BY NOW YOU may have the sense that we are sliding into war in Central America. Or perhaps you have concluded that the president really doesn't know what he wants.

You would be at least half wrong. We may indeed be moving toward a military involvement in that part of the world, but not because Ronald Reagan does not know what he wants.

He wants the rebels in El Salvador to stop being radicals and guerrillas. He wants the Marxist revolutionaries in Managua to stop being Marxist revolutionaries. And he wants their mentor, Fidel Castro, to be frightened by the show of U.S. military might that he has ordered for Central America.

The problem is that the president does not want to say what he would have to say, or do what he would have to do, to achieve those results.

He is afraid of unsettling the American public, and still more the Congress while it is voting on money for military and economic aid and covert activities for Central America. But unless he can provide a clear sense of what he wants to achieve, and at what ultimate costs, he risks losing the support of the American military. Already the generals are giving signs of reservations about a U.S. military involvement in Central America that isn't backed up by the perception that the U.S. commitment could be unlimited and is accepted by the American public.

At the same time, the president faces another dilemma. He cannot accomplish his evident purpose of scaring the Russians, Cubans and Nicaraguans while simultaneously soothing American voters. That didn't work during the Vietnam war, and there is little reason to believe it would work in Central America's war.

Once you are clear about this background, the contradictions -- and confusion -- that seem to stand out in so many of the president's statements are understandable.

Over the last few weeks and months, the president has described what is happening in El Salvador as "the first real communist aggression on the American mainland." He has said, "There is a war in Central America that is being fueled by the Soviets and Cubans. They are arming, training, supplying and encouraging a war to subjugate (El Salvador) to communism. . . . The Soviets and the Cubans are operating from a base called Nicaragua."

But this is the same president who said at a televised press conference Tuesday night that the public was paying "entirely too much attention to the efforts that we're making to provide (a) security shield." He went on to say that he had "no desire" to use combat troops -- and no plans to do so. The United States is "not seeking a larger presence in that region" -- so Americans should not worry about the largest joint land-and-sea training exercises in Central American history.

So Ronald Reagan, the hard-eyed cold warrior, is at war with Ronald Reagan, the wide- eyed and innocent presidential politician. He isn't even sure the maneuvers will last six months, as widely reported. The commander- in-chief doesn't know how many ships are involved, he says. No one has told him about a widely reported proposal to more than double the force of 55 military advisers in El Salvador. He has never heard of another proposal, also widely reported, for a fourfold increase in military and economic aid to the region. Do the joint military exercises in Honduras suggest that U.S. troops would help the Hondurans resist an attack from Nicaragua? "We haven't considered that," was his reply.

And yet a president seemingly well-informed about details told the longshoremen's convention two weeks ago that Cuba had sent one of its best combat generals to Nicaragua, that more Cuban soldiers and Soviet supplies had arrived as well, and that "this cannot be allowed to continue."

That kind of tough talk, common sense would tell you, should be followed up by a drastic American response. But when pressed in public, as at his news conference, the president offers no convincing one. Instead, he begs off talking about covert aid to the Nicaraguan contras on the ground that "if you discuss covert aid, it's no longer covert" -- at a moment when Congress is already debating that issue publicly and he is in the thick of the debate.

So all he is really offering the public is a Nixonesque "plan to end the war" and a Carteresque pitch to "trust me."

"I can only tell you that we're continuing on a policy that we believe is aimed at, first of all, bringing about peace in El Salvador, hopefully through negotiations with those who are presently radicals and fighting as guerrillas," he declared. "And in Nicaragua, (we are) hoping that we can persuade the Nicaraguan. . . government to return to the principles of the revolution and which they in writing guaranteed to the Organization of American States."

From a president who sees in the Central America war a direct threat to the national security, that has to be seen as a cop-out. In effect, he is asking hard-bitten revolutionaries who have fought their way into power, or are in the process of doing so, to become his kind of guys. He can't be serious.

The president's shell game, alas, is all too familiar. It is the same one that Lyndon Johnson played when he was promising that American boys should not be fighting wars that Asian boys ought to fight and then discovering that they would have to.

The president insists "there is no comparison with Vietnam and there's not going to be anything of that kind in this." He adds soothingly: "Maybe the people are disturbed because of the confused pattern that has been presented to them and the constant drumbeat with regard to the fact of suspicion that somehow there is an ulterior purpose in this."

He's right about the confused pattern -- and never mind his own contribution to it. But what is the point of a show of force of the magnitude of this one if it does not have an ulterior purpose? That is what limited war is all about -- whether we are talking about the first bombing of North Vietnam, or the first landing of combat troops in South Vietnam, or the dispatch of the battleship New Jersey to the Central American coast. It is signal- sending, a practice that is defended at home as a modest gesture but is supposedly also loaded with a message to chasten adversaries.

