Empires are characterized by the fact that they are not culturally or territorially contiguous entities, and involve the political domination of some peoples and territories by a core dominant civilization or class. In this sense imperialism is as old as agrarian civilization, and is not yet a purely historical phenomenon. However, all the evidence of this century suggests that nationalism and the democratization of the world are leading to the collapse of the world's surviving empires. The academic study of imperialism divides into economic and political schools of thought.
Economic theories are primarily Marxist in origin.
In their simplest forms they explain empires as driven by the urge to create wealth by force. In this perspective imperialism was an outgrowth of monopoly capitalism. Dependency theorists of imperialism, influenced by Marxism, suggest that the uneven nature of capitalist development led to the formation of a core of wealthy and powerful empires which competed for control over the less developed states of the periphery, a process which resulted in the purposeful 'non-development' or 'underdevelopment' of the peripheral states. The economic approach to explaining imperial formation is not universally accepted. The difficulty of balancing a costly centralized authority over distant territories with the need to maintain economic and political stability in the centre proves a herculean task for all great imperial powers and is normally the root cause of imperial decline.