Are you asking for some examples of challenging the color barriers? if so then some examples are the Little Rock Nine trying to desegregate schools, and the Greensboro Four who sat at a counter in a diner marked only for whites and they were harassed but they were always polite.
Invasions by Barbarian tribes
Economic troubles and overreliance on slave labor
The rise of the Eastern Empire
Overexpansion and military overspending
Government corruption and political instability
The answer to your question is the letter B