Recent U.S. history does not recommend it. One can think of only two American shows of military force, with no bloodshed, that worked in our favor. One was the landing of U.S. troops in Lebanon in 1958 under circumstances that enabled them to serve as a useful buffer while diplomacy played out. The same may be said for the U.S. military intervention in the Dominican Republic in 1965, under conditions which, by comparison with today's Central American turmoil, were very nearly ideal; the combatants were almost eager to draw back while American diplomats did their thing.

But the savagery and complexity of today's Central American bloodletting does not lend itself to neat and surgical military intervention. The model now, rightly or wrongly, is Vietnam, and nobody has better reason to understand that than the veterans of Vietnam who are now today's military leaders.

There's no easy way to measure the depth or breadth of the current crisis of confidence among the top men in uniform. But it says quite a lot about the Pentagon problem confronting the Reagan administration when the recently retired army chief of staff, Gen. Edward C. Meyer, raises serious questions about the foundations of the administration's policies in Central America.

"One of the great lessons of Korea and one of the great lessons of Vietnam is that you cannot win a war until you challenge (the enemy's) heart and soul," Meyer said in a recent interview.

How? Not necessarily with shot and shell, but with a compelling demonstration of the American public's willingness to go that far.

"I think that (they) have to clearly perceive thate that is not ruled out," he said.

"I believe that up front there has to be a face-to-face discussion between the president and the secretary of state, the secretary of defense, and the military as to what the hell they want us to do," he went on to say. Meyer, in a striking departure from past military practice, sees no obligation for the military to crank up contingency plans for Central America in the absence of any clear sense of a civilian "rationale."

"To come up with a what-if plan without having clear political direction as to what its intent was, would be reverting to the old days when the military designed the national security strategy and the forces to support it," he has said.

Ronald Reagan would have us believe that the military is not expected to produce much more than it is producing. "All that our neighbors ask for is the tools to do the job themselves," he has said. But this assumes that the tools will do the job -- which is to say that gunboat maneuvers, military aid and covert subversion will be met with no response -- that the communist "aggressors" will simply give up.

This hardly squares with the record of American "graduated response" to enemy buildups in Vietnam, and still less does it square with Ronald Reagan's own definition of the magnitude of the Soviet-Cuban-Nicaraguan threat to this hemisphere. Another top-ranking American military man, Gen. Wallace H. Nutting, who recently ended a four-year tour as head of the U.S. Southern Command with responsibility for the Central American area, shares Reagan's view that there's a war on in Central America and that the United States "is engaged in that war." But he does not share the president's queasiness about openly discussing the nature of that war.

The administration has squarely committed itself to prevent a Marxist takeover in El Salvador, he believes. But "our people as a whole have not followed up that commitment with a willingness to take those steps necessary to bring that about," he also remarks.

"As long as those limits on our willingness to engage in the ultimate resolution of the problem are evident to the guerrillas, they will persist," he said. "They have the example of Vietnam to refer back to."

This leaves a question, as it did in Vietnam, of whether the generals and the admirals, when push comes to shove, will challenge their commander-in-chief out loud. Lyndon Johnson knew how to line up the brass and get them to salute.

But Ronald Reagan is not Lyndon Johnson, and we have been through Vietnam. Not only Nutting, but apparently a number of high-ranking Pentagon officers do not want to be entrapped once again in a conflict for which there is no solid American public support. Without a national consensus, they reason, the struggle will drag on because the enemy will have no incentive to negotiate.

Nutting is asking quite a lot. He is saying that a genuine readiness, recognized by an adversary, to send Americans into battle, will actually make it unnecessary to do so. If "we make the evident commitment without limit, then the cost will go down," is the way he puts it.

But we are dealing in perceptions, as with nuclear deterrence. If that signal were misperceived, there would be nothing left but to make good on the threat -- at which point the American cost in blood and treasure goes up, and history repeats itself in a terrible way. The United States image in Latin America has not yet recovered from the free-wheeling resort to "big stick" diplomacy. Americans may not remember the "nation building" role of the U.S. Marines in Nicaragua in 1912, Haiti in 1915, the Dominican Republic in 1916 and Nicaragua in 1927. Latin Americans do.

Gen. Nutting doesn't offer Ronald Reagan an easy choice. But the general does know what he wants. So do those who argue for a gentler course: military restraint, with heavier emphasis on negotiation and accommodation with the left. Ronald Reagan suggests, sometimes, that this is what he wants, too. Other times he sounds as if there is no compromising with a communist presence in Central America. The only certainty, in what otherwise is a close and complicated question, is that he cannot have it both ways.