For want of just one helicopter, President Carter's first attempt to use military force blew up on the pad.

The technological giant of the world, the United States, called off its meticulously planned operation to rescue the 53 hostages in Tehran because too many helicopters broke down on the way there.

The Pentagon, in what had to be an embarrassing background briefing for the high-ranking military officer who gave it, tried to provide an explanation.

But he ended up admitting that the breakdowns exceeded everybody's worst fears.

If this is true of a rehearsed operation, using fine-tuned helicopters with the best pilots in the military, what does that admission say about the overall readiness of the American Army, Navy, Air Force and Marine Corps to go to war?

How would other weapons perform in the mud and under fire?

Are the critics, such as Rep. Jack Edwards (R-Ala.), correct in claiming that the Pentagon is so obsessed with buying new weapons that it does not take care of the ones it already has?

Or is it that the all-volunteer military does not pay enough money to attract and keep people skilled in operating and fixing million-dollar weapons? a

Those and other broad questions flow from the narrower ones thrown at the Pentagon yesterday about its ill-fated rescue mission.

Why did the Joint Chiefs of Staff not commit more than eight helicopters to the rescue operation to hedge against the mechanical failures that ultimately doomed the mission?

The Pentagon said yesterday that eight Sikorsky RH53 "Sea Stallion" helicopters were judged to be more than enough. The same kind of choppers, in rehearsal after rehearsal in the Southwest, had performed with few mechanical problems.

Each helicopter could carry 35 passengers, or 280 in all. This would have been twice as many as needed for the 90-man rescue team and the 53 hostages to be swooped out of Tehran.

Committing more than eight choppers, said the briefer, also would have required another C130 transport to carry additional fuel to the first rendezvous point in the Iranian desert staging area, called Desert One. This would have enlarged and complicated the operation, he said, for what looked like no good reason.

As it turned out, two of the eight helicopters failed to make the 500-mile first leg of the rescue mission from the aircraft carrier Nimitz in the Arabian Sea to Desert One.

One landed on the desert, the Pentagon briefer said, because the hydraulic system for giving the pilot power over the chopper's controls failed. This would be like power steering suddenly going out on an automobile. It is not something that can be fixed on the spot with a wrench.

The second helicopter ran into a sandstorm. The pilot began to become disoriented -- a condition called vertigo -- and opted to return to the Nimitz rather than land on the desert or climb above the storm.

Why were the pilots not trained to fly through such sandstorms, as there was such a likely chance that they would run into them?

They were, according to the Pentagon. But this particular pilot was confronted with another mechanical problem -- his gyroscopic navigation instruments conked out.

When suffering vertigo, a pilot does not know whether he is flying upside down or right side up. He must depend on his cockpit instruments to determine his position in the sky. He might be diving toward the earth without realizing it. The chopper pilot evidently decided to play it safe and return to the Nimitz. He could do this because additional navigation aids aboard the carrier could assist him.

But two other pilots handled the same sandstorm differently. They touched down on the desert briefly, waited out the worst of it, then proceeded to the rendezvous point near Tabas.

Did the pilot who returned to the carrier goof? It is too early to make that judgment, the briefer said.

Why, after learning that one chopper had broken down and landed on the desert, and a second had returned to the Nimitz, did the commanders not call in other choppers to take their places at Desert One?

There was no time for this. All the flying was to be done at night to escape detection. There was not enough of that first night left, said the briefer, to launch replacements from the Nimitz and fly the five hours to Desert One.

The plan called for the eight helicopters and six C130s -- three planes carrying fuel for the choppers and three carrying the 90 troopers of the Blue Light strike team trained to take over the embassy -- to rendezvous in the dark at Desert One.

There, the eight helicopters would be refueled and fly on that same first night -- Thursday -- for another two hours to reach a mountain hideaway about 100 miles outside Tehran. The troopers would hide there during the daylight hours of Friday, then board pre-deployed buses and trucks at dark. The vehicles would then blend in to the evening traffic streaming into Tehran.

This meant that all eight choppers were supposed to fly seven hours that first night, five from the Nimitz to Desert One and two more hours to the mountain base. The timetable was tight.

Six helicopters reached Desert One and refueled there. But one of those six discovered that it, too, had lost power over its control through a hydraulic failure. That left only five helicopters able to fly to the mountain hideaway for phase two of the rescue operation

Why did the commanders not order the operable five helicopters to fly quickly to the mountain hideaway? This would have provided 175 seats, more than enough for the 90 rescuers and 53 hostages.

The plan called for using six helicopters. One reason for insisting on that margin was the planners' expectation that at least one of those six helicopters could not be started after spending a day at the mountain base. There would be no sophisticated equipment on hand to correct such a mechanical problem.

The commander on the ground at Desert One, together with his superior overseeing the operation from an undisclosed command post some distance away, agreed that it would be too risky to proceed to the mountain hideaway with only five helicopters.

If one of those five did break down at the mountain camp, the four remaining choppers could carry only 140 people without crowding, three less than the total they were supposed to swoop out of the Embassy compound after the strike team had secured it on the second night.

Pentagon officials concede that four choppers could have carried out the 143, but such a plan did not allow for the possibility of having to take out wounded on stretchers.

Why were replacement helicopters not flown directly to the mountain hideaway that second night?

Even if a replacement helicopter made this 700-mile leg, it would land at the hideaway with near empty fuel tanks. It would not have been refueled at Desert One. It thus could not perform the rest of the mission of going to the compound and taking hostages to the exit base outside Tehran. Also, the timetable was extremely tight. Indications are that the United States would have some secret helpers in Tehran but that the force had to arrive at the pre-planned time.

Who made the no-go decision?

President Carter, on the recommendation of the commanders of the operation and the concurrence of the Joint Chiefs of Staff and Defense Secretary Harold Brown.

Was the secrecy of the mission blown by the fact that a bus with 44 Iranians happened to drive into Desert One during the refueling operation?

Not necessarily. The Pentagon briefer said the troopers forced all the Iranians off the bus, and would have loaded them onto one of the C130 transports and flown them out of Iran until after the rescue mission had been completed. With the mission canceled, the bus and its stunned passengers were released.

Also, an Iranian who looked like a smuggler drove onto the scene that first night. The Iranian was driving a tank truck -- presumably filled with black market fuel -- and towing a pickup truck. He rammed through the barricade the troopers had stretched across the road.The troopers fired at the truck, and the engine burst into flames. The suspected smuggler jumped into his pickup truck, and raced off across the desert. The soldiers fired low, hoping to blow out the pickup truck's tires, but missed.

Pentagon officials indicated the fact that the smuggler saw the Americans and then escaped would not have been enough to cancel the rescue mission.

What was the single biggest reason the mission failed?

"Bad luck," the Pentagon said yesterday.