The Defense Department issues a 12-page warning on the pitfalls of diplomacy in Central America. The State Department says the study "has no standing as a U.S. government document." The undersecretary of defense for policy, Fred Ikle', says the State Department is "plain wrong." A White House spokesman wonders out loud what all the fuss is about.

This collapse of coherence in the administration's approach to Central America is only the latest evidence that in matters having to do with national security we are, as the humorist H. Allen Smith put it, "lost in the horse latitudes" -- the stretches of ocean notorious for periods of dead calm. In the days when sailing vessels had horses in their cargo and the supply of fresh water ran low, horses crazed by thirst would kick out the wooden timbers of the hull. In like fashion, policy makers downtown and kibitzers on Capitol Hill are lashing out in crazy, contradictory ways.

The public seems not to care. Polls show record majorities approve Ronald Reagan's conduct of foreign policy. Longtime Reagan watchers trot out shopworn reassurances that laid-back management served the president fine in California. (Never mind that California does not have a foreign policy.)

For its part, the administration says the disarray is caused by "leaks" to the media; drastic plumbing is in order. But this stands the problem on its head: the leaks derive from the disarray -- the breakdown of discipline and loyalty that has always been the mark of a loss of control over the policy-making apparatus.

So how do you square this with the popular perception that Ronald Reagan has resurrected the American presidency, restoring confidence in the prestige and power of the United States as well? I think the answer lies in a failure to make the necessary distinction between the twin responsibilities of the American presidency.

Reagan has unquestionably invested the role of head of state with a new ceremonial majesty. His every gesture is so reassuring that one hardly notices what has happened to that other role of head of government: what has happened is that it has been allowed to atrophy. The president does not exercise the elemental powers of his office to resolve internal quarrels with a view to getting things under way.

The result is a bedlam level unmatched since the days of, well, Al Haig. The onset of executive atrophy showed itself plainly, early on, in such things as the on-again/off-again sanctions against the European gas pipeline to the Soviet Union. It has since showed itself repeatedly in policy for the war in Lebanon, for counterterrorism, for arms control.

Nicaragua, then, is only the latest case. In theory, there is a policy: the president wants $100 million in military and economic aid to the Nicaraguan counterrevolutionaries, variously known as "contras" or "freedom fighters." He insists this will pile pressure on the Sandinistas to negotiate. To give this two-track approach credibility, he has made veteran troubleshooter Philip Habib his special envoy. Habib is working with the only diplomatic instrument available: the so-called Contadora process by which key countries in the region would broker a peace treaty, supposedly stopping Nicaragua's threat to the neighborhood.

The trouble is that, realistically, no conceivable peace treaty can achieve the hard-liners' true purpose: a full flowering of Nicaraguan democracy. And the hard-liners know it: the Sandinistas are not going to be dislodged or "reformed" without the use of more force than the administration is ready to advocate or Congress is ready to apply. The advocates of accommodation, in turn, must know that the Sandinistas are not going to negotiate away their power.

But so long as the argument centers on means, rather than a forthright debate about ends, both sides are free to beg the hard questions. The State Department can plug a combination of covert aid and diplomacy while Defense can sneer at the futility of diplomacy, at the same time putting prudent limits on the force it is willing to bring to bear. Small wonder that partisans on one side or the other in Congress find much to exploit in the administration's internal tugging and hauling.

That's not to say the administration would necessarily prevail in Congress even if it spoke with one voice. The point is that the restoration of a coherent policy for Nicaragua has to start where foreign policy-making is supposed to start: in the executive branch. And the element that's missing in the executive branch is Ronald Reagan's firm, guiding hand.