Joseph A. Schumpeter: The Sociology of Imperialism (1918)
For it is always a question, when one speaks of imperialism, of the assertion of an aggressiveness whose real basis does not lie in the aims followed at the moment but an aggressiveness in itself. And actually history shows us people and classes who desire expansion for the sake of expanding, war for the sake of fighting, domination for the sake of dominating. It values conquest not so much because of the advantages it brings, which are often more than doubtful, as because it is conquest, success, activity. Although expansion as self-purpose always needs concrete objects to activate it and support it, its meaning is not included therein. Hence its tendency toward the infinite unto the exhaustion of its forces, and its motto: plus ultra. Thus we define: Imperialism is the object-less disposition of a state to expansion by force without assigned limits.
Our analysis of historical material show: First, the undoubted fact that object-less tendencies toward forceful expansion without definite limits of purpose, nonrational and irrational, purely instinctive inclinations to war and conquest, play a very great role in the history of humanity. As paradoxical as it sounds, innumerable wars, perhaps the majority of all wars, have been waged without sufficient reason. Secondly, the explanation of the martial, functional need, this will to war, lies in the necessities of a situation, in which peoples and classes must become fighters or go under, and in the fact that the physical dispositions and social structure acquired in the past, once existent and consolidated, maintain themselves and continue to work after they have lost their meaning and their function of preserving life. Thirdly, the existence of supporting elements which ease the continued life of these dispositions ans structures can be divided into groups. Martial dispositions are especially furthered by the groups ruling the internal relationships of interests. And with martial dispositions are allied the influences of all those who individually stand to gain, either economically or socially, by martial policy. Both groups of motives are in general overgrown by another kind of foliage which is not merely political propaganda but also individual psychological motivation. Imperialism is an atavism. It falls in the great group of those things that live on from earlier epochs, things which play so great a role in every concrete situation and which are to be explained not from the conditions of the present but from the conditions of the past. It is an atavism of social structure and an atavism of individual emotional habits. Since the necessities which created it have gone forever, it mustthough ever martial development tends to revitalize itdisappear in time.
Modern Imperialism is one of the heirlooms of the absolute monarchical state. The "inner logic" of capitalism would have never evolved it. Its sources come from the policy of the princes and the customs of a pre-capitalist milieu. But even export monopoly is not imperialism and it would never have developed to imperialism in the hands of the pacific bourgeoisie. This happened only because the war machine, its social atmosphere, and the martial will were inherited and because a martially-oriented class (i.e., the nobility) maintained itself in a ruling position with which of all the varied interests of the bourgeoisie the martial ones could ally themselves. This alliance keeps alive fighting instincts and ideas of domination. It led to social relations which perhaps ultimately are to be explained by relations of production but not by the productive relations of capitalism alone.
(From The Internet Modern History Sourcebook - Imperialism.)