Why do empires rise? Richard Drayton asks what motivates one nation to subordinate and impose its value system on others

All empires are expressions of inequality hidden behind a mask of community. By "empire" we may mean something inclusive, a space of shared peace, sovereignty or rights. Thus in 1533, Henry VIII's law proclaimed, justifying his pre-eminence over the English church, that "this realm of England is an empire". Or we may mean, as we have increasingly done over the past two centuries, a system through which a core population dominates a periphery, as Athens presided over the Delian League, and as the US dominates, in different ways, places such as Haiti or "allies" such as Britain. A given system will usually meet both criteria, depending on whether you look out from the centre or inwards, meaning different things to the dominant castes on the one hand, and to those who hew wood or sugar cane and gather water or oil on the other.

In both cases, there is a myth of shared virtue, usually for the consumption of the empire's elites, but one that may also attract those peripheral collaborators without whom no expansion is possible. That idea of community is usually proclaimed by those at the surface of imperial advance: as Spain's conquistadores burnt Arawak villages they carried the cross; today we drop cluster bombs, murder civilians and torture prisoners of war in the name of democracy and human rights.

The essential kind of inequality underlying all empires is access to violence. At the Roman origin of the word, the "imperator" was the military leader whose capacity to extract consent was the premise of the system.

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Empires, in the sense of a kingdom or a state, begin in the subordination of one community to a power that has the monopoly on legitimate violence within a frontier. Imperial systems grow rapidly at those moments when one community acquires particularly efficient tools or techniques for waging war, and can then include its neighbours in its territory or subordinate distant peoples.

By inventing the modern stirrup, the Mongols were able to fight with both hands and to dominate Eurasia. Roads and military organisation gave Incas supremacy in the Andes. Breakthroughs in maritime skill and artillery allowed the Portuguese, Dutch, English and French to muscle in on the trade of Africa and Asia after 1500. The classic "high imperialism" of the late 19th century depended directly on the weapons end of the industrial revolution, which gave Europeans those repeating rifles and machine guns that allowed them to command the interior of Africa (and indeed of North and South America) for the first time. The latest predatory phase of the American empire, with its ambitions for "full-spectrum dominance", is essentially a consequence of how satellite and electronic technologies opened up a new arms gap in the late 20th century.

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Why does efficiency in making violence so often turn into a programme of subordinating others? For the optimistic historian and economist Joseph Schumpeter, modern imperialism was only an atavism, the last expression of a medieval warrior aristocracy, soon to be swept away by the peace-loving cosmopolitanism of capitalism. I see it as a more ancient, and so far enduring, perversion of the human will for survival. We are driven as animals to reproduce and to secure the means of life for our kin. We have long taken the short cut to survival by seizing resources from those we perceive as not like us and therefore not deserving of equal treatment.

Leviticus XXV put the matter frankly: "The heathen that are round about you" would be the servants of the Hebrews, but "over your brethren... ye shall not rule one over another with rigour". Or as Tony Blair's confidante Robert Cooper (a senior British diplomat) put it in 2002: "The challenge to the postmodern world is to get used to the idea of double standards. Among ourselves, we operate on the basis of laws... But when dealing with (others), we need to revert to the rougher methods of an earlier era - force, pre-emptive attack, deception, whatever is necessary." Panic about scarcity or security, channelled through a racial or cultural idea of self and other, has generated over millennia that combination of ruthlessness, greed, self-righteousness and hypocrisy of which every empire is an expression.

A polity's will for power beyond its frontiers grows relative to its appetite for foreign resources. These may include fertile land for settlement (if you are willing to do a little "ethnic cleansing", as were British settlers in North America and Australia), slave or cheap labour (which might include fertile women), minerals, plunder (seizing gems and objets d'art , or imposing a tax regime, as did the British in India and the Germans in France), control of the supply of exotic commodities and markets for exports. The demand for these things, and thus the lunge outwards of Vikings, or Spanish hidalgos or Zulus, may be the expression of some new environmental pressures, population surges, agrarian crises, ages of ice or drought. The taste for luxuries may, quite independently, impel further aggression. But beyond material advantage, empires also offer symbolic and psychological rewards. Expansion gives the officer class of a society a space in which to exhibit its right to lead, while the men and women who rise to political power often seek exotic theatres in which to swagger.

