When the 1970s were just four months old, Richard M. Nixon -- explaining his invasion of Cambodia -- made one thing clear:

"If, when the chips are down, the world's most powerful nation acts like a pitiful, helpless giant, the forces of totalitarianism and anarchy will threaten free nations and free institutions throughout the world."

Now, with the 1970s at an end a band of fanatical young Iranians holds about 60 Americans hostage in Tehran, and many Americans at home are protesting that their country is a pitiful, helpless giant. From prophecy to reality in one quick decade, or so it may seem.

Oh, those 1970s -- not the sort of decade Americans like to identify with. Lost power, lost innocence, lost self-esteem: No more No. 1.

Bob Higgins of Joplin Mo., says this about the events in Iran:

"It gives you an unsettling feeling to think that something like this can really happen and you're powerless to stop it. Your first reaction is very angry. You think you're really going to do something about it. Your second reaction is just to sit back and wait."

The country may be following the pattern Bob Higgins described. The initial reaction was emotional and angry. Radio talk shows were suddenly flooded with enraged phone calls; irate letters deluged newspaper offices. The White House reports that it has not seen any remotely comparable manifestation of public emotion in Jimmy Carter's presidency.

One of President Carter's senior aides was stunned by a snippet of film on the CBS Evening News 10 days ago. It was life mimicking art, a scene from violent anti-Iranian protests in Los Angeles. Angry Americans beat and stomped on demonstrating Iranian students, then burned an Iranian flag. Suddenly one of the Americans, a man who looked about 30, turned squarely into the camera and shouted:

"We're not going to take it any more! We're sick of it!"

And then, thrusting his fist into the air as he shouted the word, he yelled at the camera: "America! America!"

Soon afterward, the White House asked the television networks to try to avoid inflaming the situation with footage that might contribute to more violence.

In the days since, much of the initial electricity has ebbed. Instant polls show strong support for President Carter's line of response: restraint tinged with marginal vengeance. The large crop of presidential candidates has generally avoided challenging Carter's approach or proposing anything harsher. Now, like Bob Higgins, the country seems to have decided to "sit back and wait."

America has changed profoundly during the 1970s, as has America's place in the world. This is a matter of facts, and figures, and national psychology.

In 1970 an ounce of gold could be bought for $35. Today it costs aboutt $400. A dollar bought nearly four German marks in 1970; today it buys less than two. In 1970 Americans produced 78 percent of all the footwear they puchased; today they produce about 45 percent. Even the national pastime (the real one) has passed into foreign hands; we import more than half the television sets sold in the United States; in 1970 we imported about a third.

Partly the shifting status of the national currency and the nation's industry has reflected failed or botched poliicies, but mostly these changes are a tribute to American success. The United States fundamental policy after World War II was to make the world prosperous -- prosperous and safe for American exports and American corporations. The success of this can be summarized with a single statistic: In 1950 the United States produced 47 percent of the world's steel; to that figure is less that 15 percent..

America shared its wealth so successfully, and the urge to prosper has proven so strong in so many diverse cultures, that a world capitalist economy once utterly dominated by the United States is now truly multipolar as well as mutiprosperous.

In effect, America created its own competition, then in many fields got out-competed. This was all according to plan.

So was the process of decolonization. In 1947 the United Nations had 51 members. Today it has 152. Each member has unique needs and aspirations; after all, each one is a nation.

America came out of World War II with the world's only strong, advanced economy, and with a presumption of power and influence that has infected the national personality ever since. It took only a few years for the United States to encounter its first frustration, in Korea. But somehow Americans drew no profound lesson from that unsatisfying war. The presumption of power prospered along with the booming capitalist economy.

Questions about America as No. 1 will almost certainly be a central theme of the 1980 presidential election. They alreaday are. Carter can accurately proclaim that he is the first president in a generation who has not spilled any American blood in foreign soil. Meanwhile, his opponents -- like John Connally and Ronald Reagan or Howard Baker and George Bush -- will flog the Carter administration for its supposed weakness in international affairs. They already are.

But it is not clear as yet that the old-style rhetoric of American power and pride will sweep all before it. Attitudes are more complicated now, and changing.

