Empires are built for a variety of motives -a quest for wealth, an urge to dominate, a wish to impose order on haphazard conquest, even a desire to do good. But, says Hugh Thomas, they do share one characteristic, they usually end when no one expects them to

There is much talk of empire at the moment as we debate whether we are entering a new age of imperialism. But what does the term mean in history? An empire is understood as a political undertaking in which one nation, one people or one tribe rules others for a variety of reasons. It would be wrong to restrict the use of the term to European political entities of which we (or some people) currently disapprove. The Mali empire on the Niger, the Chinese empire, the Mongol empire and the Aztec empires were all, in their way, empires owing nothing to Rome.

Nonetheless, all empires seem to have several things in common. First, most have been established, as John Seeley said of the British empire in a famous lecture in Oxford in 1870, more in "a fit of absence of mind" than as a deliberate scheme carefully contrived. Spain, for example, embarked on its American adventures in a haphazard way -Columbus founded the first city of the Spanish empire, Navidad, in what is now Haiti because one of his ships was wrecked near there and he did not have enough space on his other vessels to carry home the inhabitants who had settled there. One exception might be the German empire under Hitler, but that did not last long.

Second, empires usually have at least some economic motivation. The British, like the French, wanted sugar; hence the plantations in the Caribbean. We also needed, or thought we did at the time, tobacco; hence Virginia. The Aztecs needed more maize to eat and feathers for their celebrations; hence the expansion of Tenochtitl¨¢n to the coast. The Romans needed grain; hence their conquest of Egypt. Large customs-free zones, such as Austria-Hungary, had many benefits.

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There have almost always been strategic considerations, too. Rome conquered most of the cities of Italy, from Clusium onwards, because it feared alliances forming against it, and the same frame of mind was a decisive influence in its decision to expand to France, Spain, the Balkans, even Asia Minor and North Africa.

Britain and France quarrelled during the late 19th-century grab for Africa, and their territorial gains can partly be explained by a desire to compensate for what their rival was seizing. Much the same explanation can be found in Brazil in the 16th century, when France and Portugal were in dispute over different tracts of the Brazilian coast.

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One should always recognise, too, an element of genuine philanthropy in imperial motivation. The French general Andre Beaufre wrote of Morocco in his splendid memoir: "We carried through these colonial wars with a free conscience, free of all reserve, sure that we were carrying with us civilisation and progress, in order to help backward peoples out of the backward state in which they were. We fought adversaries whom we respected, certain that tomorrow they would become our friends... Our adversaries also respected us. They fought honourably but they knew that we stood for their future."

The British empire in West Africa is also a good example of muscular philanthropy in action. Having decided to bring the slave trade to an end, we found that the only way to subdue some African slave traders, such as the ruler of Lagos, was to overthrow them. This also explains the expansion inland in the Gold Coast after the purchase of the Dutch castle of Christiansborg a few years before.

I know from first-hand experience from my father and my uncles -who all served in Africa and East Asia -that they had views similar to those of Beaufre. I went once to Nigeria as an adviser to Lord Carrington, Margaret Thatcher's foreign secretary in the early 1980s. I was placed at dinner in Lagos next to the chief justice who, it turned out, had once been an office boy in the office of my uncle when he had been the governor's secretary in the 1920s. He remembered my uncle from his handwriting. "Not T. S. W.

Thomas?" he asked. He said to me: "Remember, in those days, the 1920s, we were not talking of 'independence' for Nigeria. We talked of preparing Nigeria for 'self-government'. There is a difference."

I don't think that Spain was ever concerned with preparing its conquered territories for self-government, and nor was the Mexican empire of Moctezuma. But there was a strong missionary element to Spanish expansion from the very beginning. The crown thought that it needed gold, but it also needed more Christian souls. I am sure that many Spaniards in the expedition of Cortes thought that in conquering the Aztecs, they were bringing a superior pantheon of deities (the Virgin Mary, St Martin, St Christopher and other saints, as well as the Trinity) and beliefs to a people who had suffered for several generations under the blight of a religion based on human sacrifice. Cortes thought that the Aztecs and their subjects were religious people, but they needed to be baptised. Even Russia, when conquering Samarkand in the 19th century, thought it was bringing the prospect of ordered civilisation.

One further point about empires. They usually end at a time when no one expects them to and when no one knows how to react. The Soviet historian Richard Pipes made that observation to me about the Soviet empire in 1981.

The same could have been said about Britain in the 1930s. One of the charms of being 70 years old is that I can remember a visit I made to Accra on the Gold Coast to see my father in 1938. How vivid the memories are: the polo club on Sunday, with delightful captains sweating in the sun -all my father's friends seem to have been captains -coming in, after a final chukka -utterly unaware that all that world would vanish within 20 years.

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In 1947, I took part in a public-school essay competition on the theme "The British empire has been the greatest force for peace and prosperity the world has ever seen". The prize: a month's holiday in Kenya. Even then, with India slipping from our imperial grasp, the Middle East and Africa seemed securely in our benign hands.

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In 1800, there was no sign that the rich and recently reformed Spanish Europe would collapse. The German polymath Humboldt was there about that time and what happened after 1808 would have astonished him -just as I suspect what happened after 1770 would have astonished a visitor to the 13 American colonies in 1763.

The Spanish empire has one other special characteristic. Its foundation by Columbus and his successors in the Caribbean, to which I devote much of my new book, was followed by soul-searching of a type that no other empire has ever experienced. What was the justification for the Spanish conquests? asked Bartolome de las Casas, the tireless defender of the indigenous people. Did Indians in the Caribbean and the New World have souls, and, if so, could they become Christians? In what circumstances, if any, could they be enslaved? The debates on these and related issues went to the very top of Spanish political life, for Las Casas was able to put his case to the King-Emperor Charles V himself.

But you will be asking: what about the American empire? I will have to decline a direct answer because I never like biographies of living people and, if there is an American empire, it is presumably something of this moment.

Second, the modern US is a unique phenomenon: a single power of a character never seen before in world history that is at any time able to act alone around the globe with the force it considers appropriate.

Third, the US certainly seems to have a desire for power. But "empire"? Does it have the trained administrators, the gifted linguists, the patient lawgivers, say, who are needed if their seemingly Roman position is to be balanced by Roman achievements? I see no sign of it. When Britain was training its subject people for self-government, including the Iraqis in the 1920s, there were many such men whose education was based on the classics and who knew far more about ancient Rome than they knew of modern America.

There are obvious disadvantages for us in the present US position. There could be benefits of a single-world empire, as suggested forcefully by the historian W. H. McNeil in his The Pursuit of Power . How to come to terms with that possibility should be the main subject of debate in politics now.

Hugh Thomas is a distinguished historian and author of many books.

His latest book, Rivers of Gold: The Rise of the Spanish Empire , is published this week by Weidenfeld & Nicolson, ?25.00.

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