Final answer:
The anchor that justified American slavery was the idea of divine mandate, proven through religious texts.
Step-by-step explanation:
The idea that it's a divine mandate, proven through religious texts, was an anchor used to justify American slavery. According to historical sources, the Curse of Ham, found in the Bible, was repeatedly used as the most authoritative justification for slavery. It was believed that African Americans were descendants of Ham, and their enslavement was seen as a divine punishment.