Or just a feeble excuse for one of this country's two most monstrous crimes (along with Native American genocide)?

Slavery began here in 1619. So "slavery in the American colonies and U.S." lasted from 1619 until emancipation in 1863, or 244 years. It wasn't finally accomplished until two years later, of course, when the Civil War ended.

Nor did racism and abuse end with the Civil War, either.

Here's an excellent article by Henry Gates in Time:

https://time.com/5562869/reconstruction-history/