Slavery in the USA.
It seems to me that much of the destruction of America can be traced to "civil rights" arguments stemming from slavery. I often wonder what our society would be like if we never had slaves in the US. Unfortunatetly, we did and have suffered for it ever since.
It's almost like some sort of Greek tragedy, with America as the great hero, but with a fatal flaw from birth that will be its ultimate downfall.