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Of course the Civil War was about slavery, and everyone knew it at the time. No, Nikki Haley, it wasn't about states' rights, except to the extent that Southern states were trying to force Northern states to help maintain slavery — something that, as I'll explain in a bit, has echoes in the current fight over abortion rights.

So Haley deserves all the condemnation she received for initially refusing to acknowledge the obvious in a campaign stop last week.

But it may be worth delving a bit deeper into the background here. Why did slavery exist in the first place? Why was it confined to only part of the United States? And why were slaveholders willing to start a war to defend the institution, even though abolitionism was still a fairly small movement and they faced no imminent risk of losing their chattels?

Let me start with an assertion that may be controversial: The American system of chattel slavery wasn't motivated primarily by racism, but by greed. Slaveholders were racists, and they used racism both to justify their behavior and to make the enslavement of millions more sustainable, but it was the money and the inhumane greed that drove the racist system.

Back in 1970, Massachusetts Institute of Technology economist Evsey Domar published a classic paper titled "The Causes of Slavery or Serfdom: A Hypothesis," which started with a historical observation that probably surprised most of his readers. Everybody knew that Czarist Russia was a nation where serfs were tied to the land; but Russian serfdom, it turned out, wasn't an ancient institution dating back to the depths of medieval history. It was, instead, introduced in the 16th and 17th centuries — after gunpowder finally gave peasant infantry the military upper hand over nomadic horse-archers, allowing the Russian Empire to expand into vast, fertile new territories.

As Domar pointed out, there's little reason to enserf or enslave a worker (not quite the same thing, but let's leave that aside) if labor is abundant and land is scarce, so that the amount that worker could earn if he ran away barely exceeds the cost of subsistence. But if land becomes abundant and labor scarce, the ruling class will want to pin workers in place, so they can forcibly extract the difference between the value of what workers can produce — strictly speaking, their marginal product — and the cost of keeping them alive.

Hence the rise of serfdom as Russia expanded east, and the rise of slavery as Europe colonized the New World.

In fact, the real historical puzzle is why high wages didn't always lead to widespread slavery or serfdom. As Domar himself pointed out, serfdom in the West had more or less withered away by around 1300, because Western Europe was overpopulated given the technologies of the time, which in turn meant that landowners didn't need to worry that their tenants and workers would leave in search of lower rents or higher wages. But the Black Death caused populations to crash and wages to soar. In fact, for a while, real wages in Britain reached a level they wouldn't regain until around 1870.

Yet serfdom wasn't reimposed, for reasons that aren't entirely clear. One thought, however, is that holding people captive in order to steal the fruits of their labor isn't easy. (Escaped serfs were a significant issue in Russia, as were escaping and rebelling slaves in America — the Second Amendment was largely about making it easier to hold slaves down. A slave rebellion led in 1848 to emancipation on St. Croix, where President Joe Biden spent his most recent vacation.) Which brings us to the story of the U.S. Civil War.

Labor was scarce in pre-Civil War America, so free workers earned high wages by European standards. Australia — another land-abundant, labor-scarce nation — more or less matched America; elsewhere, workers earned much less.

Landowners, of course, didn't want to pay high wages. In the early days of colonial settlement, many Europeans came as indentured servants — in effect, temporary serfs. But landowners quickly turned to African slaves, who offered two advantages to their exploiters: Because they looked different from white settlers, they found it hard to escape, and they received less sympathy from poor whites who might otherwise have realized that they had many interests in common. Of course, white southerners also saw slaves as property, not people, and so the value of slaves factored into the balance sheet of this greed-driven system.

So, again, the dynamic was one in which greedy slaveholders used and perpetuated racism to sustain their reign of exploitation and terror.

Because U.S. slavery was race-based, however, there was a limited supply of slaves, and it turned out that slaves made more for their masters in Southern agriculture than in other occupations or places. Black people in the North were sold down the river to Southern planters who were willing to pay more for them, so slavery became an institution peculiar to one part of the country.

As such, slaves became a hugely important financial asset to their owners. Estimates of the market value of slaves before the Civil War vary widely, but they were clearly worth much more than the land they cultivated, and may well have accounted for the majority of Southern wealth. Inevitably, slaveholders became staunch defenders of the system underlying their wealth — ferocious and often violent defenders (remember bleeding Kansas), because nothing makes a man angrier than his own, probably unacknowledged suspicion that he's actually in the wrong.

Indeed, slaveholders and their defenders lashed out at anyone who even suggested that slavery was a bad thing. As Abraham Lincoln said in his Cooper Union address, the slave interest in effect demanded that Northerners "cease to call slavery wrong, and join them in calling it right."

But Northerners wouldn't do that. There were relatively few Americans pushing for national abolition, but Northern states, one by one, abolished slavery in their own territories. This wasn't as noble an act as it might have been if they had been confiscating slaveholders' property, rather than in effect waiting until the slaves had been sold. Still, it's to voters' credit that they did find slavery repugnant.

And this posed a problem for the South. Anyone who believes or pretends to believe that the Civil War was about states' rights should read Ulysses S. Grant's memoirs, which point out that the truth was almost the opposite. In his conclusion, Grant noted that maintaining slavery was difficult when much of the nation consisted of free states, so the slave states in effect demanded control over free-state policies. "Northern marshals became slave-catchers, and Northern courts had to contribute to the support and protection of the institution," he wrote.

This should sound familiar. Since the Supreme Court overturned Roe v. Wade, states that have banned abortion have grown increasingly frantic over the ability of women to travel to states where abortion rights remain; it's obvious that the right will eventually impose a national abortion ban if it can.

For a long time, the South actually did manage to exercise that kind of national control. But industrialization gradually shifted the balance of power within the United States away from the South to the North. So did immigration, with very few immigrants moving to slave states.

And the war happened because the increasingly empowered people of the North, as Grant wrote, "were not willing to play the role of police for the South" in protecting slavery.

So yes, the Civil War was about slavery — an institution that existed solely to enrich some men by depriving others of their freedom. And there's no excuse for anyone who pretends that there was anything noble or even defensible about the South's cause: The Civil War was fought to defend an utterly vile institution.

Paul Krugman has been a columnist for the New York Times since 1999. He is distinguished professor in the Graduate Center Economics Ph.D. program and distinguished scholar at the Luxembourg Income Study Center at the City University of New York. In addition, he is professor emeritus of Princeton University's Woodrow Wilson School. In 2008, he was awarded the Nobel Memorial Prize in Economic Sciences for his work on international trade theory.