Foreign adventures are sought for their political value, for wars easily generate a patriotic stupor: the ¹ó¨¹³ó°ù±ð°ù becomes an adored patriarch, and dissent is silenced as opposition to any elite interest becomes cast as taking the side of the foreigner. Empire, in any event, eases domestic social tensions, as subordinate groups at home are appeased by the idea that, relative to others, they are the master class.

The experiences of violence and inequality generate, in themselves, a terrible momentum. " Proprium humani ingenii est odisse quem laeseris ," wrote Tacitus: "It is in the nature of men to hate those whom we hurt" - to which he might have added, to imitate those who hurt us. The planning, justification and execution of that brutality, which is the basis of each empire, lead to a hardening of racisms and a decay of compassion.

The ability to cause pain and death to people who are unable to respond in kind encourages the powerful to see themselves as different from the others and becomes part of their identity. A cycle begins: it is no longer enough to be rich and safe at home - the ambition for an extrovert power, which demonstrates its virtue through war, is awakened.

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This taste for blood is harder to lose than colonies, as dangerous myths about Britain's need to "punch above its weight in the world" have recently shown. Imperial tradition is a hardy weed that plants roots deeply in a culture and takes hold easily in the cultures of those who collaborate with, and even those subordinated to, an empire. The seed of Rome is alive in all western imperialisms, while post-colonial elites, whether in 19th-century Latin America or 20th-century India, often extend to their more marginal compatriots the authoritarian paternalism of the old regime.

In time, the brutalised may become the brutalisers, and nations that began in an experience of oppression or a war of liberation against colonial tyranny may export a parasitic despotism.

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Empires do not live by violence alone, however. They need equally the idea of community: the myth of some universal interest of which the imperial power is the vanguard. At its simplest, the predators may identify their own community's happiness as a supreme end, relative to which others'

misery is insignificant. Power and the affluence that plunder makes possible at the imperial centre then come in a circular way to justify the idealistic violence that keeps expansion growing. They may confuse their own temporary advantages with the idea that they have been divinely elected to dominion over exotic lands or peoples. Religious arrogance is then often translated into the faith that the empire's laws, manners and styles of politics or economy are the best possible.

Peripheral people become, at a stroke, Kipling's "half devil and half child", the stubborn and backward who need to be guided by a strong hand towards progress. Those who do not yield to the natural superiority of Jehovah, or Roman law, or Jesus Christ, or Westminster-style parliaments, or privatisation - those, in other words, who refuse to give their resources and labour freely and to pray or play like their overlords - deserve to be broken. The most successful empires, however, will also bestow, as Gibbon wrote of Rome, "the freedom of the city... on all the gods of mankind", their flexibility allowing the energy of the conquered to be absorbed and harnessed in the arts of war or peace.

At some point, internal contradictions within imperial universalism may cause a moral crisis. People start to believe in its mask and to demand the same treatment for metropolitan and exotic peoples. But this in itself may generate new imperial causes, as the idea of the "rights of man" propelled French revolutionary conquests across Europe in the 1790s, and as Victorian anti-slavery after 1850 turned African kingdoms, which British slave traders had made rich and strong c. 1750, into objects of conquest.

The myth of an emancipatory imperialism, of conquest as liberation, that we come, in Palmerston's boast, "not to enslave but to set free", has proved a potent ideological talisman. It has lent comfort, at different moments, to those who bore the swastika into the Sudetenland, the hammer and sickle into Poland and the Union Jack and Stars and Stripes into Iraq. Violent is their awakening from this delusion.

Richard Drayton is senior lecturer in imperial and extra-European history since 1500 at Cambridge University.

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