By no means is it alone but it is absolutely unique in having imported millions of slaves across an ocean, predicating its initial society and economy on a race based caste system. Yes a civil war was fought but, even afterwards, many supported continuing this race based caste system and so it persisted in the form of segregation, which existed during the lives of our parents and grandparents. Without a legitimate act of national reflection and reconciliation these wounds and fault lines won't heal.