Why did the United States...

...fight a brutal civil war in the nineteenth century, resulting in the deaths of over 600,000 Americans? If you're an economic determinist, i. e., it's all about the money, you may be interested in this quote from David Blight, via Ta-Nehisi Coates (my emphasis):

By 1860 there were approximately 4,000,000 slaves in the United States, the second largest slave society--slave population--in the world. The only one larger was Russian serfdom. Brazil was close. But in 1860 American slaves, as a financial asset, were worth approximately three and a half billion dollars--that's just as property. Three and a half billion dollars was the net worth, roughly, of slaves in 1860. In today's dollars that would be approximately seventy-five billion dollars.

In 1860 slaves as an asset were worth more than all of America's manufacturing, all of the railroads, all of the productive capacity of the United States put together. Slaves were the single largest, by far, financial asset of property in the entire American economy. The only thing worth more than the slaves in the American economy of the 1850s was the land itself, and no one can really put a dollar value on all of the land of North America.