Hundreds of U.S. soldiers are to be deployed during the next two months to this exposed corner where southwestern Honduras touches the borders of both Nicaragua and El Salvador, in the initial phases of the largest and most controversial joint U.S.-Honduran military exercises ever conducted.

Less than 20 miles west of here is one of the most fought-over sections of neighboring El Salvador. The border crossing itself, El Amatillo, was destroyed by the attack of several hundred Salvadoran guerrillas on April 29.

Due east is the part of northern Nicaragua where U.S.-backed anti-Sandinista guerrillas are most active. Due south is the Nicaraguan province of Chinandega and the port of Corinto, some of the Sandinistas' most productive and strategically vital territory.

"The Devil's Backside" is what some Hondurans call this hot, flat stretch of land where the air hangs stifling and still, trapped between mountains and the little Gulf of Fonseca.

Military planners in the capital talk of it as the most obvious path for an overland invasion force by either Honduras or Nicaragua.

"If you read a map you can see it is the one avenue of approach," said one foreign military adviser in Tegucigalpa, the Honduran capital.

This small chunk of land, moreover, is the one place the Hondurans say they fear Nicaragua's armored units. The Sandinistas are estimated to have more than 50 heavy Soviet-made tanks. The Hondurans have 13 very light British tanks, all of which are based in the town of Choluteca.

In a press conference last week, President Reagan said the joint maneuvers in this country "are not going to put Americans in any reasonable proximity to the border" of Nicaragua. A military briefing officer in the Honduran capital said, "Every precaution is going to be taken to stay at sufficient distance from the border areas and so as to ensure the safety of the personnel and to preclude any incident happening."

But he declined to specify what constitutes a "sufficient distance" in this highly volatile area. Training being conducted by U.S. Green Berets working here with the Honduran 11th Battalion in a program separate from the upcoming exercises say they generally stay about seven miles from either border when they lead their Honduran trainees through nearby mountains reportedly used as clandestine supply routes by the Salvadoran rebels.

"We go on patrols with them the Hondurans all the time. That's the only way you can train them," explained one adviser based in the capital. "But it's been judged not to be a combat mission with the goal to find" arms smugglers.

"Of course you never know when you're going to bump into somebody," the adviser said, adding that thus far none of the U.S. trainers in San Lorenzo or Choluteca have been present at the relatively infrequent capture of suspected gunrunners.

The question of what constitutes U.S. "combat" troops is another semantic fine point with regard to the upcoming exercises.

The U.S. soldiers destined to operate in this particular area during the maneuvers will include U.S. Army engineers, helicopter pilots and several teams of Special Forces, or Green Berets. The briefer declined to specify the number of U.S. troops to be assigned what are primarily training tasks here during the maneuvers, but indicated that it would be in the hundreds and "probably less than a thousand."

In another phase of the maneuvers scheduled for November near an isolated new Regional Military Training Center at Puerto Castilla on the north coast, thousands of U.S. Marines are supposed to storm ashore in an amphibious landing.

But except for the Marines, who are only supposed to be on Honduran soil for a few days, "there are no U.S. infantry or armor troops of the kind that close in on , whip and kill or capture the enemy," said the briefer. "So technically the president can say he hasn't deployed any combat troops in Honduras."

As outlined by military sources in Tegucigalpa familiar with the overall planning of the maneuvers, they are intended to help upgrade a Honduran Army that is particularly "very, very thin" on leadership and the technical expertise needed to carry out large-scale maneuvers.

The average Honduran soldier, moreover, is poorly educated and often has been forcibly recruited in roundups at movie theaters and other public places.

In some cases, according to military sources, mortar crews must be trained from men who do not know how to count. Some junior officers cannot read maps and some sergeants cannot use a compass, these sources said.

As tentatively scheduled, the joint maneuvers to be known as Ahuas Tara II ("Big Pine II") will begin gradually during the next month.

The smaller Ahuas Tara I exercises in early February were conducted in the remote section of eastern Honduras known as "The Mosquitia," near Puerto Lempira. U.S. and Honduran military officials said at the time that this was partly because such operations would disrupt civilian life if they were held in an area as relatively populous as this.

But Honduran officers have said consistently that it is this region of southwestern Honduras where they feel most vulnerable and most in need of support and training.

According to one official briefer connected with the exercises, the Army engineers here will be building an airstrip capable of handling U.S. C130 transport planes and instructing Honduran units in the engineering aspects of antitank warfare.

Several Honduran battalions will be rotated through this region during September and October, the briefer said in order to receive antiarmor, counterinsurgency and arms interdiction instruction.

Operations in the north of the country will include maneuvers by a U.S. artillery battalion providing "fire support coordination" with towed 105-mm howitzers in September.

Asked how this would benefit the Honduran Army, which he said has no howitzers, the briefing officer said it may be getting some in the future. The high point of U.S. presence on the ground is supposed to come with the amphibious landing at Puerto Castilla in November. At that point about 4,000 American troops will be in Honduras, the briefer said.

Asked how this would benefit the Hondurans, who have no amphibious capability, the briefing officer said it would help them to develop one and to learn how to defend against such an invasion.

"They feel they have a need to do this," the briefer said of the Honduran commanders.

The military climax of the operation is supposed to be a brigade-size "field training exercise" in northeastern Honduras tentatively slated for December or January.

A U.S. aviation unit is supposed to provide "transport, logistics, support and air mobile training" during these activities. Its aircraft will include CH47 Chinook transport helicopters and the sleek, new $8 million Blackhawks now used as utility and attack helicopters by the U.S. Army.

A new radar station to be built near here, probably on Tiger Island in the Gulf of Fonseca, is supposed to help coordinate these flights. There will also be Air Force exercises in an as yet unspecified area to teach "forward air controlling" and "fire support."