While distracting myself during the task of selecting a T-shirt to wear today, I pulled a collection of political writings by Marx and Engels off the shelf and found their review essay on world events of 1850. Here's what they have to say about slavery in the United States:
The golden age and the iron age are long past; it was reserved for the nineteenth century, with its intelligence, world markets, and colossal productive resources, to usher in the cotton age. At the same time, the English bourgeoisie has felt more forcefully than ever the power which the United States exercises over it, as a result of its hitherto unbroken monopoly of cotton production. It has immediately applied itself to the task of breaking this monopoly. Not only in the East Indies, but also Natal, the northern region of Australia and all parts of the world where climate and conditions allow cotton to be grown, it is to be encouraged in every way. At the same time, that section of the English bourgeoisie kindly disposed towards the Negro has made the following discovery: 'That the prosperity of Manchester is dependent on the treatment of slaves in Texas, Alabama and Louisiana is as curious as it is alarming.' (Economist, 21 September 1850) That the decisive branch of English industry is based upon the existence of slavery in the southern states of the American union, that a Negro revolt in these areas could ruin the whole system of production as it exists today is, of course, an extremely depressing fact for the people who spent 20 million Pounds a few years ago on Negro emancipation in their own colonies. However, this fact leads to the only realistic solution of the slave question, which has recently been the cause of such long and violent debate in the American Congress. American cotton production is based on slavery. As soon as the industry reaches a point where it cannot tolerate the United States' cotton monopoly any longer, cotton will be successfully mass-produced in other countries, and it is hardly possible to achieve this anywhere today except with free workers. But as soon as the free labour of other countries can deliver sufficient supplies of cotton to industry more cheaply than the slave labour of the United States, then American slavery will be broken together with the American cotton monopoly and the slaves will be emancipated, because they will have become useless as slaves. Wage labour will be abolished in Europe in just the same way, as soon as it becomes not only unnecessary for production, but in fact a hindrance to it.
When reading the neocons who note that all of Marx's predictions were wrong, it's important to distinguish between his and Engels's more exhortative writings and the concreteness of pieces like the one quoted above. Our own highly-paid economic analysts are no different, really.
Having said that, though, a consistent weakness of Marx and Engels's thinking does manifest itself in this passage. They struggle to account for political action that runs counter to short-term economic reason, such as the drive to abolish slavery in both the British Empire and the United States. While their analysis would surely have proven right had economic "nature" taken its course, the Civil War accelerated matters considerably.
We should also pay attention to the ambiguity of the phrase "free workers." While wage labour may have triumphed over outright slavery in many cases, the wages of those "free workers" are often so low that they don't even provide the basic requirements of food and shelter. The global economy continues to derive its energy from the differential between haves and have-nots.
Still, I find this passage quite useful in the context of current debates about sweatshop labor and the like.