Vladamir Lenin was a thinker already overdue by his time of writing. The traditional Marxist theory of history seemed to be less and less viable after a more obvious entrenchment of liberal government arose accompanied by the failure of the lower class to languish into poverty, atrophy and thus revolt against their masters. Socialist thought had fallen from the grace originally afforded to it; in the aftermath of the failed European Spring of 1848, the same year of the publishing of the Communist Manifesto, the possibility at least existed that a radical overthrow of society was probable, and perhaps with the vanguard of Communist Parties, such a revolution could be successful.
Albeit revolutionary fervor seemed to be on decline in Europe in the decades afterwards. For all the more radical thinking, radical action among the poorest was alleviated. Instead of being indued by the fires of class consciousness, national sentiments were on the rise and, even worse for the purposes of the Revolution, the material standard of living of the proletariat seemed to be on the rise in the most heavily industrialized areas.
Lenin's contribution to socialist thought is generally two-fold. On one hand, he needed to vindicate these apparent contradictions borne between the communist prediction and recent history as it had played out. Yet on the other, Lenin made the effort to explain contemporary political developments, which he noted for the apparent rise in imperialist ambitions among the most economically developed, by building imperialism into the framework of socialist eschatology.
The pre-Leninist narrative was composed of the following. Capitalism was a society in which all people could be reasonably divided into two groups, the property-less or generally poor proletariat and the bourgeoisie, the owners of capital, the means of production. Economic value stems, in this view, from the physical labor expended in production, and as the capitalist market functions, the capital-owning member of the bourgeoisie need not contribute his own effort due to the utilization of capital in his stay. The proletarian conversely must compete with his fellow proletarians for work, and must be reduced to marketing his labor for ever more competitive prices. Capitalists avoid this by inertly owning machinery.
History therefore is a struggle of the property-less to gain a legitimate share of the output of the means of production. Marx however would digitate that the end to this struggle was inevitable due to the inherent property of capital to aggregate. That is, those bourgeois with more capital could outproduce and undercut their other classmen with less capital; eventually more of the bourgeoisie would be bought out and reduced to members of the proletariat as well. With more and more people joining the lowest ranks, and with competition for jobs becoming more fierce as capital replaces more and more workers, people are forced lower than subsistence and must resort to force to rectify these economic injustices. Hence comes the Revolution as foretold by Marx.
Lenin's problem, and the problem of all other revolutionary socialists of the time was that this dream seemed further and further away from realization what with seemingly content masses with a visibly improved quality of life. The explication that he created was that the Revolution had been temporarily forestalled by capitalist markets seeking out foreign demand.
That is, although presumably the proletariat of industrialized capitalist countries proved too poor and too minor to buy the enormous quantity of products manufactured by the capitalist engine, the leading capitalists would direct their governments to engage in imperialist expansion to open up new markets to maintain such a capitalism which produced much, but impoverished even more. Thus imperialism was the solution to the aggregation of capital; since the domestic working class in increasing penury could not put forth enough money to buy the fruits of capitalism thus maintaining it, new consumers would need to be sought out.
Hence comes the encapsulation Imperialism is the final stage of capitalism. It was appealing to intellectually vagrant Marxists looking for a explanation of the apparent failure of socialist analysis, but it was even more appealing to those leftist and reformist movements in lands which fell victim to various forms of imperialism. Perhaps most strongly, it unified anti-government and anti-bourgeoisie sentiment as it proposed that in capitalist society, governing bodies only pursue the interests of the influential and wealthy owners of capital in their bouts of expansion overseas.
It needs no noting then that this idea is prevalent in our day and even amongst non-Marxists, although it also does not abide by the original specifications. Developed nations, especially the United States rarely have opportunities to interfere in the affairs of smaller nations without accusations of imperialism; wars and their discontents such as in the recent overthrow of the Hussein regime in Iraq exemplify that American policy is commonly thought of as selfish and acquisitive. However even the most formulaic accusations of malfeasance, for example that the United States invaded Iraq to secure stable oil reserves, doesn't match with the Leninist narrative at all; indeed, it is nearly diametrically opposed to it.
The problem of international capitalism as according to Lenin is not that the bourgeois machinery lacks the resources necessary to produce, but that it is so productive that its innate over-production necessitates the exportation of products to poorer countries. Strangely enough, imperialism is deplorable in Leninist thought because it consists in the expansion of capitalist methods of manufacturing and moving goods, not at all in the further expropriation and utilization of raw materials as is intuitively assumed by the common thinker.