"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." http://www.marxists.org/archive/marx/works/1861/10/25.htm
"the number of actual slaveholders in the South of the Union does not amount to more than three hundred thousand, a narrow oligarchy that is confronted with many millions of so-called poor whites, whose numbers have been constantly growing through concentration of landed property and whose condition is only to be compared with that of the Roman plebeians in the period of Rome's extreme decline. Only by acquisition and the prospect of acquisition of new Territories, as well as by filibustering expeditions, is it possible to square the interests of these poor whites with those of the slaveholders, to give their restless thirst for action a harmless direction and to tame them with the prospect of one day becoming slaveholders themselves." http://www.marxists.org/archive/marx/works/1861/10/25.htm
“In several parts of Capital I allude to the fate which overtook the plebeians of ancient Rome. They were originally free peasants, each cultivating his own piece of land on his own account. In the course of Roman history they were expropriated. The same movement which divorced them from their means of production and subsistence involved the formation not only of big landed property but also of big money capital. And so one fine morning there were to be found on the one hand free men, stripped of everything except their labour power, and on the other, in order to exploit this labour, those who held all the acquired wealth in possession. What happened? The Roman proletarians became, not wage labourers but a mob of do-nothings more abject than the former ‘poor whites’ in the southern country of the United States, and alongside of them there developed a mode of production which was not capitalist but dependent upon slavery. Thus events strikingly analogous but taking place in different historic surroundings led to totally different results. By studying each of these forms of evolution separately and then comparing them one can easily find the clue to this phenomenon, but one will never arrive there by the universal passport of a general historico-philosophical theory, the supreme virtue of which consists in being super-historical.” http://www.marxists.org/archive/marx/works/1877/11/russia.htm
He describes "military service" as one of the conditions "which hastened to so great an extent the ruin of the Roman plebeians".
"We are reminded [by the effect of the enclosures on 'the lower ranks of men'] of ancient Rome. 'The rich had got possession of the greater part of the undivided land. They trusted in the conditions of the time, that these possessions would not be again taken from them, and bought, therefore, some of the pieces of land lying near theirs, and belonging to the poor, with the acquiescence of their owners, and took some by force, so that they now were cultivating widely extended domains, instead of isolated fields. Then they employed slaves in agriculture and cattle-breeding, because freemen would have been taken from labour for military service. The possession of slaves brought them great gain, inasmuch as these, on account of their immunity from military service, could freely multiply and have a multitude of children. Thus the powerful men drew all wealth to themselves, and all the land swarmed with slaves. The Italians, on the other hand, were always decreasing in number, destroyed as they were by poverty, taxes, and military service. Even when times of peace came, they were doomed to complete inactivity, because the rich were in possession of the soil, and used slaves instead of freemen in the tilling of it.' (Appian: “Civil Wars,” I.7.) This passage refers to the time before the Licinian rogations. Military service, which hastened to so great an extent the ruin of the Roman plebeians, was also the chief means by which, as in a forcing-house, Charlemagne brought about the transformation of free German peasants into serfs and bondsmen." http://www.marxists.org/archive/marx/works/1867-c1/ch27.htm#n23
Ted