"The cultivation of the southern export articles, cotton, tobacco, sugar , etc., carried on by slaves, is only remunerative as long as it is conducted with large gangs of slaves, on a mass scale and on wide expanses of a naturally fertile soil, which requires only simple labour. Intensive cultivation, which depends less on fertility of the soil than on investment of capital, intelligence and energy of labour, is contrary to the nature of slavery. Hence the rapid transformation of states like Maryland and Virginia, which formerly employed slaves on the production of export articles, into states which raise slaves to export them into the deep South.
Even in South Carolina, where the slaves form four-sevenths of the population, the cultivation of cotton has been almost completely stationary for years due to the exhaustion of the soil. Indeed, by force of circumstances South Carolina has already been transformed in part into a slave-raising state, since it already sells slaves to the sum of four million dollars yearly to the states of the extreme South and South-west. As soon as this point is reached, the acquisition of new Territories becomes necessary, so that one section of the slaveholders with their slaves may occupy new fertile lands and that a new market for slave-raising, therefore for the sale of slaves, may be created for the remaining section. It is, for example, indubitable that without the acquisition of Louisiana, Missouri and Arkansas by the United States, slavery in Virginia and Maryland would have been wiped out long ago. In the Secessionist Congress at Montgomery, Senator Toombs, one of the spokesmen of the South, strikingly formulated the economic law that commands the constant expansion of the territory of slavery. 'In fifteen years,' said he, 'without a great increase in slave territory, either the slaves must be permitted to flee from the whites, or the whites must flee from the slaves.'
"As is known, the representation of the individual states in the Congress House of Representatives depends on the size of their respective populations. As the populations of the free states grow far more quickly than those of the slave states, the number of Northern Representatives was bound to outstrip that of the Southern very rapidly. The real seat of the political power of the South is accordingly transferred more and more to the American Senate, where every state, whether its population is great or small, is represented by two Senators. In order to assert its influence in the Senate and, through the Senate, its hegemony over the United States, the South therefore required a continual formation of new slave states. This, however, was only possible through conquest of foreign lands, as in the case of Texas, or through the transformation of the Territories belonging to the United States first into slave Territories and later into slave states, as in the case of Missouri, Arkansas, etc.
[...] "The whole movement was and is based, as one sees, on the slave question. Not in the sense of whether the slaves within the existing slave states should be emancipated outright or not, but whether the twenty million free men of the North should submit any longer to an oligarchy of three hundred thousand slaveholders; whether the vast Territories of the republic should be nurseries for free states or for slavery; finally, whether the national policy of the Union should take armed spreading of slavery in Mexico, Central and South America as its device."
-- Karl Marx, Oct. 1861, on the causes of the American Civil War.
See also: Karl Marx and the American Civil War in the International Socialist Review; Marx on Lincoln and the Emancipation Proclamation ("Reason nevertheless prevails in human history"); and Marx's relationship with Lincoln in An Unfinished Revolution.