IT IS OBVIOUS by now that the United States is caught in a deepening morass in Central America. Every week brings more bad news: political and military reverses for the groups our government is backing in the region; further evidence that many elements of Central America's security forces are brutal and corrupt; desperately incompetent attempts by the Reagan administration to influence public opinion; and a widening gap between what is at stake in Central America and the means available to advance U.S. interests.

Is there any way out of this disturbing mess? To answer that question, we need to focus on the range of choice left to the United States, to define which results would be acceptable and achievable -- and which would be dangerous and unacceptable.

The Reagan administration's approach to Central America is based primarily on exaggerated fears and unrealistic aims. It derives more from a desire to display national strength than from a sense of national self- confidence. And it stems more from projecting extraneous concerns upon Central America than from assessing the region's own realities and significance.

The welter of contradictory statements emerging from various parts of the administration suggests that the U.S. government thinks of Central America as a stack of dominos, being tipped primarily by external (Cuban and Soviet) pressure. The administration believes that a revolutionary triumph in El Salvador would lead almost ineluctably to leftist victories in the other Central American nations. The possibility of several insurgent victories is thought ultimately to threaten oil-rich Mexico (and problems for Mexico, it is said, would inundate the United States with refugees). More immediately, the administration apparently fears that leftist regimes in Central America would jeopardize U.S. security and other interests: maritime routes, the Panama Canal, and other assets.

Taken together, those interests are regarded as "vital" -- more so, according to Secretary of State Alexander Haig, than those for which so many tens of thousands died in Vietnam. To protect these interests -- and to preserve U.S. influence and prestige -- the Reagan administration provides military training and equipment, sends advisers, steps up economic assistance, considers (and probably undertakes) covert paramilitary intervention, refuses to rule out direct military involvement, and eschews negotiations that might involve redistributing power.

The Reagan administration's fears are largely unfounded, its concept of what is at stake in Central America is unwarranted, its assessment of Central America's dynamics is inadequate, and its chosen instruments are ill-chosen to achieve even those goals that are attainable.

The realities are as follows:

* The insurgencies in Central America are primarily indigenous, not inspired or controlled by the Soviets or the Cubans. Cuba is no doubt supporting the guerrillas, and a panoply of other external actors is involved: the United States and the Soviet Union, Israel and the PLO, Argentina and Venezuela, the European Social Democrats and the Christian Democrats, the human rights activists and the anti-communist international movement. But none of these actors is as important as what is happening internally. Whatever their source, guns do not fire themselves.

* Each Central American country is different. Events in any one nation surely will affect trends in another, sometimes importantly, but internal conditions in each country are crucial. Costa Rica's future, for example, will be more affected by its own economic quandary than by El Salvador's civil war.

* The probable effect on Mexico of a region-wide turn to the left in Central America would be to strengthen conservative forces. The most likely way to increase the chances of a left-nationalist anti-U.S. movement in Mexico would be to undertake U.S. military action or detectable paramilitary intervention in Central America.

* The real threat to U.S. security, even from region-wide leftist victories, is strictly limited. The one clear imperative is to prevent a strategic threat from being introduced in Central America. Strategic weapons have been kept out of Cuba for 20 years by agreement between the United States and the Soviet Union. There is no reason to believe that it would be easier for the Soviet Union to introduce such highly provocative (indeed, unacceptable) weapons into the vulnerable nations of Central America than into its close and consolidated ally in Cuba.

* U.S. economic interests in Central America are scant. No other significant and tangible U.S. interests are engaged, except insofar as Central American migration expands and creates a new set of U.S. interests. Central America's future need not be any more "vital" for the United States than we make it. The more the administration escalates its rhetoric, the more it creates a self-fulfilling prophecy.

The central issue for the Reagan administration is its perception that "losing" a confrontation in Central America will further undermine U.S. influence elsewhere and -- not incidentally -- weaken the administration's own political standing. To point out that this wound would be largely self-inflicted -- the administration chose, after all, to "draw a line" in El Salvador, perhaps because Sovet expansionism is easier to "stop" where it is not occurring than where it is -- does not solve the problem. The way to reduce the troubling prospect that a setback in Central America will weaken the United States elsewhere, however, is not by forcing an unwinnable confrontation, but by seeking a diplomatic solution.

