Paul Johnson. `The most important thing which the first quarter of the 21st century will decide is whether the United States is in a pattern of decline or not.'

`IT wouldn't at all surprise me,'' warns author and columnist Paul Johnson, ``if the first decade, or at any rate the second decade, of the 21st century is a decade of illusions - grand expectations, all kinds of rosy dreams.'' The reason? Settling into an easy chair in his third-floor London pied-`a-terre, Mr. Johnson noted that two trends now in the making will by then have peaked. One involves demographics. The other centers on technology.

The demographic trend, says Johnson, will bring into positions of authority a group of leaders for whom the lessons of the recent past will be no more than ``ancient history.'' These lessons, he explains, arise from a study of the past three decades.

First came what he calls an earlier ``Decade of Illusions'' in the 1960s, when Western nations imagined that they could ``increase state spending, welfare spending, defense spending, all kinds of investment, and at the same time increase personal spending almost indefinitely - with full employment, too.''

``Then in the 1970s came the Decade of Disillusionment,'' he continues, marked by the end of the postwar economic boom, the energy crisis, and the ``particular crisis of government and confidence in the United States'' known as Watergate.

With the 1980s, he says, came ``the Decade of Realism,'' during which Margaret Thatcher's government in Britain and Ronald Reagan's administration in the United States encouraged a return to ``old-fashioned virtues.''

The net effect of these three decades, according to Johnson, has been salutary. ``If you are old enough today to remember how very unpleasant hyperinflation was in the '70s,'' he says, ``it's unlikely that you will ever fall for policies which tend to induce very high rates of inflation.'' People now in their 20s, he says, share those memories. And they will be in leadership positions in the 1990s.

What concerns Johnson, however, is ``the generation after that - the generation that will take over and occupy important positions between the year 2000 and the year 2020 - because they won't have had this experience.''

At about the time they come into leadership positions, the second trend will make itself felt: the marriage of electronic communications and biotechnology.

Through biotechnology, says Johnson, ``it will be possibly to produce materials on which the number of minicircuits will be multiplied almost indefinitely.'' The result, he says, will be a generation of machines having ``more of the characteristics and flexibility of the human mind'' and being able to ``analyze things very closely and carefully and take decisions accordingly.''

Those developments, he concludes, will coincide with the coming to power of ``the new generation that doesn't know the experience of the 1970s'' - and therefore has ``the kind of idealistic illusions which will fit in rather well with the period of seemingly indefinite expansion'' ushered in by the new electronic revolution. The result, he warns, could be a repetition of ``the sort of mistakes that were made in the '60s.''

As these developments arrive, they will in turn be shaped by the political landscape of the 21st century. In a brief tour d'horizon, Johnson charts that landscape:

Japan. Johnson says Japan is poised to enter its ``really creative period.'' Until now, he says, the Japanese have ``principally been taking developments they've inherited from the West and improving them.'' In the future, he foresees them increasingly ``leading in technology.''

Yet Japan remains, for Johnson, an ``anomaly.'' Historically, he explains, the development of economic leadership by an emerging nation has ``inevitably and quite quickly been followed by an increase in political and military power.''

But Japan, although its economy is second only to that of the United States, has ``very little political power, virtually no military power, and an apparent disinclination to exercise either.''

In the 21st century, he says, it will be ``almost inevitable'' that Japan will ``reemerge as a major political and military power.''

United States. ``The most important thing which the first quarter of the 21st century will decide,'' he says, ``is whether the United States is in a pattern of decline or not.''

His hunch, he says, is that it is not, since the United States has ``enormous physical, territorial, and psychological resources'' and has proved itself to be ``capable of periodic renewals and recharging of energy.''

America, he notes, is ``the best- known and the least-known country in the world.'' Although Europeans think they know it well through movies, television, and periodic visits to its major cities, he notes that ``it's a very different country, with all kinds of things which we simply don't possess over here [in Europe] and [with] an absence of certain hang-ups.''

``My guess is that America will survive as a major power'' in the 21st century, he says, and will remain ``the anchorman of the West for the foreseeable future.''

Western Europe. Since 1945, says Johnson, ``Europe has had a marvelous quarter-century.'' Yet it remains ``a sleeping princess.'' Having emerged from ``a terrible period'' of wars earlier in this century, Western Europe now enjoys ``a far higher living standard than it ever had in the whole of its history. And for the first time every single country in Western Europe is a democracy.''

Like Japan, however, Europe remains an anomaly: Economically strong, it lacks commensurate military might and international influence. ``It has a much greater collective economy than the Soviet Union,'' Johnson notes. ``Yet it always has to rely upon the United States for its basic strategic defense.'' The question for the 21st century, he says, is whether this anomaly is ``going to be corrected.''

What Europe most needs, he says, is ``a new literary, artistic stimulus'' which he expects will develop during the first decades of the 21st century.

Looking back at the Europe of Dante, Shakespeare, and Goethe, he notes that ``it is the poets, the dramatists, the great writers, who provide the big ideas which later the statesmen and the generals, as it were, put into practice.''

``Who is going to be the next European writer who will kiss the sleeping princess and awaken her?'' he asks.

