THE UNITED STATES has a myopic, ideological foreign policy that really isn't a policy at all, but a collection of maneuvers produced by prejudice and instinct. The men responsible for American diplomacy, it seems, often fail to grasp the have put us into grave trouble around the world.

A harsh judgment? Certainly. But the events of recent days have provided considerable new evidence to support it.

In just one week President Reagan has vastly and recklessly increased America's stake in Lebanon, a fragile nation whose fate is probably beyond American control. He has turned international opinion against the United States by invading diminutive Grenada, squandering the moral high ground the Soviets had granted him by their bad behavior in Poland, Afghanistan and the destruction of Flight 007. He has angered and undermined his closest ally in Europe, Margaret Thatcher, and he has aggravated the gravest problem facing the United States, a problem symbolized by the largest protest demonstrations in Europe since World War II, which occurred last Sunday.

In these few days we have seen the fruits of Ronald Reagan's deeply ideological approach to the world -- the tendencey, as Seweryn Bialer of Columbia put it last week, "to know the answer to the question before it is even asked." The tiny island of Grenada was a danger because it might spread the infection of Cuban communism; the situation in Lebanon was grave because "a force" was poised to take over the entire Middle East. And if some critics cited legal niceties to pick on the United States, Ronald Reagan gave us Soviet-style assurances that the ends justify the means.

Until this past week Ronald Reagan had an extraordinary run of good luck in world affairs. The absence of obvious disasters or highly controversial actions muffled his potential critics. Though the administration was still looking in vain for its first significant accomplishment in international affairs, it had escaped fiascos.

As long as it avoided serious trouble, many people who are appalled by Reagan diplomacy kept quiet, whether out of deference to a president or fear of his political power or uncertainty about just how one declares that the emperor is nude -- or is wearing Teddy Roosevelt's old clothes. But that may change now. Very few people in this town outside the ranks of the administration and its diehard supporters have much respect for Reagan's handling of international affairs -- and no wonder.

Consider one foolish remark that the president made last Monday: "We have vital interests in Lebanon." In little more than a year, he has transformed the tragic Lebanese situation from an agony of significant but hardly crucial interest to the United States into a test of our manhood and steadfastness. Lebanon is now "central to our credibility on a global scale," Reagan announced.

At the same time, he transformed the ancient struggle of religious factions in Lebanon into a Soviet-American face-off. "Can the United States. . .stand by and see the Middle East incorporated into the Soviet bloc?" he asked, as though this unlikely development was the only alternative to the continued presence of our Marines. So the Marines stay.

To what end? The ceasefire in Lebanon is already breaking down. The chance of the various factions coming together under President Amin Gemayel is small. (Gemayel has already called the scheduled negotiations among factional leaders "a camouflage, a distraction.") Of course we would wish it otherwise, but the outcome in Lebanon is likely to be bad. And now the president of the United States has declared that his country has "vital interests" in a situation it might only be able to control by going to war against Lebanese Moslems who refuse to accept the Christian Gemayel as their leader.

And what would be the results of such a war? The lasting enmity of Arabs all over the Middle East, for one; the very real danger of a direct confrontation with the Soviet Union, for another. The Lebanese Moslems would have the support of Syria; Syria is actively supported by the Soviets, including 7,000 Soviet military advisers in their country.

Short of going to war, what will the United States do? No one knows, including the president. His own words last week have made it difficult to pursue a sensible, cautious policy.

In his speech Thursday night, the president disposed of all these problems and dangers by remaking the Lebanon crisis in his own image. Just as Cecil B. DeMille created a melodramatic Hollywood version of the Book of Exodus in "The Ten Commandments," so Reagan redid the Lebanon story in a new form more suitable for a television speech.

Instead of trying to divorce the intractable internal problems of Lebanon from the fate of the entire Middle East, Reagan made Lebanon the key to it all. He offered the preposterous notion that the unique historical and ethnic influences that comprise Lebanon are really "a microcosm" of the entire region. He claimed brilliant success for his negotiators' failed efforts to work out a Syrian-Israeli withdrawal from Lebanon, adding -- incorrectly -- that Syria had promised to leave Lebanese territory. Then he claimed the Marines had to stay there to protect Israel. This is the same Israel which had its own forces in advanced positions in Lebanon, but withdrew them into a small southern enclave out of its own perceptions of Israeli security interests.

The United States faces a genuine crisis at the end of this year, a moment of supreme importance in the history of the Cold War and the western alliance. It will begin in December with the deployment of new American missiles in West Germany, Britain and Italy. If, as now seems certain, no negotiating breakthrough heads off that deployment, the Soviets will then walk out of the Geneva negotiations and begin a new round of deployment of their own missiles. This breakdown in superpower relations will put an unprecedented strain on NATO. Many in Europe will question whether their interests are best served by an alliance led by the United States.