A new Washington Post survey of public opinion, completed during the Iranian turmoil, asked a national sampling of Americans how strongly they care, for instance, about whether the United States wins the 1980 Olympic games. The response: 72 percent of the people care about winning; 26 percent said it doesn't matter to them.

On the other hand, when they were asked about America's power position in the world, the response was somewhat different -- inward looking and not especially belligerent. The poll offered two conflicting views and found that only 22 percent endorsed this one: "The U.S. should maintain its position as the world's most powerful nation at all costs, even going to the brink of war if necessary." In contrast, 58 percent chose this statement: t"We should concentrate on problems here at home and not think so much in international terms."

Of course, the United States cannot return to an insulated existence -- even if that is what citizens would prefer. Those days are gone, and economic fact spoken most forcefully by oil. But, in the Post survey, citizens seem sharply divided on whether to accept that new reality of global interdependence.

The survey asked whether people approved or disapproved of this statement: "The United States should take all steps, including the use of force if necessary, to insure that we have an adequate supply of oil from the Middle East." The response: 39 percent agreed, 49 percent disagreed, the rest were undecided.

There are 250 Iranians studying at the University of Michigan in Ann Arbor, but the campus was calm last week. (so were most campuses where large numbers of Iranians are studying.) A reporter who went around the campus on Wednesday interviewing students found none who advocated military action against Iran. On the contrary, the respondents in this unscientific poll of 18- and 19-year-olds (people who were 7 or 8 years old at the time of the Tet offensive) seemed remarkably calm about the events in Tehran.

This is what Charles Thompson, a 19-year-old sophomore, had to say:

"A lot of people are fascinated by the idea of sending in the bombers now that we've turned off the oil spigots. My own opinion is that that would be short-sighted and very foolish. If the people of Nicaragua or Iran truly are against their regime -- if there is a despotism and the people want to overthrow it -- I'm all in favor of that.

"It may be that in our time the giants cannot help but be pitiful and helpless. We probably are the strongest nation on earth, but the strongest nation on earth could not win in Vietnam.

"Maybe the times now define superpowers not in terms of what they can do, but what they can't. Pakistan right now probably has more influence on Iran than the United States, and Pakistan is by no means a military power.

"We are going to have to be very careful. We're going to have to watch whose toes we step on. Having this enormous amount of power will not automatically mean from now on we can have anything we please. Maybe this is all good."

Katie Herzfeld, another sophomore, said this:

"What I think of the United States is that we're overextending ourselves and we got our foot in our mouth. If we just trusted our own strength enough that our own democracy could survive even with people like Khomeini around, then that is true proof of power.

"There is no such thing as a strongest country. Each country has influence where others don't. Power is a state of mind, and the world's state of mind is changing."

On other campuses young people have been demonstrating patriotically for America during these tense days. At Northwestern University in suburban Chicago, the Iranian events led to formation of Students for Reestablishment of American Pride. The group sponsored a rally on Thursday night, and 150 students showed up.

"The idea that college kids are holding pro-American deomonstrations now should not come as a surprise to anyone," beamed Ben Wattenberg last week. Wattenberg is chairman of the Coalition for a Demoncratic Majority, a group eager to restore some of the old verities of American life. "Thishis is still a patriotic country," Wattenberg insisted.

Scholars and samplers of public opinion endorse his conclusion unreservedly.

"The latent patriotism in this country is very strong," said Angus Campbell of Michigan's Institute for Social Research.

Several dozen phone conversations last week with intellectuals of different stripes around the country suggested that the American establishment was feeling pretty patriotic, too. John McConahay, a professor of psychology at Duke University who is spending this term on leave in Ocean Isle Beach, N.C., summarized the spirit of many of these conversations:

"I'm surprised at myself," McConahay said. "I have fantasies about rounding up all the Iranian students in the United States and just puttingg them in one place -- not doing anything to them, just putting them there.But that's not me . I got to admit, it angers me."

The Rev. Andrew Greeley, Catholic priest and sociologist, said, "I don't think that the American public perceives that we have lost power. They see our leaders unwilling to use it."

