These quotes from Karl Marx’s writing show that Marx did, despite WSWS claims, take a politically responsible position on the Civil War. He did not act as if Lincoln led a revolution but proved he ended up in power, swept in unconsciously by larger forces. These same forces caused the Civil War to take a “revolutionary turn”, that is, to break from the constitutional issues of state’s rights and secession towards the revolutionary issue of the rights of labor. This meant for Marx that a world socialist revolution against capital could now come to power, freeing European labor from indirect slavery under capitalism at the same time that it freed black labor from direct slavery.

This internationalism defined Marx’s position on the revolutionary potential unleashed by the Civil War. It did not signify Marx breaking from Marxism and designating Lincoln as an unconscious revolutionary or accepting bourgeois revolution as progress towards socialism. Marx did not see freeing the slaves as a bourgeois revolutionary task. On the contrary, he saw the cheap cotton provided by the Southern US as an absolutely necessary part of the development of European and world capitalism. The end of slavery in the South did not mean new hope for capitalism and bourgeois democracy but proved the approach of socialist revolution and the end of all exploitation of labor.

Marx argued that the WSWS is incorrect in assuming that capitalism ends slavery. History has proven Marx correct, even though the WSWS has the advantage of hindsight.

During the 300 years of transatlantic slave trade, about 12.5 million people were enslaved in the Americas. Today, an estimated 40.3 million people are enslaved globally. That means that there are 5.4 victims of modern slavery for every 1000 people in the world…. Evidently slavery has not merely endured – it has thrived.
-Nolan and Boersma, Addressing Modern Slavery, 2019

https://www.humanrights.unsw.edu.au/research/current-research/modern-slavery

October 12, 1862:

Lincoln is not the product of a popular revolution. This plebeian, who worked his way up from stone-breaker to Senator in Illinois, without intellectual brilliance, without a particularly outstanding character, without exceptional importance— an average person of good will, was placed at the top by the interplay of the forces of universal suffrage unaware of the great issues at stake. The new world has never achieved a greater triumph than by this demonstration that, given its political and social organization, ordinary people of good will can accomplish feats which only heroes could accomplish in the old world!

Battistini, Matteo. (2021). Karl Marx and the Global History of the Civil War: The Slave Movement, Working-Class Struggle, and the American State within the World Market. International Labor and Working-Class History. 100. 158-185. 10.1017/S0147547921000089. P. 161

1886 letter to Lafargue:

Until the colored man, yellow, red, brown, and black, becomes free, articulate, intelligent and the receiver of a decent income, white capital will use the profit derived from his degradation to keep white labor in chains. (Mateo 2021, P. 162)

Also found in Capital, Chapter 10, 1867:

In the United States of North America, every independent movement of the workers was paralysed so long as slavery disfigured a part of the Republic. Labour cannot emancipate itself in the white skin where in the black it is branded. But out of the death of slavery a new life at once arose. The first fruit of the Civil War was the eight hours’ agitation…

https://archive.org/stream/KarlMarxCapitalVolumeI-III/Karl%20Marx%20-%20Capital%20%28Volume%20I%29_djvu.txt

The Poverty of Philosophy, 1847:

Direct slavery is just as much the pivot of bourgeois industry as machinery, credits, etc. Without slavery you have no cotton; without cotton you have no modern industry. It is slavery that gave the colonies their value; it is the colonies that created world trade, and it is world trade that is the precondition of large-scale industry. Thus, slavery is an economic category of the greatest importance. (Mateo 2021, P. 164)

New York Daily Tribune, October 14, 1861:

As long as the English cotton manufactures depended on slave-grown cotton, it could be truthfully asserted that they rested on a twofold slavery, the indirect slavery of the white man in England and the direct slavery of the black men on the other side of the Atlantic. (Mateo 2021, P. 164)

New York Daily Tribune, November 7, 1861:

The first grand war of contemporaneous history is the American war. The peoples of Europe know that the Southern slaveocracy commenced that war with the declaration that the continuance of slaveocracy was no longer compatible with the continuance of the Union. Consequently, the people of Europe know that a fight for the continuance of the Union is a fight against the continuance of the slaveocracy—that in this contest the highest form of popular self-government till now realized is giving battle to the meanest and most shameless form of man’s enslaving recorded in the annals of history. (Mateo 2021, P. 165)

Letter to Engels, October 8, 1858:

The proper task of bourgeois society is the creation of the world market, at least in outline, and of the production based on that market. Since the world is round, the colonization of California and Australia and the opening up of China and Japan would seem to have completed this process. For us, the difficult question is this: on the Continent revolution is imminent and will, moreover, instantly assume a socialist character. Will it not necessarily be crushed in this little corner of the earth, since the movement of bourgeois society is still in the ascendant over a far greater area? (Mateo 2021, P. 179)