Last week, whether deliberately or not, the Reagan administration put its anxieties about the islet of Grenada ahead of any concern about the coming crisis in our most important alliance. Whatever the ultimate effect of the invasion of Grenada on the Caribbean and Central America, it will be a disaster in Europe.

Already it has caused severe embarrassment to Reagan's one genuine ally in Europe, Mrs. Thatcher. No European government or leader will endorse our elephantine swat at the Grenadan flea. The invasion will help only those Europeans who are critical of Reagan and his policies. Former West German Chancellor Willy Brandt has already remarked that after the American invasion of Grenada, U.S. criticism of the Soviets' invasion of Afghanistan "loses credibility." Even the conservative German government moved quickly to criticize the U.S. invasion and ask for its speedy conclusion.

Similarly, the invasion of Grenada will complicate the chances of any negotiated solution to the wars in El Salvador and Nicaragua. Mexico is already leading the chorus of Latin American critics of the invasion. Their strongly negative reaction grows from an old and genuine anxiety about Yankee interventionism. It is now much less likely (it was never likely) that the so-called Contadora group of Central American states will be able to help the United States extricate itself from those two wars through negotiations.

Last week's events in Lebanon and Grenada revealed fundamental flaws in the Reagan administration's diplomacy. Perhaps they were predictable; at least they were characteristic.

Both grew out of an eagerness to use American military power to show our mettle. Both were the products of narrow preoccupations with local problems that blinded policy makers to the broader implications of their decisions.

How much effort did the Reagan administration make to discover, last year, if there was a way to pursue American interests that did not involve the enormous risk of putting Marines on the ground in Lebanon? No, the administration welcomed a chance to show that it was unburdened by any "Vietnam syndrome," and was ready to play an activist world role, with Marines if necessary.

And in Grenada, did the White House explore the possibilities for diplomacy before reverting to an invasion? On the contrary, the decision to invade apparently cut off diplomatic efforts by Grenada's neighbors. It also cut off efforts to evacuate Canadians, Americans and British citizens that the Grenadan authorities had apparently encouraged. The Reagan administration acknowledges that it felt the situation in Grenada was too risky to bother with diplomacy. Not even the trauma of 229 dead Marines in Beirut could deter the administration from its appointed rounds in the Eastern Caribbean.

This eagerness to practice gunboat diplomacy is typical of the Reagan administration. Perhaps it is an inevitable consequence of the extraordinary starting point this administration chose for itself. Reagan and his defense secretary, Caspar Weinberger, came into office trumpeting America's military inferiority, surely an unprecedented gambit for leaders of a great nation. They took positions that the Joint Chiefs of Staff would not endorse, arguing that the Soviets were superior to us in almost every category of military strength. This was a great exaggeration and a bizarre political act.

By declaring American inferiority, then boasting of their determination to end it, Reagan and Weinberger established conditions that -- in their eyes -- probably required them to look for ways to exert American muscle. They were so convinced that the world had lost respect for our power that they went out of their way to show off a new American militancy.

Thus, when it first took office the Reagan administration had no intention of soon opening arms negotiations with the Russians. Reagan's first secretary of state, Alexander M. Haig Jr., threatened to "go to the source" -- Cuba -- to deal with Central American unrest. Perhaps carried away with enthusiasm for military action, the Reagan administration did nothing to stop -- and may have passively encouraged -- Israel's invasion of Lebanon. (Last week it was no longer an invasion. President Reagan, invoking his own version of history, said "Israel had been forced to cross its own border.")

Reflect for a moment on that episode. Had the United States simply enforced its own law that theoretically prohibited Israel from using American arms for offensive purposes, there would have been no Israeli invasion of Lebanon. This would have saved Israel the 518 killed and 3,000 wounded it suffered in that adventure, and perhaps headed off the national nervous breakdown that now seems to have afflicted our most important Middle Eastern ally. There would not be 7,000 Soviet advisors in Syria, or anti-aircraft missile batteries manned by Soviet troops. There would be no U.S. Marines in Lebanon -- and no funerals for 229 young Americans this week.

The preoccupation with the use or threat of force seems anti-historical. Where is the model in history that demonstrates the utility of throwing around one's military might? In Afghanistan? Vietnam? Israel's invasion of Lebanon? No, the best use of military force seems to be to repel another's mistaken use of it -- the British defense of the Falklands, for example, or our defense (clumsy as it was) of South Korea.

History? It has no apparent place in Ronald Reagan's view of the world, except for the caricatured version he has carried around in his head for years. It popped out in an answer at his press conference 10 days ago, when he was asked if he expected to get an arms control agreement with the Russians during his current term.