Ronald Steele, liberal whose 1967 book, "Pax Americana," suggested that American power in the world was overextended for ill-considered purposes, said he too was upset by what was going on Tehran. "It's the kind of behavior that nobody should have to put up with," Steele said. "The embassy of El Salvador shouldn't have to put up with it either."

Steele also spoke to a proposal that blossoms on every radio talk show these days: "Send in the Marines!" Steele thinks we have seen the last of that, except perhaps in Latin America.

Americans, he said, have to come to terms with the fact that they do not have usable military power. "We haven't had it for a long time," he said. "I don't think we'd do anything [militarily in Iran] even if they weren't sitting on all that oil, because I don't think we could do anything."

The Carter administration disagrees. It is spending millions now to improve "fast reaction" military forces that could intervene quickly in farflung corners of the world.

According to one reading of the 1970s, the United States has been "losing" all over the world -- in Angola, in Ethiopia, in Afghanistan, in Nicaragua and Iran. According to a different view, the United States has been harvesting the consequences of earlier mistakes -- for example, supporting unpopular dictators or colonial regimes without thinking that perhaps Angolans or Nicaraguans might share some of the values that Americans trumpet so eagerly themselves.

So there is an argument about those "losses." But there is one real loss that is not disputed, though it is not much discussed either -- America's loss in Vietnam.

In vietnam an investment of 50,000 lives and 130 billion dollars came to naught. This was no Korean-style standoff; it was total defeat, with nothing enduring to show for the effort except the losses. There has been nothing like this in all of American history, a humiliation of unprecedented pain and magnitude that happened during this decade just ending and that few now mention.

By the Pentagon's admission, the United States' failure to stay ahead of the Soviet Union in many categories of military strength during the 1970s can be blamed directly on money squandered in Indochina. Economists agree almost unamimously that two presidents' unwillingness to tax the country to pay for the war was the single most important cause of the inflation of the 1970s. And despite the general silence, it seems beyond dispute that Vietnam is the principal source of the widespread popular feeling that America has lost its way in the world.

One of the countless "lessons of Vietnam" was that America jumped into an alien country and culture without adequate knowledge of its traditions and realities. A superpower should have known more, and known better. It is a lesson unlearned. According to a recent presidential commission, during the 1970's American students have all but abandoned the study of foreign languages. In universities, "area studies" programs concentrating on foreign regions and cultures have been starved for funds and students.

According to the commission, 10,000 English-speaking representatives of Japanese companies are stationed in the United States. There are fewer than 900 comparable Americans in Japan; almost none speak Japanese

Both the State Department and the CIA have shrunk during the 1970s, the State Department by 15 percent. Fewer political officers are reporting from foreign countries to the secretary of state than in 1970, despite the ever-increasing complexity of international diplomacy.

Americans are proud, perhaps excessively proud of themselves and their country. Japanese are proud too, but not too proud to learn English.

The Americans presumption of power and stature is deeply ingrained in the national character. So when, during the 1970s, Europeans began to look down on the American dollar sometimes refusing to accept it, Americans came home angry, hurt.

But Americans are also pragmatic and adaptable people. Andy Spielman, a Washington travel agent, summed it up when asked about his clients' resentments over foreign snubs. "You do get people complaining, "Spielman said. "But shortly thereafter they're booking for next year."

The taking of American hostages in Iran has struck American pride hard, but it has also evoked American pragmatism. Daniel Yankelovich, the pollester and student of public opinion, suggested last week that after Iran "we may end up in a sense better off," because the country seems suddenly to have pulled together.

"This is the first thing that's happened in a long time where Americans think without qualification that we're in the right," Yankelovich said. He predicted that a new political issue is being born in these events -- -- "restoring American presitge abroad."

That sounds like 1960, Yankelovich acknowledged. And it may not be possible to go back down the path with out an argument. One who will argue is Professor Hans Morgenthau, 75, author of the classic text "Politics Among Nations" and a leader of the teach-ins against the Vietnam War in the 1960s.

Vietnam, Morgenthau said last week, was the watershed event of modern American history, one whose significance is only slowly sinking in to the national consciousness. The events in Tehran might stimulate the absorption process, he said: "We have to realize finally that we are just one nation among others that has temporarily been more fortunate than the others."