Historical materialism describes history “dialectically” by positing the dynamic interaction between the two necessary conditions--reflecting our human nature--that determine our human, historical project: (1)Natural necessity-- our condition as natural beings that absolutely depend on our natural environment to sustain and reproduce ourselves, and (2) historical necessity: our condition as self-conscious natural beings, who, in our own minds, objectify the world and are able to create societies to mediate our metabolic interaction with nature--and, when necessary, change them. These changes within and between societies--interacting, contradicting, colliding with one another--constitute the complex, non-linear, often paradoxical story of this evolution--our consciousness of the human condition and its "project": history.  Engels put in this way in a letter to Marx in 1853:

“I have the feeling that one fine day, thanks to the helplessness and spinelessness of all the others, our party will find itself forced into power, whereupon it will have to enact things that are not immediately in our own, but rather in the general, revolutionary....interest; in which event, spurred on by the proletarian populous, and bound by our own published statements...--more or less wrongly interpreted...and impulsively pushed through in the midst of party strife--we shall find ourselves forced to make communist experiments which...[will be]...untimely.  One then proceeds to lose one’s head--only physically speaking I hope--,  a reaction sets in and, until such time as the world is capable of passing historical judgement on this kind of thing, one will be regarded, not only as a brute beast...but also as stupid...  I don’t very well think of how it could happen otherwise...”

This could have been Lenin on the eve of the Bolshevik seizure of power-- despite the backward conditions in Czarist Russian and their isolation following the collapse of the 2nd International. As human, conscious, responsible human beings, they had not choice. Tragedy has been a necessary part of human history. 

Historical materialism, as a scientific tool and a philosophical stance is strongly influenced by Hegel, Darwin, radical (often idealistic) French and English egalitarianism, and the philological anthropology of the 19th century.  

It adopts Hegel’s dialectic and his view that human history is driven by a kind of collective consciousness (a “zeitgeist”), rippling through the human race, unequally, but resolving itself into various settled (often competing) social orders. It discards Hegel’s idealistic belief that the process is divinely directed, and adopts Darwin’s notion of evolution through adaptation and natural selection, to human social and intellectual development, human history--viewing the motivating drive in human social development as both the desire and the need of human beings to shape the societies (through which they mediate their metabolic relationship with the natural environment) to secure and enhance their lives, individually and collectively.  

Historical materialism’s philosophical stance can be traced to many strands: Greek materialist and epicurian philosophy; radical Christian antinomianism (anti-baptist, egalitarian Christian peasant revolts, Levellers, Diggers, etc.); the Enlightenment, and the French Revolution; romantic poets like Shelley and Heine; French utopian socialist thinkers; pioneering 19th century work on anthropology by researchers like Bachofen (Mother Right) and William Henry Morgan (Ancient Society). 

All these intellectual and spiritual currents, came together in the minds of serious thinkers in Europe faced with the effects of the early industrial revolution: what to make of the power of the new capitalist elite, of the political and social power they wielded through their ownership of incredibly productive (and destructive) machinery, of the social disruption and misery unleashed on the working population. 

Marx wrote Capital as a critique of political economy, but mainly in order to understand how the capitalist system worked--in order to find a way of changing it that would succeed.  He admired the spirit and morality of the anarchists, but not their nihilistic methods. He was Hegelian enough to know that one did not create something new from nothing, but, for better or worse, out of the old, for the old is what we inherit, materially and spiritually. The mind of a child is not a tabula rasa, it is the physical/material potential of the child to absorb the world and come to understand it through the experience of those who have gone before us, and, as circumstances suggest or demand, to imaginatively form something new out of what is still progressive (rather than destructive) in that intellectual and material inheritance. I think it was Istvan Meszaros who first named this dialectical process, out of which history is created,"historical necessity." Certainly there is a dire "historical necessity today.

No one who has read Capital will have missed the footnotes, accounting for what seems to be half of the book. These are full of detailed reports of the actual lives of uprooted people. Capital provides the necessary figures and calculations, but this witnessing of the human costs of industrial capitalism is the heart of Capital, the heart of this heartless, profoundly described and rigorously analyzed, world. Marx condemned mainstream followers of classical political economists like Adam Smith for treating surface market phenomena (and their calculations of “value”) as if they represented eternal forces of nature rather than merely reflecting--and obscuring--historically created, very unequal, social relations. He considered most of the work of their followers to be “vulgar,”  bad-faith apologetics, and not science. They returned, and their modern mainstream followers return the favor, dismissing "marxism" as “history” or “philosophy” or even “religion,” anything but economics. They are wrong. The economics in Capital is profound and rigorous science.  But bourgeois apologists are also right: an unshakable egalitarianism based upon human sympathy is the driving emotional force behind Capital and historical materialism, not cold calculation. 

Primitive accumulation--how industrial capitalism came into being will be taken up next. That will conclude my failed attempt to explicate basic marxism in 100 words or less.