Vieques Island, Puerto Rico — As Capt. August Fucci peers down through high-powered binoculars and barks directions into a portable radio, the plain below is taking a horrific pounding. A flight of four Air Force F-16 Falcon jets scream toward the ground, pilots concentrating fiercely on bombsights. Two Cobra helicopter gunships take turns peppering targets with 2.75-inch rockets and cannons. M-551 Sheridan tanks are blasting away, along with the 81-millimeter and 107-millimeter mortars and the 155-millimeter howitzers.
Every so often, the husky, brutish roar of the 20-millimeter Vulcan Gatling guns up the hill can be heard ripping above the thump and crash of the other weapons. Fire-breathing dragons, they spit out knee-deep piles of brass casings.
Captain Fucci, a short man with sleeves rolled up his thick arms and helmet chin strap tightly cinched, is fire-support officer for the 2nd Brigade, 82nd Airborne Division, United States Army. It's his job to direct his unit's arsenal against the enemy below. His opponent, he says, is ''a guerrilla-type force coming at us with weapons from another country.''
In this case, the enemy was fictitious and the fireworks were part of a ''live-fire exercise'' winding up the recent Ocean Venture '84 military maneuvers in the Caribbean.
But there was no doubt who the enemy was meant to be, or where the ''weapons from another country'' had come from. Nor, for all the typical soldierly kidding , was there any question of the earnestness with which this exercise was being conducted.
The United States is flexing its military muscles throughout the Caribbean and Central America these days, holding an unprecedented series of military exercises designed to test US capabilities and demonstrate Uncle Sam's resolve to those - Nicaragua, Cuba, the Soviet Union - who might have other plans for the region.
Aside from the geopolitical elbowing, the exercises also give US armed forces a chance to hone jungle-fighting and counterinsurgency skills that had slipped in Pentagon priorities after Vietnam. ''The benefits are really tremendous for our forces, especially for our readiness,'' enthuses one officer charged with carrying out US military policy in Latin America.
Does this mean the US military is spoiling for a fight in Central America, as some critics charge? Or that the colonels and generals are preparing to fight the last war - another Vietnam - not remembering how militarily difficult and politically sticky counterinsurgency can be?
''You fight the way you train,'' explains another senior officer in the region. Then, seeking to clarify what could be read as a prediction that the US inevitably will be at war there, he adds: ''Or you ought to train the way you're going to fight.''
What strategists call a ''low-intensity conflict,'' this veteran of air combat in Vietnam explains, is more likely these days than a massive conventional or nuclear war. Many civilian analysts and military reformers agree that the US needs more preparation here.
Ocean Venture '84 was, in many ways, a replay of the US invasion of Grenada. It included an amphibious assault, paratroopers, the ''rescue'' of 100 embassy personnel and other civilians, and close coordination among Army, Air Force, Navy, and Marine Corps. It was a reflection of growing concern about the Caribbean, where there are two strategically important choke points through which much US military force and supplies would have to transit in wartime and through which much civilian trade passes.
''We lose control of this area, and the people in Chicago are going to eat a different supper at night,'' warned Lt. Comdr. William D. Barron, a spokesman for US forces in the Caribbean. ''It's important that people realize this.''
But the exercise was also a preview of how well the US military might perform in those parts of Latin America where the threat of war is not merely theoretical. As Commander Barron said, ''An amphibious assault is an amphibious assault . . . the techniques are pretty much the same.''
Critics of the US operation in Grenada say that because every branch of the service wanted a piece of that rather limited action, communications problems and intelligence gaps occurred that may have prolonged the engagement and even cost lives. Officers here strongly rejected this analysis.
''We plan on fighting like we train,'' said Col. Stephen J. Silvasy Jr., commander of the 2nd Brigade, 82nd Airborne Division, as he stood near a rough jungle airfield on the Vieques Island military reservation during Ocean Venture '84.
''The days are over when the Marines or the Army can do it on their own,'' said Colonel Silvasy, who parachuted with the first US troops into Grenada. ''The only way we're going to win is in a joint environment.''
''It's the only way to do it,'' agreed Col. William W. Bahnmaier, commander of the 26th Marine Amphibious Unit, the other major ground force taking part in the 16-day exercise.
To make the war game as realistic as possible, some 600 marines from a California-based reserve unit (many of whom spoke Spanish) took the part of the guerrilla force resisting the US invasion. Referees with white tape on their helmets and shirt sleeves sorted out the ''casualties'' and winners in various encounters. Since the guerrillas were outnumbered 10 to 1 in ground troops (and nearly 50 to 1 when Air Force, Navy, and Coast Guard personnel were added in), there wasn't much doubt about who would ''win'' when the exercise was over.
''Psy-ops'' (psychological warfare) units practiced their interrogation methods, riflemen and tankers ''buttoned up'' against mock chemical attack, and military lawyers in jungle fatigues instead of three-piece suits sorted through ''atrocities'' on both sides. Not far off the coast of Vieques Island snooped a real-life Soviet intelligence ship that, in turn, was shadowed by a US destroyer. Helicopters hustled back and forth from the carrier Iwo Jima.
There were also the foul-ups the military expected on maneuvers involving 32, 500 men, 392 aircraft, and 37 ships: a Marine tank that threw a tread in the rough jungle, the Vulcan gun that got a ''hot'' round jammed in one of its chambers, a limping paratrooper with his foot in a camouflaged cast.
As with any military operation, there was also a lot of ''hurry up and wait, '' as troops lounged in the tropic sun, eating meals from plastic pouches or grabbing a quick swim at ''Blue Beach.'' Most, at least according to their senior officers, didn't give much thought to the controversial nature of US military activities in Latin America.
''They're not concerned about the politics involved,'' said Colonel Silvasy. ''They're thinking about chow and mail . . . about honing those basic soldier skills.'' As he waited in line for breakfast, however, an enlisted marine indicated otherwise. His family ''had given him the cold shoulder,'' he said, when they heard he was flying to Puerto Rico to help demonstrate US resolve in the region.
But in the view of those in charge of US military policy here, the institutional memory from Vietnam and even Grenada is fast fading. And such exercises are the key to preparedness, they say, particularly in a tropical setting that can't be matched in the United States without ruffling local political feathers or rankling localenvironmentalists.
Ocean Venture '84 was the third annual exercise of its type in the Caribbean, and more are expected to follow. Pentagon officials last week reported to Congress their plans to conduct military exercises in Central America through 1988.
''US forces are supposed to maintain the capability to operate around the world,'' says a senior officer. ''For this, the games are fantastic.''