When I did my degree we looked at why one society should create an empire.  What is necessary for one civilisation to dominate another. Empires are the product of imbalance between one civilisation and another.  Lets assume that we can judge society as a level of par and that this par is a measurement of technology, social strength and exploitation of local natural resources.  This imbalance is can be the result of one of the  civilisation being below par which allows it to be dominated or one of the civilisations being above par which allows it to dominated.  The British Empire, of the 19th century, was able to dominate India because the Maharajahs were unable to form a united front to stop them, Africa was only dominated when the technology gulf, between the British and the Africans, was at its greatest.

But what motivated the empire builders, what motivations can span the ages that separate the British, French and Americans empires of 19th and 20th centuries from the ancient empires of Persia, Nineveh, Tire, Roman and Greece.  Racial ideas of supremacy, war and then need to stabalise an unstable region or the needs of the belly.

Sounds odd but the belly, consumerism and the need to feed is possibly one of the more powerful illuminations for the explanation of imperialism.  Ever notice how empires do not spread to places where where there is nothing, no resources, no opposition and no food.  There are thousands of places all across the world where no empire has trod because there is nothing there to liberate, exploit or consume.  Empires spread into places where there is a spot of lunch.