Shortly after the hostage rescue attempt went up in flames in Iran's back country last spring, the Pentagon's Joint Chiefs of Staff ordered plans prepared for a second raid to free the captive Americans.

Details of this second rescue plan are still top secret. But government sources, some of whom have left office since the guarded conversations took place, revealed the general outline, perhaps to take the sting out of hostage charges that the U.S. government should have tried harder to free them.

Rescue Mission Two, not its code name, called for a pincer movement involving paratroopers racing from the Tehran airport to the U.S. Embassy compound at the same time commandos were airlifted to round up the hostages imprisioned elsewhere in Iran.

But the Joint Chiefs decided in August the risks were too high, and that a substantial number of hostages might be killed in this desperation try.

The biggest obstacle, intelligence sources said, was the fact that the fractured command and erratic behavior of the Iranian militants screened both their intentions and the locations of some of the hostages. There were no one command center to put under electronic and human surveillance, for example, they said.

The Joint Chiefs' decision against using the U.S.'s vastly superior military power for fear of killing relatively few people dramatizes how a small band of lightly armed terrorists can exploit the historic American ethic of putting human lives first.

Even though President Carter was prepared to take considerable risks on the April 24 raid, he still used only a sliver of American military power. And even that was never brought to bear inside the U.S. Embassy compound in Tehran. The commandos came up one helicopter short at the first launching point inside Iran, a bit of back country called Desert One. The rest of the mission was scrubbed. Eight Americans were killed during the aborted operation.

No one can say for certain whether the commandos would have gotten the hostages out of Tehran if they had executed the rest of this first top-secret plan (code-named Rice Bowl in hopes of throwing off any unauthorized people hearing about it).

Whatever might have happened had Rice Bowl gone all the way, the Joints Chiefs and their planning staffs knew the aborted mission would cost them surprise on any second rescue attempt. Therefore, the planners opted for more force, including a strong thrust from paratroopers -- almost certainly the 82nd Airborne expressly designed for flying into trouble spots.

Planners apparently remained confident, as they had in designing Rice Bowl, that a spearhead of American troopers could rush into the embassy and the nearby foreign ministry at night and hold it until more help arrived. This confidence suggests detailed knowledge on how, when and perhaps with whose help, those two places could be penetrated.

Under Rice Bowl, helicopters would have swooped into the embassy compound and/or the nearby stadium at night to pick up all 53 hostages rounded up by Blue Light commando team. But as planners of Rescue Mission Two sat down in May to map their operation they could not count on such a concentration of hostages.

All through the spring and early summer of 1980 the Central Intelligence Agency, Defense Intelligence Agency and National Security Agency struggled to pin down how many hostages had been moved, and where, following the April 24 mission.

But their "assets", intelligence jargon for agents and other informers on the ground in Iran, had been devastated by the raid. Documents captured by Khomeini forces blew the cover on some people; Tehran's massive roundups of suspects sent others underground and what little was left of the CIA network from the days of the shah was shredded into even smaller bits.

Although electronic eavesdropping and spy satellites helped U.S. intelligence officials see through the claims of Iranian militants that the hostages had been scattered widely around Iran, the so-called "fine grain" information on the Americans' whereabouts remained elusive.

But Rescue Mission Two planners kept working as if the operation would go ahead. Intelligence officials continued to press for a breakthrough on the locations. But the hostages proved to be a moving target, even though most of the shuttling was in Tehran rather than to the distant countryside as the militants kept declaring.At most, about three-quarters of the hostages were pinpointed for any length of time.

Spring gave way to early summer, and there was still no information breakthrough. How could the Blue Light outfit, under Chargin' Charlie Beckwith or any other commando leader, rescue hostages they could not find? Going house-to-house in the dark through the suspected locale would be courting disaster. Even Col. Beckwith, the gung-ho leader, had demonstrated on the April raid that he would not go along with something that risky.

Diplomatic negotiations with Iran kept breaking down, making military action look more and more like the only answer. But putting all the pieces together for another rescue effort was extremely complicated and time was of the essence. Secret arrangements would have to be made with foreign governments to fly through their air space and use their bases to refuel. Commandos would have to be sneaked into launching positions.

The Joint Chiefs' deliberations by midsummer had boiled down to this crucial question: Is Rescue Mission Two too risky to try? Put another way, was it likely more hostages would be killed than saved? The chiefs' finally gave their answer in August: "No go; too risky."

The Pentagon's Rescue Mission Two plan had been rejected before Columnist Jack Anderson released on Aug. 18 a controversial column which said: "A startling, top secret plan to invade Iran with powerful military force has been prepared for President Carter . . . The tentative invasion date has been set suspiciously for mid-October. Sources say the president has assesed the political consequences and has concluded the invasion would be popular with the electorate." In a subsequent column for release Aug. 20 Anderson wrote that "the primary invasion target is Kharg Island in the Persian Gulf . . . ."

Pentagon officials insist their rescue mission plan bore no resemblance to the operation described by Anderson. Rescue Mission Two was never even formally recommended to the president.

The American reluctance to use military force where lives of Americans might be endangered is not a new phenomenon. North Korea hijacked the U.S. spy ship Pueblo in 1968 and its crew was held captive 11 months. After their release was negotiated, some Pueblo sailors complained that the Johnson administration withheld military retaliation against North Korea in the interest of getting the crew back alive. Richard Nixon, vowed in campaigning for president in 1968 that he would not tolerate a Pueblo, only to find himself holding back any military response when North Korea shot down a Navy electronic eavesdropping EC121 plane on April 14, 1969.

Now, some of the former hostages reportedly have complained to Carter about Washington's restraint during their long captivity. The Reagan administration has vowed to take an unrelentingly tough line against any such future acts of terrorism.