Just because a guy's in uniform and touts West Point credentials, don't assume he's giving you the facts without spin. History can be complicated.
The historical record shows a northern controlled Congress passed a steep tariff targeting southern states 40 days prior to southern shots being fired at Fort Sumter. The southern states, feeling themselves treated as step-children, grew tired of funding the lion's share of the federal budget, and chose to walk away.
In the Prager video, Col. Ty Seidule, a senior West Point history prof, makes effective use of Southern political quotes to support his slavery-was-the-cause Civil War narrative, but Col. Seidule fails to share comments from northern politicians that might run counter to his position.
As long as the south was paying the bills, Union politicians weren't so eager to kill their cash cow.
Here's some quotes from one noted Northerner:
"I do not now, nor ever did, stand in favor of the unconditional repeal of the Fugitive Slave law... I do not now, or ever did, stand pledged against the admission of any more slave States into the Union... I do not stand to-day pledged to the abolition of slavery in the District of Columbia... I do not stand pledged to the prohibition of the slave-trade between the different States." - Abraham Lincoln, 1858, in his second debate with Stephen Douglas.The American Civil War was well underway before Lincoln moved to rebrand it was an anti-slavery crusade. Interestingly enough, socialists like to brag it was Karl Marx who inspired Lincoln's shift.
One more point of contradiction for the "it was all about" slavery notion: As the Civil War raged on, West Virginia was admitted to the Union as a slave holding state. It happened in 1863, six months after Lincoln's Emancipation Proclamation touted the ending of slavery in the break-away states.
West Virginia's state legislature eventually abolished slavery there in 1865, anticipating passage of the 13th Amendment to the U.S. Constitution.
I'm not arguing right or wrong of any of the actions detailed here, merely offering up examples from America's historical record that the path to the American Civil War, and its embrace of emancipation, wasn't clean cut as some make it out to be.
Like I said from the start, history can be complicated; it's the norm rather than the exception.