The Reagan administration appears to desire in Central America a cluster of friendly, stable nations which hold honest elections and respect human rights, welcome U.S. private investment, and support Washington internationally. In short, the Reagan administration wants congenial, prosperous neighbors.

That goal is not realistic. No matter what the U.S. government does (or does not do), Central America in the next several years will be unstable, economically distressed, strife- torn, and unable to build and sustain effective political institutions. U.S. investors will not flock to Central America. Some of the region's most stable countries may turn out to be the most independent of the United States. Hegemony will not be easily reimposed.

What, then, should be done? How can the United States reverse the drift toward disaster in Central America?

The first step is to focus on how to avoid the worst.

The worst that could possibly happen in Central America would be to blunder into a global war with the Soviet Union. That ultimate tragedy is improbable, but the chain of consequences leading from a possible U.S. invasion of Cuba all the way to Armageddon is not harder to imagine than the process which led from the assassination of an Austrian archduke to World War I.

The United States should immediately take steps, therefore, to reassure the Soviet Union that our government will continue to respect Cuba's territorial integrity, as we expect full adherence by the Soviet Union to other aspects of the 1962 agreements which settled the missile crisis.

Apart from world war, the worst reasonably feared course in Central America would be prolonged U.S. military involvement in Central America's civil turmoil. U.S. military intervention would strengthen anti-American forces throughout Central America, Latin America and the whole Third World, fuel major conflicts with key allies, fracture consensus within the United States, and distract attention from more serious national and international problems.

A quick, decisive and "successful" U.S. military intervention is simply not in the cards. Even if it were, it would be a course to avoid. U.S. military intervention would blur the important distinction between the international role of the United States and that of the Soviet Union, and would perpetuate precisely the forms of international behavior we are trying to end. Ultimately, too, the regime imposed or reinforced by U.S. intervention would likely be forced out by intransigently anti-U.S. forces, enraged by the interventionist role of the United States.

The way to avoid all these profoundly undesirable results is by making it crystal clear that the United States will not undertake military or paramilitary intervention in Central America. The administration should voluntarily and formally clarify that force will only be contemplated to remove a clear and present security threat, and then only under multilateral auspices of the Organization of American States. If the administration persists in keeping its (rhetorical) options open, Congress should impose further restrictions on the executive.

Another result the United States should be trying to avert is the decisive military victory of anti-U.S. leftist forces over those with whom we have been aligned in Central America. The more clearly military a victory of the insurgents, the less influence the United States and other forces for moderation will have after a leftist triumph -- and the greater the demonstration effect elsewhere in the region. It would be in the interest of the United States, therefore, to move the forum of confrontation in El Salvador from the battlefield to the negotiation table -- an arena where the economic strength and political influence of Mexico, Venezuela, and the United States will be more relevant.

The most likely outcome in Central America in the intermediate term is protracted and expanded civil war. The United States should do all it can to help avoid this: by supporting international efforts at mediation, by working with all relevant parties to curtail arms flows from ourselves and others to the region, by supporting economic development programs in countries (like Costa Rica) where viable polities are threatened, and by trying to support moderate groups in each country. All these courses should be pursued, but they may very well not be enough to coopt the insurgents.

It may be, therefore, that the best we can realistically hope for in Central America is the establishment in the next several years of independent, nationalist, left-leaning, even Marxist-Leninist regimes. Some of these regimes, like the current Sandinista junta in Nicaragua, may well be friendly to Cuba and to the Soviet Union, especially if they come to power with Cuban help and against the will of the United States.

If U.S. policy permits, however, all these nations should still be inserted firmly within the international capitalist economy. They will still be dependent on trade, finance, techology and investment from the United States, and they will be accessible to U.S. influence. They will still be interested in U.S. economic assistance, which is likely to be far more effective as a constructive influence on behavior than predictably counterproductive threats.

Not even this result will be easily achieved. If the administration continues to "draw lines," to threaten, to reject negotiations, to undermine international efforts at mediation, and to grasp at straws to justify its stance, it will leave itself no choices but humiliation or intervention.

One can hope, however, that today's elections in El Salvador, however they turn out, will give the Reagan administration a chance to change its course.

The way to do so is clear: to rule out U.S. military intervention; to seek renewed multilateral backing to prohibit the introduction of extra-hemispheric military bases in the region; to support fully Mexico's effort to negotiate a political solution, and genuinely to accept profound changes in Central America, even when they diminish immediate U.S. influence. There is no other way out.