The US Civil War started 150 years ago. During this time, Karl Marx wrote about the war and what it meant for America and the development of capital itself. You can find various references to the war in his magnum opus where he often refers to the war as the “slave owners rebellion.”
One article he wrote that I find to be of interest is one in which he criticized the media coverage of the war (via criticizing the London Times). The arguments he refutes in the article (the war being about tariffs, sovereignty, etc.) still come up today in debates about the causes of the war, even leading some folks to go as far as to say that the civil war was not about slavery despite the overt explanation by the South that their intention was to preserve slavery (see my article on the anniversary of the secession of South Carolina that covers this topic).
Some quotes from:
The war between North and South — so runs the first excuse — is a mere tariff war, a war between a protectionist system and a free trade system, and Britain naturally stands on the side of free trade. Shall the slave-owner enjoy the fruits of slave labour in their entirety or shall he be cheated of a portion of these by the protectionists of the North? That is the question which is at issue in this war. It was reserved for The Times to make this brilliant discovery. The Economist, The Examiner, The Saturday Review and tutti quanti expounded the theme further. It is characteristic of this discovery that it was made, not in Charleston, but in London. Naturally, in America everyone knew that from 1846 to 1861 a free trade system prevailed, and that Representative Morrill carried his protectionist tariff through Congress only in 1861, after the rebellion had already broken out. Secession, therefore, did not take place because the Morrill tariff had gone through Congress, but, at most, the Morrill tariff went through Congress because secession had taken place. When South Carolina had its first attack of secession in 1831, the protectionist tariff of 1828 served it, to be sure, as a pretext, but only as a pretext, as is known from a statement of General Jackson. This time, however, the old pretext has in fact not been repeated. In the Secession Congress at Montgomery all reference to the tariff question was avoided, because the cultivation of sugar in Louisiana, one of the most influential Southern states, depends entirely on protection.
For hardly had the Kansas-Nebraska Bill gone through, which wiped out the geographical boundary-line of slavery and made its introduction into new Territories subject to the will of the majority of the settlers, when armed emissaries of the slaveholders, border rabble from Missouri and Arkansas, with bowie-knife in one hand and revolver in the other, fell upon Kansas and sought by the most unheard-of atrocities to dislodge its settlers from the Territory colonised by them. These raids were supported by the central government in Washington. Hence a tremendous reaction. Throughout the North, but particularly in the North-west, a relief organisation was formed to support Kansas with men, arms and money. Out of this relief organisation arose the Republican Party, which therefore owes its origin to the struggle for Kansas. After the attempt to transform Kansas into a slave Territory by force of arms had failed, the South sought to achieve the same result by political intrigues. Buchanan’s government, in particular, exerted its utmost efforts to have Kansas included in the States of the Union as a slave state with a slave constitution imposed on it. Hence renewed struggle, this time mainly conducted in Congress at Washington. Even St[ephen] A. Douglas, the chief of the Northern Democrats, now (1857 – 58) entered the lists against the government and his allies of the South, because imposition of a slave constitution would have been contrary to the principle of sovereignty of the settlers passed in the Nebraska Bill of 1854. Douglas, Senator for Illinois, a North-western state, would naturally have lost all his influence if he had wanted to concede to the South the right to steal by force of arms or through acts of Congress Territories colonised by the North. As the struggle for Kansas, therefore, called the Republican Party into being, it at the same time occasioned the first split within the Democratic Party itself.
The Republican Party put forward its first platform for the presidential election in 1856. Although its candidate, John Fremont, was not victorious, the huge number of votes cast for him at any rate proved the rapid growth of the Party, particularly in the North-west. At their second National Convention for the presidential election (May 17, 1860), the Republicans again put forward their platform of 1856, only enriched by some additions. Its principal contents were the following: Not a foot of fresh territory is further conceded to slavery. The filibustering policy abroad must cease. The reopening of the slave trade is stigmatised. Finally, free-soil laws are to be enacted for the furtherance of free colonisation.
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.
There is an entire section of writings by Marx and Engels on the US Civil war that can be found at the Marxist Internet Archive following this link.