This faction has agreed with that position since 1861:
https://isreview.org/issue/80/karl-marx-and-american-civil-war
“THE CIVIL War is the defining event in the history of the United States, yet also the most misunderstood. More books are written on this war than on any period of US history, yet for all the words poured across the pages, the real cause of the war—slavery—is usually missed or obscured. Rather, there are tales of chivalrous Confederate generals heroically leading charges, drunken Union generals butchering their men in horrible frontal assaults, brothers fighting brothers in a pointless war that ravaged the land and wounded a people. Was the Civil War just a tragic mistake? A war like any other imperialist war the United States ruling class has its soldiers fighting in today? While some answer these questions with a yes, Karl Marx and Frederick Engels would have been taken aback. They would have resoundingly answered “no.” The Civil War, they believed, was not just another horrible atrocity, but rather a revolution that ended slavery and destroyed the slave-owners’ power as a class.
“Marx and Engels saw the events leading to the Civil War as momentous. In a January 1861 letter to Engels, written after the election of Republican candidate Abraham Lincoln, but before his inauguration, Marx wrote, “In my opinion, the biggest things that are happening in the world today are on the one hand the movement of the slaves in America started by the death of John Brown, and on the other the movement of the serfs in Russia.”
“During the war, Karl Marx and Frederick Engels contributed dozens of insightful articles for the New York Tribune and, later, for the Viennese Die Presse on political and military issues. Engels specialized on the military strategy of the Lincoln administration and that of the Confederate Jefferson Davis rebel government. Karl Marx had a more sweeping look at the conflict, from the economic development of the nation to the actions of the political and military leaders. Overall, Marx had a better grasp on the whole war. Both men saw the war as an extension of the American Revolution of 1776. Marx and Engels argued that Lincoln’s Emancipation Proclamation and the North’s arming of Black soldiers transformed the Civil War from a purely constitutional war to preserve the country with slavery intact, into a revolutionary war. They did not characterize the Civil War as a socialist revolutionary war, but they believed that it advanced the cause of all workers, both white and Black, by destroying chattel slavery. The revolution armed former slaves, destroyed the horrendous institution of slavery without compensation to the slave-owners, and opened the way for a struggle between the working class and the capitalist class. As a result, our next revolution in this country will be a working-class revolution.
You seem to have missed what Marx actually said about the reason for the war: “The war between the North and South is a tariff war. The war, is further, not for any principle, does not touch the question of slavery, and in fact turns on the Northern lust for power.” Karl Marx, 1861.
Marx was not alone. The hallowed author Charles Dickens wrote, also in 1861: “The Northern onslaught upon slavery is no more than a piece of specious humbug disguised to conceal its desire for economic control of the United States.” And: “Union means so many millions a year lost to the South; secession means loss of the same millions to the North. The love of money is the root of this as many, many other evils. The quarrel between the North and South is, as it stands, solely a fiscal quarrel.”