Sometime this week, the 31-piece band of the New Mexico Army National Guard will unpack its tubas, trombones and piccolos in the Honduran jungle and begin entertaining American troops in Central America.

The band's fortnight sojourn, its first deployment abroad, is a small symptom of what has become a sustained and institutionalized U.S. military and intelligence presence in Honduras. Since the summer of 1983, there have rarely been fewer than 1,000 U.S. troops in that mountainous nation the size of Ohio, while tens of thousands more have cycled through in an unbroken series of exercises.

Intelligence gathered by U.S. planes, drones and sophisticated sensors in Honduras now flows almost instantaneously to Washington, the U.S. Southern Command in Panama, El Salvador's army and, with some restrictions, the contra rebels battling Nicaragua, according to administration officials and congressional testimony.

While Congress has authorized no bases for Honduras, the administration recently revealed ambitious plans stretching through 1991 to build, among other things, ammunition caches, barracks and outdoor racquetball courts.

The buildup has been accomplished without fanfare or, at times, total candor. If one of the Army's remotely piloted reconnaissance planes is shot down over hostile territory, for example, military spokesmen are under orders simply to announce that the drone "malfunctioned, went out of control and crashed" unless asked specifically about hostile fire, according to internal documents.

None of this suggests that U.S. troops are about to enter combat, either against leftist Salvadoran rebels on one side of Honduras or the leftist Sandinista government of Nicaragua on the other. Even the most hawkish senior administration and military officials say they remain convinced that such a direct U.S. role would be a mistake.

One such official said that he has in fact helped "hold the reins" on grander schemes for U.S. facilities in Honduras because a larger U.S. presence would be "defeatist" -- an admission that the Sandinistas are unlikely to be toppled soon.

But during the past three years, the infrastructure to support such a role has been put in place. Perhaps more important, the infrastructure now exists for the United States to play a major supporting role for both the Salvadoran army and the CIA-backed Nicaraguan contra rebels, U.S. and congressional officials said.

"SouthCom now has in its tactical intelligence center the capability to monitor the war and to run the war in Central America," said one source who recently visited the region, referring to the U.S. Southern Command. "Things are being routinized."

Perhaps nothing symbolizes that development more clearly than the transition from Gen. Paul F. Gorman, the burly and self-confident Army general who reigned from Panama until last year, to Gen. John S. Galvin, the low-key new Southern Command chief.

"Gorman was the innovator. He brought intelligence capabilities and intelligence platforms into Central America like nobody could believe," said one source who knows both generals. "Galvin is the consolidator making things work."

Shortly before his tour of duty ended, for example, Gorman, who developed a reputation as the "viceroy" of Central America, obtained a used jet as big as President Reagan's Air Force One for his command. Gorman's friend Galvin, soft-spoken and silver-haired, now can use the jet to hop from one Latin American country to another.

Galvin said in a recent brief interview that Gorman's viceroy image was a "bum rap," and he praised Gorman's accomplishments. "I'm reaping the benefits," Galvin said.

But he also said that, as senior military man in the region, he now takes pains to show leaders of other nations that the State Department makes policy -- "State leads, and Defense follows up," he told a conference at the National Defense University. Unlike Gorman, Galvin is fluent in Spanish, and he is said to have developed cordial relations with leaders in the area.

Certainly, the U.S. and Salvadoran military are working closely. U.S. intelligence on Salvadoran guerrillas, one knowledgeable source said, now is sent from Honduras to Washington for processing and then back to El Salvador, where U.S. Army trainers assigned to Salvadoran brigade headquarters receive it in what the military calls "real time" -- quickly enough to be operationally useful.

Several administration officials, without discussing methods, agreed that the Salvadorans receive real time intelligence and act on it to keep the rebels on the run. "Intelligence sharing is a key element of our assistance in Central America," Defense Secretary Caspar W. Weinberger said in a recent report to Congress.

A congressional aide suggested that such direct involvement should trigger the war powers act, which requires congressional approval when U.S. troops adopt a "coordinating" role in combat. But an administration official said "we've had 375 lawyers" look at that and have concluded the assistance is legal.

U.S. intelligence has been instrumental for the contras, too, according to former contra leader Edgar Chamorro. Chamorro, who now opposes CIA efforts in Nicaragua, described the assistance in an affidavit submitted on behalf of the Nicaraguan government to the International Court of Justice last fall.

"The CIA, working with United States military personnel, operated various electronic interception stations in Honduras for the purpose of intercepting radio and telephonic communications among Nicaraguan government military units," he testified. "By means of these interception activities, and by breaking the Nicaraguan government codes, the CIA was able to determine -- and to advise us of -- the precise locations of all Nicaraguan government military units."

When Congress approved $27 million in "nonlethal" aid for the contras last fall, it also granted a classified amount -- raising the total to more than $30 million -- to expand the U.S. role in giving "information and advice" and communications equipment and training. But Congress prohibited U.S. participation in "planning or execution" of operations, and senior officials said they do far less for the contras than for the Salvadorans.

Much of the U.S. infrastructure entered Honduras during military maneuvers and then was left behind, according to Chamorro and others. Airstrips that the contras now use were built or improved, radars were installed.

The extended maneuvers were a Gorman innovation, unknown in the rest of the world. Most U.S. training exercises last a week or two, but soldiers on maneuvers in Honduras spend months -- building roads and bridges, practicing propaganda tactics and other "psychological operations" and hunting for arms traffickers, storming beaches and parachuting into airfields, digging wells and vaccinating children.

Even when no exercises are officially taking place, U.S. platoons fly in and out of Honduras on "emergency deployment readiness" drills. And Joint Task Force Bravo, U.S. headquarters in Honduras, maintains between 800 and 1,600 troops at Palmerola air base and around the country at all times.

Some critics, such as Gov. Bruce E. Babbitt of Arizona, have said that the drills are provocative and could involve U.S. troops in combat even if policy-makers seek to avoid that. The critics point to incidents such as a U.S. helicopter that was shot down on the Nicaraguan border two years ago; four Marines killed in San Salvador; and sporadic sniping episodes involving U.S. advisers who have been shot at in the Salvadoran countryside.

But Pentagon officials describe those incidents as aberrations, and the administration disclaims any intention of provoking Nicaragua to shoot at U.S. soldiers. The "political and social reactions throughout the western hemisphere would be terrible," one senior official said, if U.S. forces took on Nicaragua.

At the same time, that official described the Sandinistas as a force for evil in the region, the chief target of the U.S. buildup in Honduras and the administration's largest remaining challenge -- one that is not likely to be resolved peaceably. As a result, the administration will press Congress and other nations to give the contras more aid.

"It's important that it be enough so that it can change the dynamic in Nicaragua," the official said. "Pinpricks don't make sense . . . . It's not a matter of puttering around and keeping the Sandinistas off balance, with a settlement down the road. A Contadora regionally negotiated settlement is very unlikely."

Meanwhile, the exercises and construction continue. And Col. Tom Tays, chief of staff of New Mexico's National Guard, said his troops are "excited and very proud" to take part in the General Terencio Sierra maneuver.

The band will not only entertain U.S. troops and Honduran dignitaries, a New Mexico spokeswoman said, but will also offer a "musical clinic" to Honduran civilians.

"We keep our instruments ready at all times," Tays said last week. "We're ready to go."