DANIEL PATRICK MOYNIHAN sermonized a few years ago on the deplorable tendency in Washington to oversimplify every problem. What is needed, he said, are people who can "complexify."

In the case of Central America, he was wrong. This town is now full of men and women who have taken the relatively simple problem of El Salvador and complexified it outrageously. They have elevated an insurgency of moderate proportions into an international crisis, while baffling their countrymen and themselves. The fleet has sailed, Henry Kissinger has been recalled from his various business enterprises, the only televised "covert" war in history has been launched and the End of the Earth cult has been revved up for yet another Armageddon: all this over a country of 4.5 million people confronted with five quarreling bands of guerrillas in the number of 5,000 to 7,000.

The problem for the government of El Salvador and its patron, the United States, is not Nicaragua or Cuba or the Soviet Union which provide the guerrillas with arms, sympathy and advice. That kind of assistance cannot be interdicted anymore than the United States can inderdict the flow of dope over our own borders. Furthermore, it can be offset by arms, sympathy and advice flowing from Washington to San Salvador.

The guerrillas are the problem and that problem, in all probability, can only be solved by the army of El Salvador. There is of course talk of a negotiated settlement based on "power sharing," but that is no more likely in the short run than a "power sharing" settlement between the Sandinistas in Nicaragua and the contras we have launched against them.

There is also idealistic talk in the American Congress about eliminating the "roots of war" -- poverty, injustice, illiteracy and so on. But our record of rebuilding and reforming countries in our own image is not good under the best of circumstances. And in the midst of a war, those goals are wishful thinking. One of the cardinal principles of guerrilla warfare is to destroy or severely damage the strategic assets and infrastructure of the society under attack, whether that be power plants and transmission lines, roads, bridges, crops, factories, police stations, courthouses, schools or medical clinics. That is happening all over the world today, wherever guerrilla wars are being fought.

A military solution to such problems is ultimately possible if the masses of the people have not been "revolutionized" or disaffected to the point of popular uprisings, as occurred in Nicaragua and Cuba. From all the evidence, that point has not been reached in El Salvador. It is indeed a poor country; the per capita GNP is only $650 per year. But poverty is relative. Nearly 50 countries in the world are worse off -- including China and India where per capita GNP is about half that of El Salvador, which the World Bank classifies as "lower-middle income." This reinforces the demonstrable truth that there is no necessary correlation between revolution and economic deprivation. If it were otherwise, China would be in flames.

The El Salvador problem is a military problem. No guerrilla war in modern times with the exception of Zimbabwe has had anything but a military solution. If the United States would keep its eye on that mark instead of engaging in diversionary sideshows, a solution might be found.

The first requirement is to train the Salvadoran army in infantry and small-unit tactics. The task of the soldier is to find, kill or capture his enemy and he needs to know how to do that if he is to succeed.

The second requirement is to get those soldiers into the bush and into the hills to find, engage and otherwise harass the guerrillas. The Salvadoran soldiers are courageous and sometimes "valiant to a fault," according to an American officer who has observed them up close in combat. But they provide no useful service in barracks or on parade grounds.

A third requirement is to keep them in the army once they have been trained and have experienced combat. Today, too many of them leave the service after a two-year tour. The guerrillas, on the other hand, are in for the duration.

A fourth requirement is for the United States to provide its ally with the necessary tools of war on a consistent and reliable basis. The requirements are modest; no exotic weapons are needed.

Finally, two reforms must somehow be achieved internally or imposed by Washington. The officer corps must be overhauled to break up the old-boy networks that create political commanders. The institution of the army must be made subservient and responsive to civilian control. At the same time, the private armies and death squads must be eliminated or brought under control. That is a tall order but it is essential to the army's ultimate success.

The recent adventurism of the Reagan administration in Central America has put all of these modest proposals at risk. By alienating public opinion and dividing the Congress, the entire assistance program to El Salvador has been imperiled.

That is the danger in overly complexifying problems.