House official explained:

"He meant that even if there were casualties, it was preferable to letting the situation continue indefinitely. Presidents have to make decisions that will cost lives. Everybody here knew that it was possible that there would be a loss of life of some of the hostages."

For months the president had pursued a policy of trying to win the safe return of all the hostages with patient diplomatic negotiation. In the beginning, this restraint had won him the highest marks from the American people, as his low standing in the public opinion polls soared and his campaign to win reelection was rolling impressively over the challenge of Sen. Edward M. Kennedy (D.-Mass.).

But by March, the nation's frustration was mounting, its confidence in Carter's handling of the crisis was falling, and the Kennedy campaign was finding new life, in the form of landslide wins in the Democratic primaries of New York and Connecticut.

The president based his hopes for the negotiated return of the hostages on a secret, jerrybuilt diplomatic effort founded upon contacts between his chief of staff, Hamilton Jordan, a specialist in domestic politics but not diplomacy, and three French lawyers, who were also novices in international policy but who could serve as intermediaries to the unstructured and uncertain leadership of the Iranian government.

As the month of March ended, the president's hopes soared. He thought he had assurances that Ayatollah Ruhollah Khomeini would approve the transfer of the hostages from the militants to the Iranian government, the crucial first step to securing their safe return to the United States.

The president was anxious to get the good news out on April 1, the day of the Wisconsin primary he hoped would restore his sagging political fortunes. He called an extraordinary 7 a.m. news conference, timed to make the television network morning news shows, and hailed a "positive step" in Iran.

But Khomeini and his followers promptly pulled the rug out from under Carter. The negotiations collapsed and the president, who had gotten himself so far out front with his own peace-is-at-hand proclamation, now turned toward the military option.

Even as the final diplomatic effort was in progress, the president had met with his top national security advisers at Camp David to review the various military recourses available. These included the mining of Iran's harbors or the imposition of a naval blockade. And there was the rescue operation that had been conceived in November and had been refined steadily since.

In the days ahead, the president would know well that his secretary of state, Cyrus R. Vance, opposed all military options for Iran, favoring instead further pursuit of the paths of negotiation and consultation.

By this time, a number of those closest to the president had come to blame Vance, with some bitterness, for the president's landslide defeat in the New York primary, by mishandling the controversial vote on the anti-Israeli resolution at the United Nations.

Carter concluded, according to top advisers, that the crisis in Iran had dangerously heightened world tensions. It had inflamed the already explosive Persian Gulf region. It had diverted world attention from the Soviet invasion of Afghanistan.

A naval blockade or mining of the harbors would not solve the crisis, but inflame it further, he felt. It would only broaden and intensify the crisis and perhaps cause the Islamic world to turn against the United States.

"Ending the crisis -- once and for all -- became the major factor in the president's decision-making," one of his senior advisers said.

Just before his diplomatic effort collapsed, Carter had derided former president Ford's decision to undertake the Mayaguez rescue as more tempting than wise. "I've decided to use political and economic options and forgo the military options altogether," Carter told Meg Greenfield, editor of the editorial page of The Washington Post.

"We've not had any loss of life during this administration because of people being sent into combat. It happens to be the first time in 53 years that that has happened," he said.

Days later, with his diplomatic effort reduced to rubble, Carter changed his mind. He reviewed the military rescue plans again with his National Security Council. He knew the plan could result in the deaths of some of the hostages, and he knew approving it would result in the certain loss of his secretary of state.

Knowing all this, Carter ordered the military rescue.

"It was," said one of his most senior advisers, "a classic presidential decision."