Iconoclastic historian re-evaluates our century; Modern Times: The World From the Twenties to the Eighties, by Paul Johnson. New York: Harper & Row. 817 pp. $27.95.

Even before the past several years of Thatcher conservatism, Britons were noting a shift from left to right in intellectual fireworks. Paul Johnson is one of the writers who not only observed it but joined it. He used to edit the left-wing New Statesman. These days he writes for the right-wing Spectator, in which he recently acknowledged without false modesty that ''most considerable thinkers in Britain now incline to the Right of the political spectrum.''

''Modern Times'' is itself a considerable example of inclining to the right in writing a book of international history. Its brimming pages evoke most of the century so far, often standing customary views on their heads in the kind of intellectual shakeup that can make readers think twice, even if they then dash back to what they thought before.

A touchstone for understanding the volume may be what Johnson wrote this spring in tribute to a senior British writer who earlier switched to the rightward path: Malcolm Muggeridge. Johnson recalls a Muggeridge article he did not wholly agree with when it came in to the New Statesman. Now he sees it as having great wisdom. It argued that the most lasting destructive force of the 20 th century had not been fascism or Nazism or even Stalinism but the heedless pursuit of liberal ideas.

Not that ''Modern Times'' is soft on the totalitarian ''isms.'' It both spells out their monstrous crimes and deplores their corrupting effects on free and humane nations. Indeed, Johnson traces a critical stage in humanity's moral decline to his own country's World War II policy of bombing German civilians, a policy legally arrived at and ''enthusiastically backed by the bulk of the British people.'' It was a point at which ''the moral relativism of the totalitarian societies invaded the decision-making process of a major legitimate power.''

''Moral relativism'' is the key phrase here. It is the fundamental error Johnson sees in whatever society it appears - and, of course, in that ''heedless pursuit of liberal ideas'' where he so often finds it combined with a breakdown of order and retreat from personal responsibility. It is fascinating to follow him, as he links Einstein's concept of relativity in the physical world with Freud's and Marx's concepts in human and political relations.

With Einstein, as with today's ''sociobiologist'' Edward Wilson, Johnson sees a valuable willingness to test theories against empirical data. Not so with the moral relativists. For him their untestable theories have no authority against natural law and traditional Judeo-Christian absolutes.

And their predictions of the demise of religion have not come true. ''The outstanding non-event of modern times was the failure of religious belief to disappear,'' writes Johnson, noting that the present Pope came out of an officially atheistic state, Poland.

As for the rest of modern times, the Johnson version is different, to say the least.

The movies may be glorifying Gandhi and joining Churchill in deploring General Dyer's massacre of Indians at Amritsar; Johnson deglorifies Gandhi, brings forth every possible extenuating circumstance for Dyer, and says Churchill came to regret his speech against Dyer.

Prevailing Western opinion watches Spain and Portugal, hailing every step of democratic recovery from the dictatorships of Franco and Salazar. Johnson predicts history will take a far more favorable view of the dictators than is fashionable now.

Americans tend to recall Watergate as a massive abuse of power whose cleansing testified that ''the system works.'' Johnson minimizes it as ''the use of extra-legal means to protect the President and his policies,'' surrounded by media ''witchhunt'' and ''hysteria.''

Even conservative US President Reagan praises the peace effort of today's clergy while disagreeing with certain positions. Johnson finds a precursor in the clergy of a half century ago ''seizing on the peace issue as a remedy for declining congregations and their own flagging faith.''

Johnson's reinterpretations sometimes go so far as to seem distortions. He leaves responsible conservative opinion behind when he sees ''a return to vigorous enforcement of anti-trust legislation'' as a sign of trouble. And he gets some facts wrong, putting Cuba ''only forty miles'' from the United States (instead of 90), attributing America's constitutional protection of the press to the Fourth Amendment (instead of the First). One must hope he is more careful in matters that readers cannot so easily check.

Who wind up as the heroes of Johnson's modern times?

Not Franklin Roosevelt. Too frivolous. Not Nehru, not Hammarskjold. US Presidents Harding and Coolidge are nearer the mark. West Germany's Konrad Adenauer and Italy's Alcide de Gasperi are close to the top.

At one point Church-ill and Eisenhower are called the two greatest statesmen of the midcentury. This although Church-ill, with his World War II military aid to Stalin (at the cost of depriving Britain's base at Singapore), ''may have sacrificed a liberal empire in order to preserve a totalitarian one.'' And although Eisenhower, by failing to sign the 1954 Geneva accords calling for Vietnamese elections, may have been ''more responsible for the eventual mess in Vietnam than any other American.''

Johnson is not alone in raising Eisenhower's stock at the moment. Here is no bland emptyhead unable to control John Foster Dulles, unwilling to resist Joe McCarthy. He was ''wily and experienced.'' He kept Dulles ''on a secret, tight rein.'' He organized McCarthy's downfall behind the scenes. He demanded both congressional authorization and allied support as conditions for US military involvement anywhere. He ''concealed his gifts and activities because he thought it essential that the autocratic leadership, which he recognized both America and the world needed, should be exercised by stealth.''

Good old Ike a secret autocrat? It is part of the Johnsoniana a reader has to winnow. Then there was the public autocrat, France's Charles de Gaulle, whom Johnson seems to put above even Churchill and Eisenhower as ''the outstanding statesman of modern times.''

All this just begins to tap a volume that ranges the globe while suggesting, characteristically, that the Western democracies should have dropped the United Nations long ago.