"I realize the history of negotiations in kethe past has been long drawn out," Reagan said. "But if you will look at some of the negotiations in the past, maybe it was long drawn out because the longer the Soviets sat there, the more we unilaterally disarmed, and they found that just by waiting they could get things that they wanted."

Where did any of this happen? Only in Reagan's imagination. The United States has never unilaterally disarmed; we have steadily improved our nuclear arsenal for many years. There have been delays in arms control negotiations, but they have more often been due to American political considerations than anything else. Ronald Reagan ought to know that his own candidacy for the Republican presidential nomination in 1976 blocked completion of the Vladivostok accords in a new treaty. But specific knowledge about arms control or about dealing with the Russians in general has not been a Reagan strong suit.

At a meeting with congressmen earlier this month, Reagan said he had only recently learned how heavily the Soviets depended on their 308 biggest or "heavy" missiles for their defense. In fact, Soviet concern for those missiles is perhaps the key issue in all attempts since March 1977 to negotiate new arms agreements with Moscow. "The level of knowledge about arms issues," says a congressman who has been actively cooperating with the Reagan White House on these matters, "is really, really horrible."

This is a fact known to virtually everyone in Washington who closely follows strategic issues, including people who generally agree with Reagan's ideological position. In the same way, the specialists all know that retired Brig. Gen. Edward Rowney, Reagan's negotiator for a strategic arms agreement in Geneva, is a man of limited intellect and no real interest in making a plausible deal with the Russians.

But if the limitations of Reagan and his team are widely accepted by the expert community in Washington, the experts mostly keep still. Reporters who also realize that many officials in this administration are less than whizzes don't know how to put that into print. "We are paying the penalty of low expectations," argues one old hand who has served in both Democratic and Republican cabinets. "No one expects Reagan to know the facts, so no one picks on him when he doesn't."

This is true. Lou Cannon, The Post's White House correspondent, wrote the other day that according to congressmen who heard him discuss the invasion of Grenada, Reagan displayed "an unusually detailed grasp" of the issues involved. A reader from Mars might have thought this meant that in absolute terms, Reagan had a splendid mastery of the material. But every insider in Washington knew what Cannon acknowledges was intended -- that this time, Reagan knew something about what he was discussing.

"Everybody in this town has known that the emperor has no clothes," said that same former cabinet member, "but there has been a polite silence not to say so." Polite, or foolish? Perhaps, once again, the American people will have cause to complain that the "establishment" that is supposed to know the most about these things failed to warn its countrymen of the dangers it faced. If the people who know most say nothing, what is the good of having the freedom to speak out?

"We've really got to start talking," says George Ball, undersecretary of state in the Kennedy and Johnson administrations. "The fact is we've let these fellows get away with murder, and the situation now is much too serious for that."

To ideological men like Ronald Reagan, new information is only useful if it confirms old prejudices. Though he is shrewd enough to bend and budge under pressure (hence, for example, his abandonment of old positions on Taiwan), in his heart Reagan knows he has always been right about the nature of the world, of communism, of America's proper role.

Traditionally Americans have boasted that they were the pragmatic world power -- the ones who could cut through cant and get to the otiations in keessence of problems, see things as they really were. This has been our boast, but it is not the reality.

Perhaps the best evidence of this is our attitude toward the rule of law, particularly international law. A real pragmatist would have to understand the value of enforcing standards of international behavior. Only if a common code of civilized behavior achieves wide acceptance can we hope to move the world toward less barbarous behavior.

Not for the first time, we have an American president whose ideology tells him he can disregard such pragmatic theories. Listen to Reagan speaking the week before last about covert (in other words, probably illegal) actions by the United States inside the territory of other states:

"I do believe in the right of a country, when it believes that its interests are best served, to practice covert activity. . . ."

Or consider the administration's assertions that the invasion of Granada was justified to "restore law and order" and "government institutions" or "democratic institutions."

Do such statements have meaning? No. Reagan does not mean that any country has the right to operate covertly in another; he means only that the United States claims that right in some cases. Was law and order a serious American concern? Or should we believe the later version -- the one offered in President Reagan's speech Thursday night -- that we were heading off the creation of a big Cuban base?

"(We have been invited) to render urgent assistance, including assistance with armed forces. . . . This decision is fully in accord with the right of states to individual and collective self-defense. . . . This decision safeguards peace. . . . (Our) troops will be immediately withdrawn as soon as the threat is eliminated. . . ."

So spoke the Soviet news agency Tass on Aug. 21, 1968, the day Warsaw Pact troops invaded Czechoslovakia. The fact that the statements of the United States government this week sound eerily like Tass should warn us that we, too, have become an ideological power whose concern is not with standards of international behavior, but rather with pursuing our president's own vision of what is right.