In the view of many US military officers who live and work in Central America , the United States is already at war here. When they talk about the continued fighting, about who will win or lose in El Salvador, the pronoun used is typically ''we'' and not ''they.'' And this is not merely a rhetorical slip by those close to combat in the region.
''Why should we let what America considers its greatest opponent - communism - get a foothold in the region, when we can nip it in the bud when it's still at the early stages of guerrilla warfare?'' asks Lt. Col. Donald M. McCay, commander of the US Army Jungle Operations Training Center at Fort Sherman in Panama.
At the same time, according to officers taking part in military exercises and those responsible for carrying out US policy in Latin America, the US is unlikely to send combat troops into Central America or find itself dragged into another Vietnam, as critics have charged.
''We're not going to get enmeshed in a long, drawn-out conflict here, and the United States is not going to use military force to do anything,'' insists one senior military officer assigned to the region. But he adds: ''If the people here can't solve the problem themselves, then the whole issue has to be rethought.''
There is a paradox here explained by the definition of ''war.'' In this instance, it means what strategists call ''low-intensity conflict.'' This is war that does not seem to threaten US soil directly or include massive exchanges of conventional firepower that could lead to nuclear strikes.
But it is also the type of war that the US increasingly could find itself fighting, yet is far from fully prepared to wage. It is also, by definition, the kind of conflict least likely to win public support, since it carries heavy political baggage.
''Security assistance'' funds to El Salvador and big US military exercises in Honduras and the Caribbean are the most obvious battlefields on which this war is being fought. But there are other, less well-known areas that are quietly and steadily growing:
* The number of students at the US Army School of the Americas (which trains officers, cadets, and senior enlisted men from Latin countries) has tripled under the Reagan administration. Overall activity here, measured in ''student weeks,'' has jumped fivefold.
* The US Army's Jungle Operations Training Center in Panama is ''getting bigger and bigger and bigger,'' according to the center's deputy commander, Maj. Fred Berger. With the Army's new push for light-infantry divisions, this school for jungle warfare, specializing in counter-insurgency tactics, is expected to continue growing.
Officers here say they believe the US had better beef up its counterinsurgency skills - which dwindled after Vietnam - in case US forces should be drawn into the fighting in Central America.
* There will be increasing numbers of military exercises in the region, particularly those like ''Kindle Liberty'' and ''Blue Blade'' with the armed forces of Panama, and ''Fuerzas Unidas,'' a series of naval exercises combining US and other Latin American fleets.
* The US Navy plans to double the number of students it can teach annually at its Small Craft Instruction and Technical Training School in Panama. It will also instruct Latin American students in offensive tactics in larger craft.
* The US armed forces for at least the next several years will continue to expand their military infrastructure here - roads, airfields, port facilities, medical units - as part of a plan to ''regionalize'' area defense. But, like the increases in jungle training and joint exercises, these facilities are also designed to prepare US combat forces to move quickly if, for example, Nicaragua were to attack its neighbors.
There is also increasing interest among US military officers in having Costa Rica, which has little military capability and declares itself neutral, expand its defense resources.
''I'd like to see Costa Rica have a stronger military,'' said one senior officer who directs US military activity in Central America. ''In modern times, it's very difficult to be absolutely neutral and depend on everybody's manners and morality to leave you alone.''
US military activities in Latin America are directed by the United States Southern Command in Panama. Perched 500 feet above the Panama Canal on Quarry Heights, ''Southcom'' headquarters is a deceptively low-key and unimpressive spot. Housed in modest wood frame buildings of World War II vintage, surrounded by palm trees and other jungle flora, Southcom has fewer than 400 permanently assigned staff members. But the military assets on which it can draw and the influence it wields are extensive.
The Southcom commander, Gen. Paul F. Gorman (who does not grant press interviews), is reputed to be a strong advocate of fighting the spread of communism in Latin America. Some congressional critics of Reagan-administration policy in Central America charge General Gorman with running an expanding field of increasingly aggressive US military activities in the region.
Spokesmen for the general in Panama and other military officials deny this. But there is no doubt that the US military presence here is formidable, even when the growing number of exercises and the militarily valuable infrastructure that will remain after the exercises are discounted.
In Panama alone there are some 9,300 US military personnel permanently stationed. This includes the 193rd Infantry Brigade, about 7,000 soldiers under the operational control of General Gorman. The brigade is made up of three infantry battalions, a special forces (''Green Beret'') battalion, a military intelligence group, the jungle training center, and various support groups.
The US Naval Station, headquartered at Fort Amador, includes about 400 personnel equipped with harbor and river patrol boats. Howard Air Force Base houses the US Air Force Southern Air Division in Panama. Some 1,900 airmen fly and maintain C-130, A-7, and other aircraft.
The Southern Command oversees all US military grants and foreign military sales in Latin America. It is also in charge of 16 military groups, which are uniformed advisers assigned to US embassies in the region. Southcom also deploys about 40 mobile training teams and 20 technical assistance teams throughout Latin America.
The principal task of US forces here is supposed to be defense of the Panama Canal. But this role has been overshadowed by the growing conflict throughout Central America and the active part the forces under Southcom's operational control are taking in this ''low-intensity conflict.''
''We are engaged in a very real war today . . . best characterized as the employment of military capabilities rather than military force,'' Southcom briefers tell visitors. Sometimes, however, the distinction can be hard to determine.
The Air Force C-130s flying out of Panama, for example, have reportedly been used recently to provide intelligence for the Salvadorean Army in its fight against leftist guerrillas. This new development is not denied by US officials.
''Information is the best thing we can give these countries,'' says one officer. ''Even if you give them steel helmets and flak vests and advanced weapons, if they don't have the information, they might as well stay in garrison.''
In private conversation, some US officers are more upbeat about the state of the Salvadorean Army and less concerned about a Cuban-backed guerrilla offensive next fall than President Reagan indicated in his recent speech. They stress improvements in the US-supported Salvadorean Army and note that the guerrillas have traditionally mounted some kind of offensive in the fall months.
But they are far from sanguine about US interests being preserved in the region without continuing military assistance. And, again, there is this sense that the US is an active participant in the battle and not just a supporter of one side.
''If it keeps going the way it is, we're going to lose the war,'' one officer warned.
Beyond El Salvador, officers here note that under the Panama Canal Treaty, all US forces are supposed to leave Panama by the end of 1999. Relations with Panama remain good despite the current post-election tension.
But the military buildup in Honduras and elsewhere as well as the growing regionalization of security efforts in Central America can also be seen as a hedge against the time when the US Southern Command may no longer peer down from its redoubt at Quarry Heights.
Next Effect of the Panama Canal treaty on U.S. role.