A lot of people like to pretend that the Civil War wasn't about slavery. That theory is a way of excusing the South, of saying that the Confederate cause wasn't fundamentally about preserving an evil institution.

It's also nonsense — as this devastating video from Prager University explains:

Was the Civil War About Slavery? New Video! "Was the Civil War About Slavery?"What caused the Civil War? Did the North care about abolishing slavery? Did the South secede because of slavery? Or was it about something else entirely...perhaps states' rights? Colonel Ty Seidule, Professor of History at the United States Military Academy at West Point, settles the debate once and for all. Posted by Prager University on Monday, August 10, 2015

The video stars Colonel Ty Seidule, the head of the history department at the US Military Academy at West Point. Seidule clearly and comprehensively explains why the Civil War was, in fact, motivated by the South's desire to preserve slavery. As Confederate Vice President Alexander Stephens put it, "Our new government was founded on slavery."

Seidule's argument is especially compelling because he's mostly just quoting Confederates' own words. He points out, for instance, that the secession document in every Confederate state stated that protecting the South's "peculiar institution" (that is, slavery) was its reason for leaving the Union.

"Our new government was founded on slavery"

Seidule also debunks the idea that the war was about "states' rights." Before secession, he points out, slave states were happy to circumvent states' rights when it suited them. For example, Mississippi once complained that New York's notion of states' rights was too strong — because it prevented Mississippi slaveowners from bringing their slaves up North. This war wasn't about the principle of federal power; it was about the threat that the federal government might eventually use that power to abolish slavery.

Ultimately, Seidule's point boils down to something very simple: Be honest. Americans should be able to admit that a huge part of the country was devoted to slavery, so much so that they were willing to die for it. But at the same time, Americans should be proud that their government waged a war to end slavery.

"It is to America's everlasting credit that it fought the most devastating war in its history in order to abolish slavery," Seidule concludes. "As a soldier, I am proud that the United States Army — my army — defeated the Confederates."

Watch: Should America offer reparations?