It's the places where slavery ended first that became most prosperous. The places where the most overt forms of slavery still exist are the poorest.

The slave camps in the Soviet Union made the Soviet system unsustainable. Violent parasitic behavior is far less efficient & less productive than free & mutually beneficial exchange.

The US is now failing because slavery is returning in the form of monetarily & fiscally undermined property rights, along with a web of laws & regulations so complex that everyone is a criminal.

Reality generally gives the correct feedback for good & bad policies, but delusional & power hungry people see what they want to see.