If the Senate takes Ronald Reagan's suggestion and probes every aspect of the Iranian hostage situation, the senators won't like what they learn about the aborted rescue mission last April.

The truth is that our military and intelligene professionals planned, rehearsed and executed the raid as if it were amateur night. The breakdown of the helicopters, humiliating as that was, didn't compare with the failures of command-level brainpower that foredoomed the complex expedition.

A combined operation of the type envisioned -- a secret raid by land, sea and air forces on enemy territory -- is the trickiest military operation on the books. It requires careful planning, tight security and bold leadership if the men who must carry it off are to have a fighting chance.

There was a time when combined operations were an American specialty. D-Day in Normandy was the most spectacular example; the Inchon landing in Korea more than six years ago later showed that our armed forces still had the knack. How was it possible then that, three decades later, the Pentagon's best and brightest could produce nothing better than a fiasco that made the U.S. military a laughingstock among friend and foe alike?

Confusion and lack of inter-service coordination plagued the rescue mission from the start. And from intelligence and Pentagon sources, I've learned more incredible details that make clear the commando raid had little chance of success. Indeed, the plan itself was so complicated, and the preliminary blunders so serious, that some bitter insiders suspect the raid was never intended to be more than a show -- a predictable failure designed to convince the public that Jimmy Carter was, after all, willing to "do something" -- or perhaps to demonstrate that military action would not work.

Here are some of the undisclosed details:

The Desert One site was a well-known CIA evacuation area, dating back to the days before the shah's overthrow. It was chosen despite the fact that a high-ranking Iranian security officer, who supervised development of the CIA site, was known to have been captured by the revolutionary authorities. The officer was arrested some time before the rescue attempt. Sources said that even if he did not tell what he knew under torture before he was executed, he had a document on him at the time of his arrest that pinpointed the Desert One landing site.

Preliminary traffic in and out of Desert One included two missions to resupply Iranian agents -- on Oct. 21, 1979, and March 4, 1980. A third mission -- to collect soil samples to determine whether the C130 transports could land safely -- occurred in the second week of March. The sample collectors landed in a light plane, which was immediately spotted by a smuggler's vehicle -- an ominous portent of the April raid.

If the mission had proceeded, as scheduled, from Desert One to a mountain hideout east of Tehran and then on to the actual rescue, the prospects were even more grim. The CIA estimated informally that there would be 60 percent losses among the hostages -- in other words, about 30 of them would die. This estimate was regarded as optimistic by CIA analysts.

The commandos were wearing black knit caps with strips of reflective tape on the inside. If they found they needed firepower from the gunships that would be circling Tehran, they were to signal by pulling off the caps inside out so the reflective tape would be visible to the helicopter crews.

Operational changes continued until the last minute. In fact, two days before pushoff, officers were still sketching optional versions of the plan in the sands of Oman.

The first C130 to land activated landing lights buried in the sand on a preliminary mission. It landed in spite of the fact that the pilots saw vehicles on the road below. So when the transports touched down, the commandos had to chase and fire at one of the vehicles -- a smuggler's truck -- and capture the other, a bus full of civilians.

The raiders' maps were pitifully inadequate. For example, soldiers on motorcycles were to post themselves at a sharp curve shown on the map. But there was no such curve, and the motorcyclists overshot their assigned location, creating confusion.

A list of Iranian undercover agents in Tehran was left behind in one of the helicopters. Only three of dozens succeeded in getting out of the country, using French and Belgian papers.

There was some reason to believe the helicopters would have been blown up if they had made it to the Tehran stadium chosen as an evacuation site. Though a CIA agent had traversed the stadium and found no land mines, some experts said the Iranians would be more likely to use radio-detonated mines than pressure-sensitive ones.

One source summed up the opinion of the intelligence community when he told my associate Dale Van Atta: "This was basically a bust-in-and-shoot-'em-up kind of plan. It's a good thing it failed so early -- and the reason the administration hasn't released any details is because it would have taken much more criticism than it got on just Desert One."