Asia. If, as Johnson foresees, Japan attains new political and military power, then a major question facing the 21st century will be ``the China-Japan relationship.'' Will it be, he asks, one of enmity or one of partnership? If the latter, ``which will be the senior partner? Or could they achieve a partnership of equality?''

The latter, he says, would be ``the most desirable solution.'' Given the size of China and the economic strength of Japan, however, such a partnership ``would of course create the most important economic unit by far in the world.''

Also very much in the Asian picture, however, is India - ``a rather ramshackle social-democratic, parliamentary democracy, which has done very much better than one might have expected.'' He says India has ``a lot of weaknesses.'' But he adds that ``it has a kind of staying power which one would not have predicted'' - and which he attributes, in part, to its British heritage. He expects, in the 21st century, that living standards in India will rise faster than those in China - largely, he says, because the former nation benefits from a partially free market economy.

Middle East. Johnson says the Arab world ``missed its great opportunity'' with the oil boom. ``They could have transformed themselves. They could have brought their thinking into modern times: They could have created an industrial economy, and they failed to do it.''

``I would guess that in the 21st century the Arab world will be less important than it has been.'' He sees plenty of promise, however, for two countries: Turkey, which he expects to have ``a good century in the 21st century,'' and Iran, which will be ``a very impressive country'' when it ``gets rid of the fanatics.''

Africa. Among the 50-odd nations that make up this continent, he says, most have ``gone downhill'' in the last 25 years - making Africa ``a very sad case.'' The problem: excessive population growth, which he says always follows a ``predictable pattern.''

``When you go through these terrific population surges,'' he says, ``it always means a great deal of either imperialism or of political instability, revolution, and so on.'' Europe and Asia passed through large population surges - with their attendant instabilities - several decades ago.

More recently, Latin America passed the peak of the surge - with the exception of Central America, where population growth is still a challenge.

But he says Africa is just entering its population surge. ``There's going to be a great deal more [growth and instability] over the next 30 or 40 years,'' he warns.

Latin America. ``People have been predicting a tremendous future for Latin America since the 16th century, and it's never actually happened.'' He says the most likely success story is Brazil, which has ``an extraordinarily mixed, multiracial society'' with ``huge minorities'' from Europe and Japan. Already, ``there are large areas [of the country] where things are going well,'' leading him to think Brazil could become ``a major economic power in the 21st century.''

Soviet Union. Historically, says Johnson, the Russian people have proved themselves to be ``enormously conservative.'' Today the Soviet regime ``benefits from the instinctive feeling among the Russian people that this is the regime that they've always had, as it were, and it oughtn't to be changed.''

Can the Soviet Union undergo major reforms? Johnson does not think so. ``The most important single factor about the regime is the absolute monopoly of power exercised by the Communist Party. Until you change that monopoly, you can't have structural reform of any consequence. And if you change that monopoly, I don't believe the regime as a whole can survive.''

He sees, instead, ``a long period of stagnation for the Soviet Union'' - during which time there could be ``breakaways in Eastern Europe which [the Soviets] are unable or lack the will to prevent.''

Standing back from the world he has just sketched, Johnson sees little likelihood that the present ``nuclear deadlock'' will be changed. History teaches that ``you never actually get effective disarmament agreements unless the basic political [obstacles], which have made the powers arm in the first place, are removed.''

``I think if we could solve our ideological and political differences, there wouldn't be much trouble about a disarmament treaty. But to get the second without the first, I'm afraid, just doesn't work in practice.''

Johnson also expresses little concern about the environment. ``I think this is a 20th-century problem which we're in the process of solving.'' He adds that ``I don't see it as a major 21st-century problem at all.''

High on his agenda, however, stands the question of education.

``One of the things we've got to do in the 21st century is to rethink the whole ideology of education,'' he says. ``I think we're going to have a lot of opportunities in the 21st century to spend the profits of an expanding world economy on desirable leisure and cultural ends. And no one has yet produced, to my mind, a rational philosophy as to how those ends should be pursued.

``Perhaps the most important thing that will happen in the 21st century,'' he concludes, ``is a rebirth of classical cultural values and civilization - after a century of frenzied experiment which hasn't really produced very much.''

Next: Gen. Olusegun Obasanjo, former head of state of Nigeria, March 19. A writer keyed to history

``History is the most important thing to learn,'' says British author Paul Johnson. ``It's more important than almost any other discipline if you're involved in high-level decisions.''

In his voluminous writings, Mr. Johnson practices what he preaches. His most recent book, ``Modern Times: The World from the Twenties to the Eighties'' (1983), takes a comprehensive, sweeping, and often iconoclastic look at the 20th century.

In more than a dozen earlier books, he has examined subjects ranging from the Suez war to Pope John XXIII, from the history of Christianity to British castles.

Educated at Stonyhurst and at Magdalen College, Oxford, Johnson joined the editorial staff of the left-wing New Statesman magazine in 1955, becoming editor in 1965.

Then, in a political volte-face that stunned his contemporaries, he took up the conservative cause, threw his support behind British Prime Minister Margaret Thatcher's government, and became a columnist for the right-wing Spectator magazine - in whose pages he noted that ``most considerable thinkers in Britain now incline to the Right of the political spectrum.'' A frequent lecturer, he often discusses the future from a historical perspective.