By any reasonable standard, the violent overthrow of the largest, wealthiest slave society on earth ought to qualify as a revolution. Four million slaves were liberated during the American Civil War and with that the labor system of the South was radically transformed. Abolition was immediate and uncompensated. The “Slave Power” was overthrown, ending decades in which the South held disproportionate sway over the federal government. The Constitution was fundamentally restructured by three amendments that abolished slavery, redefined citizenship, banned racial discrimination in voting, and forever altered the relationship between the federal government and the states. The revolution secured the triumph of wage labor, paving the way for the Industrial Revolution of the late nineteenth century and with it a Gilded Age of capitalist plutocracy.

How did this happen? Ask a random group of American historians what caused the Civil War and they’re likely to reply in unison, “slavery.” Push them to elaborate and they’ll probably cite the southern secessionists who were as clear as could be that they were leaving the Union to protect slavery. But protect it from what? Was the North actually threatening slavery? Ask those questions and the same historians are likely to break out into rival and occasionally angry camps. On one side are those who insist that when the war began, northerners had no meaningful antislavery convictions to speak of. Emancipation was forced on an unwilling North and a reluctant Abraham Lincoln, either by the slaves themselves or by the exigencies of war. A few years back one historian of the secession crisis actually claimed that the slaves were freed “inadvertently.”

On the other side are those who see the rise of antislavery politics, culminating in the triumph of the Republican Party, as a major cause of the Civil War. Different historians stress different aspects of this process, but there is widespread agreement that antislavery politics not only split the nation, it also divided the North. Republicans ended up fighting a two-front war — against the South, obviously, but also against northern Democrats. This conflict within the North was epitomized in the famous series of debates between Abraham Lincoln and his Democratic rival Stephen Douglas.