Mr Bush's speech in London showed the thinking of a great world leader

For three years, George Bush has been the most powerful man in the world. But within months of becoming President, he was reminded that military predominance does not guarantee invulnerability; that America faced threats, which would test its resolve to the uttermost; that America would never be able to enjoy its might and its wealth within secure frontiers, unless it achieved a constructive engagement with the rest of mankind - unless it found a way of spreading its bounty and of helping all nations take their place in the ascent towards prosperity and freedom.

This is a task, not for a presidency, but for a century. Even so, Mr Bush is determined to set America on a course of challenge, danger - and ultimate triumph.

It is an awesome undertaking, which has involved hard thinking. Last Wednesday, a London audience had the privilege of hearing the President expound his thinking, and Mr Bush excelled himself. That was no easy task, for he always rises to a big occasion with a fine speech. But even by his standards, this was special. His text had lightness of touch, wit, and grace notes. There was also close argument on great themes, leading to sweeping and world-historical conclusions. Above all, there was idealism. It was the finest piece of political oratory since the era of Kennedy and de Gaulle.

It is worth considering the contrast between Mr Bush and his closer competitors. Richard Nixon had an outstanding intellect. He was the most accomplished analysist of foreign affairs ever to occupy the White House. Yet as President, for reasons discussed at lengths in Kissinger's memoirs, he never found his own tone of voice.

Bill Clinton and Tony Blair are more naturally gifted speakers than George Bush. That may be part of the problem. They owe their successes to an actor's quickness, not a statesman's depth. In the roar of the crowds, Messrs Blair and Clinton find it easy to re-invent themselves. Whatever the noise level, George Bush always hears the still, small voice.

Mr Bush is no actor. He can only play himself. A man of simple, if hard-won, piety, he may well be the least guileful occupant of the White House. On Wednesday, he used "Puritan" as a term of approval. George Bush himself is a fusion of Puritan New England and Gawd-day-um West Texas. Puritanism is the moral spine of the Great Republic. But the vast skies and limitless horizons of Texas are its moral vision. America is about enduring values and endless renewal. So is its President.

So was Ronald Reagan, who fitted America like a comfortable old suit. His predecessor Jimmy Carter, the most mean-spirited of Presidents, seemed to relish the prospect of reducing his fellow-Americans to his own level of gloominess, mediocrity and self-hatred. A Carter speech sounded like a sick old dog gnawing on a meatless bone. After that, Mr Reagan was a tonic. He, too, was a far more natural speaker than George Bush, and some of his one-liners are an endowment of the English language: "Mr Gorbachev, tear down that wall."

But Mr Reagan lacked Mr Bush's intellectual energy. I once suggested to Margaret Thatcher that there was a paradox. However much she admired President Reagan, she would not have employed him in her Government, because he could not grasp detail. "Ron may not understand detail," she replied "But he does understand the principles necessary for the restoration of American greatness."

She was right, yet in Ronald Reagan's day, that was a simple task. Confronted by the Cold War and the challenge of the Evil Empire, it was easy for the President to respond, with an old-fashioned arms race drawing on new-fangled technology which the Soviets could not hope to emulate. There is no equivalent arms race in the war against Islamic terrorism, which is why the world is now more dangerous than it was in the final phase of the Cold War. It was safe for President Reagan to encourage Americans to feel at ease with themselves. President Bush has had to remind them that: "The evil is in plain sight. The danger only increases with denial."

President Reagan was intellectually lazy, at a time when that did not matter. President Bush, who has had intellectualism forced on him by circumstances, has taken to it much more readily than those who knew the young George Bush would ever have suspected. As a speechmaker, Ronald Reagan reached rhetorical heights which only Kennedy equalled and Churchill surpassed. But overall, George Bush, beats Ronald Reagan in weight of intellectual firepower.

Margaret Thatcher never lacked that firepower. Though not by temperament an intellectual, she was the first Conservative Prime Minister to use the word as a term of approval without any ironic counterpoint. She was always happy to deliver speeches with an intellectual content. Even so, she must give precedence to George Bush. His intellectualism has the added weight of imperium. When he outlines the principles which ought to govern the world, he is - at least pro tempore - in a position to reinforce his intellectual argument with an imperial will.

Harold Macmillan, happy to acquiesce in a subordinate role, said the British ought to try to play Greeks to the Americans' Romans. Mrs Thatcher was not a natural consort, yet that was the most to which she could aspire. This may explain why none of her speeches, however impressive, reached the heights which President Bush attained last Wednesday. His language was enhanced by imperial purple.

If only his beneficent use of imperial authority were more widely appreciated. On Wednesday, George Bush said the following about the Middle East: "The forward strategy of freedom must also apply to the Arab-Israeli conflict ... Our commitment remains firm. We seek justice and dignity. We seek a viable, independent state for the Palestinian people, who have been betrayed by others for too long. We seek security and recognition for the state of Israel, which has lived in the shadow of random death for too long.

"Israel should freeze settlement construction, dismantled unauthorised outposts, end the daily humiliation of the Palestinian people, and not prejudice final negotiations with ... walls and fences. Arab states should ... establish normal relations with Israel."

No American President has ever expressed such sympathy with the Palestinian people, or such implicit criticism of Israel's policy towards them. Yet how much credit has George Bush received in Britain? No sane person would expect sense from the French, the Germans and the European Commission. Confronting the misery of the last century in Europe, those grumbling dotards respond, not by lamenting their own failure but by resenting anyone else's success. Still unable to acknowledge the fact that America saved Europe, they cannot bear to admit that America is now the only hope of remedying the rest of the world's wrongs.

Yet even in Britain there are cavillers. But I defy any fair-minded person to read Mr Bush's speech and refuse to recognise his pivotal role in the Middle East peace process.